论文部分内容阅读
Abstract Rice yields and nitrogen use efficiencies were studied at five sites in southwest China using two nitrogen fertilization rates and five controlled-release urea （CRU） to ordinary urea （U） ratios. The fertilizer treatments significantly increased rice yields compared with the control （no nitrogen added） yields to different degrees at different sites. Applying CRU and U increased the rice yield more than adding the same amount of nitrogen as U only. Higher increasing production rate were found using a nitrogen application rate of 105 kg/hm2 than 150 kg/hm2. A 70∶30 CRU∶U ratio increased the yield more than other four ratios. Nitrogen use efficiency was 21.9% higher using a nitrogen application rate of 105 kg/hm2 than 150 kg/hm2， and 46.6%， 38.1%， 34.7%， and 22.2% higher than when only U was applied when CRU∶U ratios of 70∶30， 50∶50， 100∶0， and 30∶70， respectively， were used. A 70∶30 CRU∶U ratio gave the highest economic output （yuan/hm2）. Applying both CRU and U gave an output 3 078.87 yuan/hm2 higher at a nitrogen application rate of 150 kg/hm2 than at a nitrogen application rate of 105 kg/hm2. Economic output was always higher using both CRU and U than using U only. The highest economic output was given using a 70∶30 CRU∶U ratio. Increasing the amount of nitrogen added decreased the output efficiency （per hm2） because CRU is expensive. Significant relationships were found between the yield increase rate and the proportion of CRU added （regression equation y =7.429x -185.7， R2 = 0.663） and between the total rainfall over the whole growth period and the proportion of CRU added （y = -0.087 1x + 112.29， R2 = 0.687 9）. These regression equations can be used to determine the appropriate proportion of CRU that should be added at a site， depending on the rainfall and target rice yield.
Key words Ecological conditions； Fertility； CRU∶U； Yield； Nitrogen use efficiency； Output efficiency
The rice yield in China was 6 810.7 kg/hm2 at the end of the 12th Five-Year Plan Period， mainly because of the use of improved rice varieties， advances in rice-growing techniques， and increases in fertilizer application rates. China has 7% of global arable land but accounts for one-third of global fertilizer use. Fertilizer consumption per hm2 in China is more than three times of the global average， but fertilizer use efficiency is about 30% lower in China than in developed countries[1-3]. This has resulted in increasingly serious environmental pollution. Considerable attention has therefore been paid to improving nitrogen use efficiency by attempting to optimize the types of nitrogen fertilizers used and the application methods. Several studies have been focused on the use of controlled-release nitrogen fertilizers[4-9]， controlled-release nitrogen fertilizer application rates[10-11]， controlled-release nitrogen fertilizer performances compared with urea[12-15]， controlled-release nitrogen fertilizer absorption characteristics， and the nitrogen use efficiency of rice grown using controlled-release nitrogen fertilizers[16-20]. Other studies have been focused on the effects of controlled-release nitrogen fertilizers on rice growth， density allocation， and yield[21-23]， and on soil biological activity[24]. These studies have provided useful information about the use of controlled-release nitrogen fertilizers to improve the nitrogen use efficiency of rice. Slow-release and conventional fertilizer application at the same site has been compared in many studies， but few studies have been focused on the different yield-increasing effects of slow-release and conventional fertilizers at different sites and the different nitrogen use efficiencies of rice grown using slow-release and conventional fertilizers at different sites. In practice， the environmental conditions and soil fertility at a site will affect the nitrogen use efficiency of the crops grown there. More nitrogen fertilizer will need to be added at some sites than others. In this study， we analyzed the yield-increasing effects of different fertilization treatments and the nitrogen use efficiencies of rice at five different sites. We provide technical guidance based on our results for the rational use of controlled-release nitrogen fertilizers to suit the soil and ecological conditions at each site.
Materials and Methods
Test varieties， test site， survey method
The hybrid rice （Orzya sativa） variety Chuanguyou 7329， bred by the Rice and Sorghum Institute， Sichuan Academy of Agricultural Sciences， and reviewed by the Sichuan Crop Variety Approval Committee in 2013， was used in the experiments.
Two types of nitrogen fertilizer， Kingenta controlled-release urea （CRU） and ordinary urea （U）， were used.
The field sites were in five counties in three Chinese provinces. The sites were in Wenjiang City， Guanghan City， and Zhongjiang County in Sichuan Province， Mangshi City in Yunnan Province， and Suiyang County in Guizhou Province. Wenjiang City， Guanghan City， and Zhongjiang County， Mangshi City， and Suiyang County sites were at altitudes of 593， 495， 546， 750， and 870 m， respectively， and latitudes and longitudes of 103°51′55′ E and 30°42′44″ N， 104°10′30′ E and 31°3′26′ N， 105°01′46′ E and 30°61′09″ N， 98°33′56″ E and 24°22′40″ N， and 107°04′34″ E and 27°54′10″ N， respectively. The annual daily average temperatures at the Wenjiang City， Guanghan City， Zhongjiang County， Mangshi City， and Suiyang County sites were approximately 6 000， 6 400， 6 350， 7 300， and 5 600 °C， respectively. The experiments were performed in 2015.
In the yield survey， rice plants were harvested at each site when the plants reached maturity. The spike number/hm2 was calculated by selecting， before the rice was harvested， 10 clusters from the fifth cluster in the third row on a diagonal line in each area. Two points were selected in each area. The average spike number per cluster was determined from 60 clusters and used to calculate the number of spikes/hm2 by multiplying the spike number of each cluster by the number of clusters/hm2. The yield components were measured by collecting three clusters at the same positions as were used to determine the spike numbers （six clusters per area）. The clusters were taken to the laboratory， where the number of kernels per spike， seed setting rate， and 1 000-kernel weight were determined （n = 3 per treatment）.
Fertilizer treatments and fertilizer application
Five CRU∶U ratios （70∶30， 50∶50， 30∶70， 100∶0， and 0∶100） were used. Two nitrogen application rates， 105 and 150 kg/hm2， were used. A control （CK） treatment at each site had no fertilizer added. The five CRU∶U ratios at a nitrogen application rate of 105 kg/hm2 were called treatments 1–5， and the five CRU∶U ratios at a nitrogen application rate of 150 kg/hm2 were called treatments 6-10. The CK treatment was called treatment 11.
In treatments 1-4 and 6-9， the fertilizer was applied once， to act as a basal fertilizer. In treatments 5 and 10， 70% of the fertilizer was applied as a basal fertilizer and the other 30% was applied 10 d after the rice plants had been transplanted. Calcium superphosphate （450 kg/hm2） and potassium sulfate （75 kg/hm2） were also applied as basal fertilizers.
The field experiments were established using a split-block design， with the ecological point as the main treatment， the nitrogen application rate as the sub-treatment， and the CRU∶U ratio as the sub-sub-treatment. Each test area was 13.34 m2， and three replicates were performed per treatment. The replicates were separated by ridges covered with plastic film to prevent leakage. The distance between neighboring test areas was 0.85 m. The rice seedlings were transplanted using a machine.
Data analysis
The data were analyzed and processed using Microsoft Excel and DPS v. 7.05 software. The nitrogen use efficiency was calculated using the equation below.
Agronomic nitrogen use efficiency （kg/kg） = （Yield for nitrogen treatment-Yield for no nitrogen used （CK））/Nitrogen application rate
Results
Effects of the CRU∶U ratio on rice yield under different ecological conditions
Table 1 shows the rice yields found at the five sites using the two fertilizer application rates and five CRU∶U ratios. The variance analysis results （Table 2） showed that there were highly significant differences between the rice yields at the different test sites， between the rice yields found using the different nitrogen application rates， and between the rice yields found using the different CRU∶U ratios. The interactions between the test site and CRU∶U ratio， between the test site and the application rate， and between the test site and CRU∶U ratio were extremely significant， but the interaction between the nitrogen application rate and CRU∶U ratio was not significant （Table 2）. The sites could be ranked from highest to lowest rice yield in the order of Wenjiang City （11 535.9 kg/hm2） > Jianyang County （10 448.6 kg/hm2） > Guanghan City （10 180.4 kg/hm2） > Suiyang County （9 597.1 kg/hm2） > Mangshi City （9 508.5 kg/hm2）. The differences between the rice yields for the Guizhou and Yunnan sites were not significant， but the differences between the rice yields for the other pairs of sites were extremely significant.
The CRU∶U ratio affected the yield. The highest yield in Wenjiang City， 11 963.7 kg/hm2， was obtained using a CRU∶U ratio of 70∶30， and the next highest （11 852.4 kg/hm2） using a CRU∶U ratio of 100∶0. The highest yield in Jianyang County， 10 869.3 kg/hm2， was obtained using a CRU∶U ratio of 50∶50， and the next highest （10 745.2 kg/hm2） using a ratio of 100∶0. The highest yield in Guanghan City， 10 637.3 kg/hm2， was obtained using a CRU∶U ratio of 70∶30， and the next highest （10 454.9 kg/hm2） using a CRU∶U ratio of 30∶70. The highest yield in Suiyang County， 9 725.1 kg/hm2， was obtained using a CRU∶U ratio of 50∶50， and the next highest （9 708.7 kg/hm2） using a CRU∶U ratio of 0∶100. The highest yield in Mangshi City， 10 035.1 kg/hm2， was obtained using a CRU∶U ratio of 70∶30， and the next highest （9 615.4 kg/hm2） using a CRU∶U ratio of 50∶50. The fertilizer ratio affected the yield differently at different sites because different sites had different fertilities and environmental conditions.
Effects of the CRU∶U ratio on yield
The yield of rice fertilized with U only was 3.99% lower at a nitrogen application rate of 150 kg/hm2 （yield 10 454.9 kg/hm2） than at a nitrogen application rate of 105 kg/hm2 （yield 10 053.2 kg/hm2）. A CRU∶U ratio of 70∶30 gave a significantly higher yield （10 572.7 kg/hm2） than a ratio of 50∶50 （10 414.3 kg/hm2）. The yields using ratios of 50∶50， 100∶0 （10 286.6 kg/hm2）， and 30∶70 （10 199.1 kg/hm2） were not significantly different. A CRU∶U ratio of 30∶70 gave a significantly higher yield than a ratio of 0∶100 （9 797.6 kg/hm2）. The best yield was clearly obtained using a ratio of 70∶30， and the second best yield using a ratio of 50∶50. The lowest yield was obtained using only U.
The yield-promoting effects of the fertilizer treatments are shown in Table 3. The yield was higher when fertilizer was applied than when fertilizer was not applied， but the yields for each treatment were different at the different sites. The nitrogen application rates of 105 and 150 kg/hm2 increased the yield relative to the CK yield by 23.52% and 34.68%， respectively， in Guanghan， 48.84% and 54.84%， respectively， in Zhongjiang， 20.16% and 22.9%， respectively， in Wenjiang， 11.14% and 8.24%， respectively， in Mangshi， and 42.02% and 50.96%， respectively， in Suiyang. The different responses were related to the different fertilities and environmental conditions at the different sites. Compared with applying only U， applying both CRU and U increased the yields at all the sites except for Mangshi. Applying CRU + U at nitrogen application rates of 105 and 150 kg/hm2 increased the yield relative to applying only U by 10.45% and 6.55%， respectively， in Guanghan， 12.58% and 7.3%， respectively， in Zhongjiang， 14.63% and 7.55%， respectively， in Wenjiang， and 5.08% and 5.15%， respectively， in Suiyang. Applying CRU + U at nitrogen application rates of 105 and 150 kg/hm2 decreased the yield relative to applying only U by 7.7% and increased the yield relative to applying only U by 3.6%， respectively， in Mangshi. At all sites except for Mangshi， the yield increased more when the nitrogen application rate was 105 kg/hm2 than when the nitrogen application rate was 150 kg/hm2 （Table 3）. These results indicate that applying both CRU and U significantly increased the rice yield relative to adding U only.
The variance analysis results for the yield components showed that there were significant differences in the spike number per hm2， kernels per spike， seed setting rates， and 1 000-kernel weights between sites and between the two nitrogen application rates. The highest spike number per hm2 was obtained at a CRU∶U ratio of 70∶30， and the highest seed setting rate was obtained at a CRU∶U ratio of 50∶50. The yields were higher because the spike number per unit area was higher and the seed setting rate was higher （Table 4）.
Relationships between the yield and other factors
Table 5 shows the correlation coefficients for the relationships between the yield and environmental factors in the rice vegetative growth stage， reproductive growth stage， and whole growth period， The yield was not significantly correlated with the daily mean temperature or daily rainfall in the vegetative growth stage， the daily mean temperature， daily rainfall， or number of rainless days in the reproductive growth stage， or the daily mean temperature or rainfall in the whole growth period. However， the yield was significantly positively correlated with the number of rainless days in the vegetative growth stage and the number of rainless days in the whole growth period. That is， many rainless days and abundant sunshine in the vegetative growth stage at the test sites led to a large population of the rice plants surviving， resulting in high yields.
The correlation analysis showed that the CRU∶U ratio was significantly negatively correlated with daily rainfall vegetative growth stage，the daily mean temperature and rainfall in the reproductive growth stage， and whole growth period （Table 5）. However， the CRU∶U ratio was not significantly correlated with the other factors. This indicates that the effects on the rice plants of high temperatures and excessive rainfall in the middle and late growth periods depended on the CRU∶U ratio. A linear regression equation （Fig. 1） was obtained by plotting rainfall during the whole growth period against the CRU∶U ratio. The relationship shown in Fig. 1 indicates that sites with higher rainfalls in the rice reproductive stage benefitted from lower CRU∶U ratios. This might be because nitrogen is released more slowly from CRU than from U， so rainfall in the late growth period would allow more nitrogen to be lost from CRU-treated soil than from U-treated soil. This regression equation can be used to determine the optimum CRU∶U ratio to use at a site based on rainfall at the site.
Relationship between initial paddy field fertility and CRU∶U ratio
The CK yields reflected rice yields for paddy fields with basic fertility （no fertilizer added）. The correlation coefficient for the relationship between the CK yields and the yields when nitrogen application rates of 105 and 150 kg/hm2 were used was -0.77 （P<0.01）.
The yields at the two fertilization levels and the CRU∶U ratio were significantly correlated （R = 0.810 0， P<0.05； Fig. 2）. As shown in Fig. 2， adding fertilizer had a stronger yield-increasing effect when the soil fertility was lower and a higher CRU∶U ratio gave a stronger yield-increasing effect. These findings indicated that the paddy fields had low nutrient-retention capacities. Large amounts of nutrients were lost after U was applied， but CRU better addressed nutrient deficiencies in the test paddy fields throughout the rice growth period. The cumulative daily mean temperature in the vegetative growth stage was positively correlated with rice yield （Table 5）. This correlation was not significant， but the daily mean temperature in the vegetative growth stage would have affected rice growth and nutrient uptake. Excessive application of chemical fertilizers in recent decades has damaged the structures of paddy fields. The water- and nutrient-retention capacities of paddy fields have decreased， and environmental pollution and rice production costs have increased. It is important to change the types of nitrogen fertilizer used to produce rice， that is， to switch to controlled-release fertilizers to decrease nitrogen losses and improve nitrogen use efficiency. This will decrease nitrogen application rate， rice production cost， and environmental pollution.
TDT（x1）， TDR（x2） and NRD（x3） represent total daily temperature， total daily rainfall， and no rain days of the vegetative growth stages， respectively； TDT（x4）， TDR（x5） and NRD（x6） represent total daily temperature， total daily rainfall， and no rain days of the reproductive growth stages， respectively； TDT（x7）， TDR（x8） and NRD（x9） represent total daily temperature， total daily rainfall， and no rain days of the whole growth stages， respectively； CRU（x10） represents Controlled-release urea； and GY（x10） represents grain yield. Agronomic nitrogen use efficiency
The nitrogen use efficiencies （Table 6） were significantly different at different test sites， for the different nitrogen application rates， and for the different CRU∶U ratios. A significant interaction was found for the nitrogen use efficiency between the test site and the CRU∶U ratio， but no significant interactions were found between other pairs of factors （Table 7）.
The nitrogen use efficiencies were different at the different sites， being 28.33 kg/（kg of N） at Zhongjiang， 24.01 kg/（kg of N） at Suiyang， 18.01 kg/（kg of N） at Guanghan， 16.36 kg/（kg of N） at Wenjiang， and 6.94 kg/（kg of N） at Mangshi City. The nitrogen use efficiencies were also different for the two application rates， being 20.6 kg/（kg of N） for a nitrogen application rate of 105 kg/hm2 and 21.9% lower [16.9 kg/（kg of N）] for a nitrogen application rate of 150 kg/hm2. The different CRU∶U ratios gave nitrogen use efficiencies that decreased in the order of 70∶30 [21.4 kg/（kg of N）]， 50∶50 [20.2 kg/kg of N）]， 100∶0 [19.7 kg/（kg of N）]， 30∶70 [17.9 kg/[kg of N）]， and 0∶100 [14.6 kg/（kg of N）]. The nitrogen use efficiencies for the 70∶30， 50∶50， 100∶0， and 30∶70 CRU∶U ratio treatments were 46.6%， 38.1%， 34.7%， and 22.2% higher， respectively， than the nitrogen use efficiencies for the U only （0∶100） treatment.
The nitrogen use efficiency patterns between the sites and fertilization treatments were different to the yield patterns. However， like yield， the agronomic nitrogen use efficiency was highest for the 70∶30 and 50∶50 CRU∶U ratios. The differences in the nitrogen use efficiencies found at the different sites were caused by the different soil fertilities and environmental conditions at the different sites.
Effects of the CRU∶U ratio on output
The sizes of the workforce and cost at the different field sites were very different. To account for these differences， we assumed that the input cost （phosphorus and potassium fertilizers， pesticides， herbicides， planting and transplanting rice seedlings， field management， rice harvesting， and rice drying） were the same for the nitrogen application rates of 105 and 150 kg/hm2 at each site. Differences in the output prices would therefore be caused by the difference in the application rate. As shown in Table 8， output for the 150 kg/hm2 treatment was 3 078.84 Yuan higher than the output for the 105 kg/hm2 treatment. Applying the same principal to the different fertilizer ratio treatments， the outputs for the 70∶30， 50∶50， 30∶70， and 100∶0 CRU∶U ratio treatments at a nitrogen application rate of 105 kg/hm2 were 2 097.44， 1 631.79， 1 235.11， and 981 yuan/hm2 higher， respectively， than the output for the 0∶100 CRU∶U ratio treatment. Similarly， the outputs for the 70∶30， 50∶50， 30∶70， and 100∶0 CRU∶U ratio treatments at a nitrogen application rate of 150 kg/hm2 were 1 685.67， 1 418.67， 1 135.72， and 477.48 yuan/hm2 higher， respectively， than the output for the 0∶100 CRU∶U ratio treatment. Therefore， the 70∶30 CRU∶U ratio provided the highest income at both application rates. Increasing the nitrogen application rate decreased the output efficiency because CRU is expensive， so the nitrogen input costs increased. Discussion
Reasonable application of controlled release nitrogen fertilizer
Excessive nitrogen fertilizer application rates and low nitrogen use efficiencies are important rice production problems in China. Using slow-release nitrogen fertilizers can address these problems by decreasing nitrogen loss and increasing the nitrogen use efficiency of crops. A high yield requires a high rice plant population at the tillering stage and steady growth to be maintained in the middle and late growth stages. This requires coordinated application of controlled-release nitrogen fertilizer and ordinary nitrogen fertilizer. If the controlled-release nitrogen fertilizer to ordinary fertilizer ratio is too high the tillering rate and the establishment of a high-yield population will be affected. If the ratio is too low the nitrogen requirements of the rice in the middle and late stages of growth and development will not be met.
We evaluated the yield-increasing effects of two nitrogen fertilizer application rates and five CRU∶U ratios on rice crops at five different sites. The rice yields were higher for the 105 and 150 kg/hm2 nitrogen application rate treatments than for the CK treatments. The yields were higher for the CRU+U treatments than for the U only treatments at all the sites except for Mangshi. The 70∶30 CRU∶U ratio treatment had a larger yield-increasing effect than other treatments ratios， similar to the results of several previous studies[12-15]. The yields were very different for the different CRU∶U ratio treatments， depending on the environmental and soil fertility conditions at the different sites. Therefore， the most appropriate CRU∶U ratio for a site should be determined to match the fertility and environmental conditions of the site.
In previous studies， the same yields were achieved using less controlled-release fertilizer than ordinary fertilizer[10-11，21-23]. In our study， the rice yields were higher for the 150 kg/hm2 nitrogen application rate than for the 105 kg/hm2 nitrogen application rate， but the output efficiency was higher for the 105 kg/hm2 nitrogen application rate than for the 150 kg/hm2 nitrogen application rate. In previous studies， lower yields have been found at very high nitrogen application rates than at lower nitrogen application rates， but the highest nitrogen application rate we used was within the range of reasonable nitrogen application rates for the study areas. Three factors should be considered when selecting the amount of controlled release fertilizer to use. These are the type of nitrogen， the initial fertility of the paddy soil， and the ecological conditions. The nitrogen fertilizer should also be balanced with phosphorus， potassium， organic fertilizer， and inorganic fertilizer to achieve optimum nitrogen absorption and minimize ammonium nitrogen loss through volatilization[25]. Nitrogen use efficiency
Like the yield， the nitrogen use efficiency depends on the nitrogen application rate and the paddy field fertility. Generally， if the initial fertility of a paddy field is low， the agronomic nitrogen use efficiency will be high. It has previously been shown that applying controlled-release nitrogen fertilizer alone or in combination with ordinary nitrogen fertilizer can significantly increase the nitrogen use efficiency[9-10，12，17]. We found that the nitrogen use efficiency was higher when CRU+U was applied than when U was applied and higher when the nitrogen application rate was 105 than 150 kg/hm2， similar to the results of previous studies. However， the nitrogen use efficiencies at the same nitrogen application rates and ratios were different at different sites because of the different soil fertilities， temperatures， light conditions， and water resources at the different sites.
A combination of controlled-release fertilizer and irrigation can decrease nitrogen volatilization and increase the nitrogen use efficiency[26]. Therefore， for optimum rice production， farmers should first determine the target yield and then select a nitrogen application rate and a controlled-release nitrogen fertilizer based on the nitrogen supply capacity of the paddy field. This， combined with appropriate water management， can give a high rice yield and optimize the nitrogen use efficiency of the crop. In our study， the output decreased as the nitrogen application rate increased because controlled-release nitrogen fertilizer is expensive. Increased use of CRU will increase both the nitrogen use efficiency and the rice yield. Our results indicated that output was best at a nitrogen application rate of 105 kg/hm2 if a moderate target yield was used instead of a high target yield. This result can help develop plans to achieve the long-term goal of decreasing fertilizer use and increasing nutrient use efficiency during the production of rice in southwest China.
Optimization of the CRU∶U ratio
Using controlled-release nitrogen fertilizer significantly increases rice yield per unit area and increases the nitrogen use efficiency. This conclusion has been reached by rice researchers， technology extension workers， and rice farmers. Many previous studies have been focused on the yield-increasing effects of different fertilizers using different fertilizer ratios and on the nitrogen use efficiency of crops grown using different fertilizer ratios at single sites[10-11，22-23]. Such studies have promoted the popularity and use of controlled-release nitrogen fertilizers. We studied the yield effects and nitrogen use efficiency using the same nitrogen application rates and CRU∶U ratios at different sites with different soil fertilities and environmental conditions. Significant correlations were found between soil fertility and yield （R = 0.46； P<0.01） and between yield and the CRU∶U ratio （R = 0.81； P<0.05）. A significant negative correlation was found between yield and rainfall during the rice growing period （Table 5）. Correlations were found between the yield， CRU∶U ratio， and field conditions （fertility and environmental conditions）. These relationships were used to establish a mathematical model. The equation for the relationship between the CRU∶U ratio and the increase in yield was y = 0.089x + 27.27 （R2 = 0.663）， and the equation for the relationship between the yield and rainfall during the whole growth period was y = -0.087x + 112.2 （R2 = 0.687）. For rice production， the appropriate CRU∶U ratio can be calculated using these regression equations if the yield increase resulting from applying nitrogen （i.e.， the difference in yield between the CK and fertilizer treatments） and the cumulative rainfall during the whole growth period in the area of interest are known. This provides a method for selecting the most appropriate application rate for controlled-release nitrogen fertilizer. This will play an important role in selecting the most appropriate amount and type of fertilizer to use based on the soil and environmental conditions for the site of interest and this will improve the nitrogen use efficiency of rice.
Conclusions
Field experiments were conducted at five sites. The rice yields achieved using two nitrogen application rates and five CRU∶U ratios were compared. Applying nitrogen significantly increased the rice yield relative to the CK treatment， but to different degrees at different sites. Applying both CRU and U increased the yield more than applying the same amount of nitrogen as U only， and the yields were higher for a nitrogen application rate of 150 kg/hm2 than for a nitrogen application rate of 105 kg/hm2. A CRU∶U ratio of 70∶30 gave higher yields than other ratios. The nitrogen use efficiency was 21.9% higher for a nitrogen application rate of 105 kg/hm2 than for a nitrogen application rate of 150 kg/hm2. The nitrogen use efficiencies for the 70∶30， 50∶50， 100∶0， and 30∶70 CRU∶U ratios were 46.6%， 38.1%， 34.7%， and 22.2% higher， respectively， than the nitrogen use efficiency for the 0∶100 ratio. The output （which took into account the yield， crop price， and input costs） was 3 078.87 yuan/hm2 higher for a nitrogen application rate of 150 kg/hm2 than for a nitrogen application rate of 105 kg/hm2 when both CRU and U were applied. The output was higher under all CRU+U combinations than when only U was applied， and the highest output was found for a CRU∶U ratio of 70∶30. Increasing the amount of nitrogen added decreased the output efficiency because CRU is expensive. Regression equations were obtained for the relationships between the yield increase and rainfall during the whole growth period （y = 7.429x-185.7， R2 = 0.663） and between the yield and CRU∶U ratio （y = -0.087 1x + 112.29， R2 = 0.687 9）. These regression equations can be used to determine the most appropriate CRU∶U ratio to use to match the rainfall at the site of interest. Acknowledgements
The authors thank the following researchers for their help with this study： Zheng Jiaguo （Crop Research Institute， Sichuan Academy of Agricultural Sciences）； Tu Shihua （Soil and Fertilizer Research Institute， Sichuan Academy of Agricultural Sciences）； Prof. Ma Jun （Sichuan Agricultural University）； Zhou Weijia （Rice Research Institute， Guizhou Academy of Agricultural Sciences）； and Yang Congdang （Grain Crops Research Institute， Yunnan Academy of Agricultural Sciences）. The English text of a draft of this manuscript was edited by Liwen Bianji， Edanz Group China （www.liwenbianji.cn/ac）.
References
[1] LIU J， ZHAO XJ. Development sratus and strategy of chinese slow， controlled release fertilizers[J]. Northern Horticutrue， 2011 （21）： 189-190.
[2] GU JL， XU QM， CAO B. Current situation and expectation of slow and controlled release fertilizers[J]. Journal of Anhui Agri. Sci.， 2007， 35（32）： 10369-10372.
[3] YUAN JL， LIANG XQ， LI L， et al. Response of rice yield and nitrogen uptake to enhanced efficiency nitrogen fertilizer in China： A meta-analysis[J]. Scientia Agricultura Sinica， 2014， 47（17）： 3414-3423.
[4] HU CH， LUO GB， ZENG JH， et al. Influence of different types of slow-release nitrogen fertilizer on rice yield and nitrogen fertilizer use efficiency[J]. Chinese Agricultural Science Bulletin， 2011， 27（15）： 174-177.
[5] CHANG Z， LIU SX， ZHANG ZX， et al. Study on the fertilizer efficiency and the economic benefit of new slow-release nitrogenous fertilizers[J]. Soil and fertilizer in China， 2008， 6：59-63.
[6] HE XS， LIAO ZW， HUANG PZ， et al. Characteristics and performance of a new water-absorbent slow release nitrogen fertilizer[J]. Scientia Agricultura Sinica， 2006， 39（10）： 2048-2055
[7] XU FY， QIN YP， WANG XL， et al. Effects of different novel functional urea on the yield and efficiency of nitrogen absorption and utilization of hybrid rice[J]. Hubei Agricultural Sciences， 2014， 53（10）： 2257-2263.
[8] WANG B， WAN YF， GUO C， et al. A comparison of the effects of controlled release urea， stable urea and microorganisms increasing double rice yield and nitrogen use efficiency[J]. Journal of Plant Nutrition and Fertilizer， 2015， 21（5）： 1104-1112， （4）： 918-923.
[9] LI FM， AI TC， ZHOU SB， et al. Influence of slow-release nitrogen fertilizers on lowland rice yield and nitrogen use efficiency[J]. Chinese Journal of Soil science， 2004， 35（3）：311-315
[10] SUN ZX， JIAN YY， WANG XX. Effects of controlled release nitrogen fertilizer on rice yield[J]. Shanghai agricultural science and technology， 2011， 4：98-99. [11] ZHOU J， MA GH， LONG JR， et al. Study on the rate of slow-release nitrogen fertilizer for super hybrid late rice[J]. Phosphate & Compound Fertilizer， 2009， 24（1）： 80-82.
[12] XU JK， LI XH， LI W， et al. Effects of mixture of slow-release urea and conventional urea on ammonia volatilization and soil nitrogen dynamic change[J]. Soil and fertilizer in China， 2015， （6）： 23-27.
[13] CHEN XY， WU LH， HAN KF， et al. Effects of different mixture rates of coated urea and prilled urea on Rice Grain Yield and nitrogen use efficiency[J]. plant nutrition and Fertilizer Science， 2010， 16（4）： 918-923.
[14] WANG XY， TIAN XH， WANG XL， et al. Effects of bulk blending urea on rice yield and nitrogen fertilizer agronomic efficiency[J]. Hubei Agricultural Sciences， 2010， 40（10）： 2386-2389.
[15] WU SF， HAN KF， WU LH， et al. Effects of application of controlled-release bulk blending urea on yield， quality and nitrogen utilization efficiency of rice[J]. Journal of Zhejiang agriculture， 2011， 23（5）： 983-987.
[16] SHAVIV A. Plant response and environmental aspects as affected by rate and pattern of nitrogen release from controlled release N fertilizers[J]. Springer Netherlands， 1996， 68：285-291.
[17] LI Y， LI YH， ZHAO JH， et al. Effects of slow-and cortolled-release nitrigen fertilizer on nitrogen utilization characteristics and yield of machine-transplanted rice[J]. Journal of Zhejiang University Agric.& Life Sci.， 2015，41（6）：672-684.
[18] DU J， SUN KG， ZHANG YH， et al. Effects of controlled release urea on physiological characteristics and nitrogen use efficiency of rice and NO-3-N contents in soil[J]. Journal of Agricultural Resources and Environment， 2016， 33（2）： 134-141.
[19] DAI PA ，NIE J ，ZHENG SX， et al. Efficiency of nutrient utilization of controlled-release nitrogen fertilizer for rice at different soil fertility levels[J]. 2003， 34（2）： 115-119.
[20] SUN KG， DU J， HE AL， et al. Effect of controlled release urea fertilizer on rice yield and nitrogen utilization efficiency[J]. Journal of Henan Agricultural Sciences， 2015， 44（12）：57- 60.
[21] TIAN XH， MA GH，YANG LZ， et al. Effects of slow release nitrogen fertilizer on growth and physiological function in the late stage of super hybrid rice[J]. Hybrid Rice， 2010， 25（5）： 64-69.
[22] MA GH， LONG JR， DAI QM， et al. Studies on the optimized allocation of contrlled-release nitrogen fertilizer application and planting densith for medium super hybrid rice combination Y Liang You1[J]. Hybrid Rice， 2008， 23（6）：73-77.
[23] MA GH， ZHOU J， LONG JR， et al. Effect of slow-released nitrogen fertilizer on growth and yield of super hybrid early rice[J]. Journal of Hunan Agricultural University （Natural Sciences）， 2008， 34（1）： 95-99.
[24] LI DP， WU ZJ， LIANG CH， et al. Effect of slow/controlled release nitrogen fertilizers on soil biological activities[J]. Journal ofAgro-Environment Science， 2006， 25（3）： 664-669.
[25] HUANG P， ZHANG JB， ZHU AN， et al. Coupled water and nitrogen （N） management as a key strategy for the mitigation of gaseous N losses in the Huang-Huai-Hai Plain[J]. Biol Fertil Soils， 2015， 51： 333-342.
[26] TING LAN， YONG HAN， MARCO ROELCKE， et al. Temperature dependence of gross N transformation rates in two Chinese paddy soils under aerobic condition[J]. Biol Fertil Soils， 2014， 50： 949-959.
Key words Ecological conditions； Fertility； CRU∶U； Yield； Nitrogen use efficiency； Output efficiency
The rice yield in China was 6 810.7 kg/hm2 at the end of the 12th Five-Year Plan Period， mainly because of the use of improved rice varieties， advances in rice-growing techniques， and increases in fertilizer application rates. China has 7% of global arable land but accounts for one-third of global fertilizer use. Fertilizer consumption per hm2 in China is more than three times of the global average， but fertilizer use efficiency is about 30% lower in China than in developed countries[1-3]. This has resulted in increasingly serious environmental pollution. Considerable attention has therefore been paid to improving nitrogen use efficiency by attempting to optimize the types of nitrogen fertilizers used and the application methods. Several studies have been focused on the use of controlled-release nitrogen fertilizers[4-9]， controlled-release nitrogen fertilizer application rates[10-11]， controlled-release nitrogen fertilizer performances compared with urea[12-15]， controlled-release nitrogen fertilizer absorption characteristics， and the nitrogen use efficiency of rice grown using controlled-release nitrogen fertilizers[16-20]. Other studies have been focused on the effects of controlled-release nitrogen fertilizers on rice growth， density allocation， and yield[21-23]， and on soil biological activity[24]. These studies have provided useful information about the use of controlled-release nitrogen fertilizers to improve the nitrogen use efficiency of rice. Slow-release and conventional fertilizer application at the same site has been compared in many studies， but few studies have been focused on the different yield-increasing effects of slow-release and conventional fertilizers at different sites and the different nitrogen use efficiencies of rice grown using slow-release and conventional fertilizers at different sites. In practice， the environmental conditions and soil fertility at a site will affect the nitrogen use efficiency of the crops grown there. More nitrogen fertilizer will need to be added at some sites than others. In this study， we analyzed the yield-increasing effects of different fertilization treatments and the nitrogen use efficiencies of rice at five different sites. We provide technical guidance based on our results for the rational use of controlled-release nitrogen fertilizers to suit the soil and ecological conditions at each site.
Materials and Methods
Test varieties， test site， survey method
The hybrid rice （Orzya sativa） variety Chuanguyou 7329， bred by the Rice and Sorghum Institute， Sichuan Academy of Agricultural Sciences， and reviewed by the Sichuan Crop Variety Approval Committee in 2013， was used in the experiments.
Two types of nitrogen fertilizer， Kingenta controlled-release urea （CRU） and ordinary urea （U）， were used.
The field sites were in five counties in three Chinese provinces. The sites were in Wenjiang City， Guanghan City， and Zhongjiang County in Sichuan Province， Mangshi City in Yunnan Province， and Suiyang County in Guizhou Province. Wenjiang City， Guanghan City， and Zhongjiang County， Mangshi City， and Suiyang County sites were at altitudes of 593， 495， 546， 750， and 870 m， respectively， and latitudes and longitudes of 103°51′55′ E and 30°42′44″ N， 104°10′30′ E and 31°3′26′ N， 105°01′46′ E and 30°61′09″ N， 98°33′56″ E and 24°22′40″ N， and 107°04′34″ E and 27°54′10″ N， respectively. The annual daily average temperatures at the Wenjiang City， Guanghan City， Zhongjiang County， Mangshi City， and Suiyang County sites were approximately 6 000， 6 400， 6 350， 7 300， and 5 600 °C， respectively. The experiments were performed in 2015.
In the yield survey， rice plants were harvested at each site when the plants reached maturity. The spike number/hm2 was calculated by selecting， before the rice was harvested， 10 clusters from the fifth cluster in the third row on a diagonal line in each area. Two points were selected in each area. The average spike number per cluster was determined from 60 clusters and used to calculate the number of spikes/hm2 by multiplying the spike number of each cluster by the number of clusters/hm2. The yield components were measured by collecting three clusters at the same positions as were used to determine the spike numbers （six clusters per area）. The clusters were taken to the laboratory， where the number of kernels per spike， seed setting rate， and 1 000-kernel weight were determined （n = 3 per treatment）.
Fertilizer treatments and fertilizer application
Five CRU∶U ratios （70∶30， 50∶50， 30∶70， 100∶0， and 0∶100） were used. Two nitrogen application rates， 105 and 150 kg/hm2， were used. A control （CK） treatment at each site had no fertilizer added. The five CRU∶U ratios at a nitrogen application rate of 105 kg/hm2 were called treatments 1–5， and the five CRU∶U ratios at a nitrogen application rate of 150 kg/hm2 were called treatments 6-10. The CK treatment was called treatment 11.
In treatments 1-4 and 6-9， the fertilizer was applied once， to act as a basal fertilizer. In treatments 5 and 10， 70% of the fertilizer was applied as a basal fertilizer and the other 30% was applied 10 d after the rice plants had been transplanted. Calcium superphosphate （450 kg/hm2） and potassium sulfate （75 kg/hm2） were also applied as basal fertilizers.
The field experiments were established using a split-block design， with the ecological point as the main treatment， the nitrogen application rate as the sub-treatment， and the CRU∶U ratio as the sub-sub-treatment. Each test area was 13.34 m2， and three replicates were performed per treatment. The replicates were separated by ridges covered with plastic film to prevent leakage. The distance between neighboring test areas was 0.85 m. The rice seedlings were transplanted using a machine.
Data analysis
The data were analyzed and processed using Microsoft Excel and DPS v. 7.05 software. The nitrogen use efficiency was calculated using the equation below.
Agronomic nitrogen use efficiency （kg/kg） = （Yield for nitrogen treatment-Yield for no nitrogen used （CK））/Nitrogen application rate
Results
Effects of the CRU∶U ratio on rice yield under different ecological conditions
Table 1 shows the rice yields found at the five sites using the two fertilizer application rates and five CRU∶U ratios. The variance analysis results （Table 2） showed that there were highly significant differences between the rice yields at the different test sites， between the rice yields found using the different nitrogen application rates， and between the rice yields found using the different CRU∶U ratios. The interactions between the test site and CRU∶U ratio， between the test site and the application rate， and between the test site and CRU∶U ratio were extremely significant， but the interaction between the nitrogen application rate and CRU∶U ratio was not significant （Table 2）. The sites could be ranked from highest to lowest rice yield in the order of Wenjiang City （11 535.9 kg/hm2） > Jianyang County （10 448.6 kg/hm2） > Guanghan City （10 180.4 kg/hm2） > Suiyang County （9 597.1 kg/hm2） > Mangshi City （9 508.5 kg/hm2）. The differences between the rice yields for the Guizhou and Yunnan sites were not significant， but the differences between the rice yields for the other pairs of sites were extremely significant.
The CRU∶U ratio affected the yield. The highest yield in Wenjiang City， 11 963.7 kg/hm2， was obtained using a CRU∶U ratio of 70∶30， and the next highest （11 852.4 kg/hm2） using a CRU∶U ratio of 100∶0. The highest yield in Jianyang County， 10 869.3 kg/hm2， was obtained using a CRU∶U ratio of 50∶50， and the next highest （10 745.2 kg/hm2） using a ratio of 100∶0. The highest yield in Guanghan City， 10 637.3 kg/hm2， was obtained using a CRU∶U ratio of 70∶30， and the next highest （10 454.9 kg/hm2） using a CRU∶U ratio of 30∶70. The highest yield in Suiyang County， 9 725.1 kg/hm2， was obtained using a CRU∶U ratio of 50∶50， and the next highest （9 708.7 kg/hm2） using a CRU∶U ratio of 0∶100. The highest yield in Mangshi City， 10 035.1 kg/hm2， was obtained using a CRU∶U ratio of 70∶30， and the next highest （9 615.4 kg/hm2） using a CRU∶U ratio of 50∶50. The fertilizer ratio affected the yield differently at different sites because different sites had different fertilities and environmental conditions.
Effects of the CRU∶U ratio on yield
The yield of rice fertilized with U only was 3.99% lower at a nitrogen application rate of 150 kg/hm2 （yield 10 454.9 kg/hm2） than at a nitrogen application rate of 105 kg/hm2 （yield 10 053.2 kg/hm2）. A CRU∶U ratio of 70∶30 gave a significantly higher yield （10 572.7 kg/hm2） than a ratio of 50∶50 （10 414.3 kg/hm2）. The yields using ratios of 50∶50， 100∶0 （10 286.6 kg/hm2）， and 30∶70 （10 199.1 kg/hm2） were not significantly different. A CRU∶U ratio of 30∶70 gave a significantly higher yield than a ratio of 0∶100 （9 797.6 kg/hm2）. The best yield was clearly obtained using a ratio of 70∶30， and the second best yield using a ratio of 50∶50. The lowest yield was obtained using only U.
The yield-promoting effects of the fertilizer treatments are shown in Table 3. The yield was higher when fertilizer was applied than when fertilizer was not applied， but the yields for each treatment were different at the different sites. The nitrogen application rates of 105 and 150 kg/hm2 increased the yield relative to the CK yield by 23.52% and 34.68%， respectively， in Guanghan， 48.84% and 54.84%， respectively， in Zhongjiang， 20.16% and 22.9%， respectively， in Wenjiang， 11.14% and 8.24%， respectively， in Mangshi， and 42.02% and 50.96%， respectively， in Suiyang. The different responses were related to the different fertilities and environmental conditions at the different sites. Compared with applying only U， applying both CRU and U increased the yields at all the sites except for Mangshi. Applying CRU + U at nitrogen application rates of 105 and 150 kg/hm2 increased the yield relative to applying only U by 10.45% and 6.55%， respectively， in Guanghan， 12.58% and 7.3%， respectively， in Zhongjiang， 14.63% and 7.55%， respectively， in Wenjiang， and 5.08% and 5.15%， respectively， in Suiyang. Applying CRU + U at nitrogen application rates of 105 and 150 kg/hm2 decreased the yield relative to applying only U by 7.7% and increased the yield relative to applying only U by 3.6%， respectively， in Mangshi. At all sites except for Mangshi， the yield increased more when the nitrogen application rate was 105 kg/hm2 than when the nitrogen application rate was 150 kg/hm2 （Table 3）. These results indicate that applying both CRU and U significantly increased the rice yield relative to adding U only.
The variance analysis results for the yield components showed that there were significant differences in the spike number per hm2， kernels per spike， seed setting rates， and 1 000-kernel weights between sites and between the two nitrogen application rates. The highest spike number per hm2 was obtained at a CRU∶U ratio of 70∶30， and the highest seed setting rate was obtained at a CRU∶U ratio of 50∶50. The yields were higher because the spike number per unit area was higher and the seed setting rate was higher （Table 4）.
Relationships between the yield and other factors
Table 5 shows the correlation coefficients for the relationships between the yield and environmental factors in the rice vegetative growth stage， reproductive growth stage， and whole growth period， The yield was not significantly correlated with the daily mean temperature or daily rainfall in the vegetative growth stage， the daily mean temperature， daily rainfall， or number of rainless days in the reproductive growth stage， or the daily mean temperature or rainfall in the whole growth period. However， the yield was significantly positively correlated with the number of rainless days in the vegetative growth stage and the number of rainless days in the whole growth period. That is， many rainless days and abundant sunshine in the vegetative growth stage at the test sites led to a large population of the rice plants surviving， resulting in high yields.
The correlation analysis showed that the CRU∶U ratio was significantly negatively correlated with daily rainfall vegetative growth stage，the daily mean temperature and rainfall in the reproductive growth stage， and whole growth period （Table 5）. However， the CRU∶U ratio was not significantly correlated with the other factors. This indicates that the effects on the rice plants of high temperatures and excessive rainfall in the middle and late growth periods depended on the CRU∶U ratio. A linear regression equation （Fig. 1） was obtained by plotting rainfall during the whole growth period against the CRU∶U ratio. The relationship shown in Fig. 1 indicates that sites with higher rainfalls in the rice reproductive stage benefitted from lower CRU∶U ratios. This might be because nitrogen is released more slowly from CRU than from U， so rainfall in the late growth period would allow more nitrogen to be lost from CRU-treated soil than from U-treated soil. This regression equation can be used to determine the optimum CRU∶U ratio to use at a site based on rainfall at the site.
Relationship between initial paddy field fertility and CRU∶U ratio
The CK yields reflected rice yields for paddy fields with basic fertility （no fertilizer added）. The correlation coefficient for the relationship between the CK yields and the yields when nitrogen application rates of 105 and 150 kg/hm2 were used was -0.77 （P<0.01）.
The yields at the two fertilization levels and the CRU∶U ratio were significantly correlated （R = 0.810 0， P<0.05； Fig. 2）. As shown in Fig. 2， adding fertilizer had a stronger yield-increasing effect when the soil fertility was lower and a higher CRU∶U ratio gave a stronger yield-increasing effect. These findings indicated that the paddy fields had low nutrient-retention capacities. Large amounts of nutrients were lost after U was applied， but CRU better addressed nutrient deficiencies in the test paddy fields throughout the rice growth period. The cumulative daily mean temperature in the vegetative growth stage was positively correlated with rice yield （Table 5）. This correlation was not significant， but the daily mean temperature in the vegetative growth stage would have affected rice growth and nutrient uptake. Excessive application of chemical fertilizers in recent decades has damaged the structures of paddy fields. The water- and nutrient-retention capacities of paddy fields have decreased， and environmental pollution and rice production costs have increased. It is important to change the types of nitrogen fertilizer used to produce rice， that is， to switch to controlled-release fertilizers to decrease nitrogen losses and improve nitrogen use efficiency. This will decrease nitrogen application rate， rice production cost， and environmental pollution.
TDT（x1）， TDR（x2） and NRD（x3） represent total daily temperature， total daily rainfall， and no rain days of the vegetative growth stages， respectively； TDT（x4）， TDR（x5） and NRD（x6） represent total daily temperature， total daily rainfall， and no rain days of the reproductive growth stages， respectively； TDT（x7）， TDR（x8） and NRD（x9） represent total daily temperature， total daily rainfall， and no rain days of the whole growth stages， respectively； CRU（x10） represents Controlled-release urea； and GY（x10） represents grain yield. Agronomic nitrogen use efficiency
The nitrogen use efficiencies （Table 6） were significantly different at different test sites， for the different nitrogen application rates， and for the different CRU∶U ratios. A significant interaction was found for the nitrogen use efficiency between the test site and the CRU∶U ratio， but no significant interactions were found between other pairs of factors （Table 7）.
The nitrogen use efficiencies were different at the different sites， being 28.33 kg/（kg of N） at Zhongjiang， 24.01 kg/（kg of N） at Suiyang， 18.01 kg/（kg of N） at Guanghan， 16.36 kg/（kg of N） at Wenjiang， and 6.94 kg/（kg of N） at Mangshi City. The nitrogen use efficiencies were also different for the two application rates， being 20.6 kg/（kg of N） for a nitrogen application rate of 105 kg/hm2 and 21.9% lower [16.9 kg/（kg of N）] for a nitrogen application rate of 150 kg/hm2. The different CRU∶U ratios gave nitrogen use efficiencies that decreased in the order of 70∶30 [21.4 kg/（kg of N）]， 50∶50 [20.2 kg/kg of N）]， 100∶0 [19.7 kg/（kg of N）]， 30∶70 [17.9 kg/[kg of N）]， and 0∶100 [14.6 kg/（kg of N）]. The nitrogen use efficiencies for the 70∶30， 50∶50， 100∶0， and 30∶70 CRU∶U ratio treatments were 46.6%， 38.1%， 34.7%， and 22.2% higher， respectively， than the nitrogen use efficiencies for the U only （0∶100） treatment.
The nitrogen use efficiency patterns between the sites and fertilization treatments were different to the yield patterns. However， like yield， the agronomic nitrogen use efficiency was highest for the 70∶30 and 50∶50 CRU∶U ratios. The differences in the nitrogen use efficiencies found at the different sites were caused by the different soil fertilities and environmental conditions at the different sites.
Effects of the CRU∶U ratio on output
The sizes of the workforce and cost at the different field sites were very different. To account for these differences， we assumed that the input cost （phosphorus and potassium fertilizers， pesticides， herbicides， planting and transplanting rice seedlings， field management， rice harvesting， and rice drying） were the same for the nitrogen application rates of 105 and 150 kg/hm2 at each site. Differences in the output prices would therefore be caused by the difference in the application rate. As shown in Table 8， output for the 150 kg/hm2 treatment was 3 078.84 Yuan higher than the output for the 105 kg/hm2 treatment. Applying the same principal to the different fertilizer ratio treatments， the outputs for the 70∶30， 50∶50， 30∶70， and 100∶0 CRU∶U ratio treatments at a nitrogen application rate of 105 kg/hm2 were 2 097.44， 1 631.79， 1 235.11， and 981 yuan/hm2 higher， respectively， than the output for the 0∶100 CRU∶U ratio treatment. Similarly， the outputs for the 70∶30， 50∶50， 30∶70， and 100∶0 CRU∶U ratio treatments at a nitrogen application rate of 150 kg/hm2 were 1 685.67， 1 418.67， 1 135.72， and 477.48 yuan/hm2 higher， respectively， than the output for the 0∶100 CRU∶U ratio treatment. Therefore， the 70∶30 CRU∶U ratio provided the highest income at both application rates. Increasing the nitrogen application rate decreased the output efficiency because CRU is expensive， so the nitrogen input costs increased. Discussion
Reasonable application of controlled release nitrogen fertilizer
Excessive nitrogen fertilizer application rates and low nitrogen use efficiencies are important rice production problems in China. Using slow-release nitrogen fertilizers can address these problems by decreasing nitrogen loss and increasing the nitrogen use efficiency of crops. A high yield requires a high rice plant population at the tillering stage and steady growth to be maintained in the middle and late growth stages. This requires coordinated application of controlled-release nitrogen fertilizer and ordinary nitrogen fertilizer. If the controlled-release nitrogen fertilizer to ordinary fertilizer ratio is too high the tillering rate and the establishment of a high-yield population will be affected. If the ratio is too low the nitrogen requirements of the rice in the middle and late stages of growth and development will not be met.
We evaluated the yield-increasing effects of two nitrogen fertilizer application rates and five CRU∶U ratios on rice crops at five different sites. The rice yields were higher for the 105 and 150 kg/hm2 nitrogen application rate treatments than for the CK treatments. The yields were higher for the CRU+U treatments than for the U only treatments at all the sites except for Mangshi. The 70∶30 CRU∶U ratio treatment had a larger yield-increasing effect than other treatments ratios， similar to the results of several previous studies[12-15]. The yields were very different for the different CRU∶U ratio treatments， depending on the environmental and soil fertility conditions at the different sites. Therefore， the most appropriate CRU∶U ratio for a site should be determined to match the fertility and environmental conditions of the site.
In previous studies， the same yields were achieved using less controlled-release fertilizer than ordinary fertilizer[10-11，21-23]. In our study， the rice yields were higher for the 150 kg/hm2 nitrogen application rate than for the 105 kg/hm2 nitrogen application rate， but the output efficiency was higher for the 105 kg/hm2 nitrogen application rate than for the 150 kg/hm2 nitrogen application rate. In previous studies， lower yields have been found at very high nitrogen application rates than at lower nitrogen application rates， but the highest nitrogen application rate we used was within the range of reasonable nitrogen application rates for the study areas. Three factors should be considered when selecting the amount of controlled release fertilizer to use. These are the type of nitrogen， the initial fertility of the paddy soil， and the ecological conditions. The nitrogen fertilizer should also be balanced with phosphorus， potassium， organic fertilizer， and inorganic fertilizer to achieve optimum nitrogen absorption and minimize ammonium nitrogen loss through volatilization[25]. Nitrogen use efficiency
Like the yield， the nitrogen use efficiency depends on the nitrogen application rate and the paddy field fertility. Generally， if the initial fertility of a paddy field is low， the agronomic nitrogen use efficiency will be high. It has previously been shown that applying controlled-release nitrogen fertilizer alone or in combination with ordinary nitrogen fertilizer can significantly increase the nitrogen use efficiency[9-10，12，17]. We found that the nitrogen use efficiency was higher when CRU+U was applied than when U was applied and higher when the nitrogen application rate was 105 than 150 kg/hm2， similar to the results of previous studies. However， the nitrogen use efficiencies at the same nitrogen application rates and ratios were different at different sites because of the different soil fertilities， temperatures， light conditions， and water resources at the different sites.
A combination of controlled-release fertilizer and irrigation can decrease nitrogen volatilization and increase the nitrogen use efficiency[26]. Therefore， for optimum rice production， farmers should first determine the target yield and then select a nitrogen application rate and a controlled-release nitrogen fertilizer based on the nitrogen supply capacity of the paddy field. This， combined with appropriate water management， can give a high rice yield and optimize the nitrogen use efficiency of the crop. In our study， the output decreased as the nitrogen application rate increased because controlled-release nitrogen fertilizer is expensive. Increased use of CRU will increase both the nitrogen use efficiency and the rice yield. Our results indicated that output was best at a nitrogen application rate of 105 kg/hm2 if a moderate target yield was used instead of a high target yield. This result can help develop plans to achieve the long-term goal of decreasing fertilizer use and increasing nutrient use efficiency during the production of rice in southwest China.
Optimization of the CRU∶U ratio
Using controlled-release nitrogen fertilizer significantly increases rice yield per unit area and increases the nitrogen use efficiency. This conclusion has been reached by rice researchers， technology extension workers， and rice farmers. Many previous studies have been focused on the yield-increasing effects of different fertilizers using different fertilizer ratios and on the nitrogen use efficiency of crops grown using different fertilizer ratios at single sites[10-11，22-23]. Such studies have promoted the popularity and use of controlled-release nitrogen fertilizers. We studied the yield effects and nitrogen use efficiency using the same nitrogen application rates and CRU∶U ratios at different sites with different soil fertilities and environmental conditions. Significant correlations were found between soil fertility and yield （R = 0.46； P<0.01） and between yield and the CRU∶U ratio （R = 0.81； P<0.05）. A significant negative correlation was found between yield and rainfall during the rice growing period （Table 5）. Correlations were found between the yield， CRU∶U ratio， and field conditions （fertility and environmental conditions）. These relationships were used to establish a mathematical model. The equation for the relationship between the CRU∶U ratio and the increase in yield was y = 0.089x + 27.27 （R2 = 0.663）， and the equation for the relationship between the yield and rainfall during the whole growth period was y = -0.087x + 112.2 （R2 = 0.687）. For rice production， the appropriate CRU∶U ratio can be calculated using these regression equations if the yield increase resulting from applying nitrogen （i.e.， the difference in yield between the CK and fertilizer treatments） and the cumulative rainfall during the whole growth period in the area of interest are known. This provides a method for selecting the most appropriate application rate for controlled-release nitrogen fertilizer. This will play an important role in selecting the most appropriate amount and type of fertilizer to use based on the soil and environmental conditions for the site of interest and this will improve the nitrogen use efficiency of rice.
Conclusions
Field experiments were conducted at five sites. The rice yields achieved using two nitrogen application rates and five CRU∶U ratios were compared. Applying nitrogen significantly increased the rice yield relative to the CK treatment， but to different degrees at different sites. Applying both CRU and U increased the yield more than applying the same amount of nitrogen as U only， and the yields were higher for a nitrogen application rate of 150 kg/hm2 than for a nitrogen application rate of 105 kg/hm2. A CRU∶U ratio of 70∶30 gave higher yields than other ratios. The nitrogen use efficiency was 21.9% higher for a nitrogen application rate of 105 kg/hm2 than for a nitrogen application rate of 150 kg/hm2. The nitrogen use efficiencies for the 70∶30， 50∶50， 100∶0， and 30∶70 CRU∶U ratios were 46.6%， 38.1%， 34.7%， and 22.2% higher， respectively， than the nitrogen use efficiency for the 0∶100 ratio. The output （which took into account the yield， crop price， and input costs） was 3 078.87 yuan/hm2 higher for a nitrogen application rate of 150 kg/hm2 than for a nitrogen application rate of 105 kg/hm2 when both CRU and U were applied. The output was higher under all CRU+U combinations than when only U was applied， and the highest output was found for a CRU∶U ratio of 70∶30. Increasing the amount of nitrogen added decreased the output efficiency because CRU is expensive. Regression equations were obtained for the relationships between the yield increase and rainfall during the whole growth period （y = 7.429x-185.7， R2 = 0.663） and between the yield and CRU∶U ratio （y = -0.087 1x + 112.29， R2 = 0.687 9）. These regression equations can be used to determine the most appropriate CRU∶U ratio to use to match the rainfall at the site of interest. Acknowledgements
The authors thank the following researchers for their help with this study： Zheng Jiaguo （Crop Research Institute， Sichuan Academy of Agricultural Sciences）； Tu Shihua （Soil and Fertilizer Research Institute， Sichuan Academy of Agricultural Sciences）； Prof. Ma Jun （Sichuan Agricultural University）； Zhou Weijia （Rice Research Institute， Guizhou Academy of Agricultural Sciences）； and Yang Congdang （Grain Crops Research Institute， Yunnan Academy of Agricultural Sciences）. The English text of a draft of this manuscript was edited by Liwen Bianji， Edanz Group China （www.liwenbianji.cn/ac）.
References
[1] LIU J， ZHAO XJ. Development sratus and strategy of chinese slow， controlled release fertilizers[J]. Northern Horticutrue， 2011 （21）： 189-190.
[2] GU JL， XU QM， CAO B. Current situation and expectation of slow and controlled release fertilizers[J]. Journal of Anhui Agri. Sci.， 2007， 35（32）： 10369-10372.
[3] YUAN JL， LIANG XQ， LI L， et al. Response of rice yield and nitrogen uptake to enhanced efficiency nitrogen fertilizer in China： A meta-analysis[J]. Scientia Agricultura Sinica， 2014， 47（17）： 3414-3423.
[4] HU CH， LUO GB， ZENG JH， et al. Influence of different types of slow-release nitrogen fertilizer on rice yield and nitrogen fertilizer use efficiency[J]. Chinese Agricultural Science Bulletin， 2011， 27（15）： 174-177.
[5] CHANG Z， LIU SX， ZHANG ZX， et al. Study on the fertilizer efficiency and the economic benefit of new slow-release nitrogenous fertilizers[J]. Soil and fertilizer in China， 2008， 6：59-63.
[6] HE XS， LIAO ZW， HUANG PZ， et al. Characteristics and performance of a new water-absorbent slow release nitrogen fertilizer[J]. Scientia Agricultura Sinica， 2006， 39（10）： 2048-2055
[7] XU FY， QIN YP， WANG XL， et al. Effects of different novel functional urea on the yield and efficiency of nitrogen absorption and utilization of hybrid rice[J]. Hubei Agricultural Sciences， 2014， 53（10）： 2257-2263.
[8] WANG B， WAN YF， GUO C， et al. A comparison of the effects of controlled release urea， stable urea and microorganisms increasing double rice yield and nitrogen use efficiency[J]. Journal of Plant Nutrition and Fertilizer， 2015， 21（5）： 1104-1112， （4）： 918-923.
[9] LI FM， AI TC， ZHOU SB， et al. Influence of slow-release nitrogen fertilizers on lowland rice yield and nitrogen use efficiency[J]. Chinese Journal of Soil science， 2004， 35（3）：311-315
[10] SUN ZX， JIAN YY， WANG XX. Effects of controlled release nitrogen fertilizer on rice yield[J]. Shanghai agricultural science and technology， 2011， 4：98-99. [11] ZHOU J， MA GH， LONG JR， et al. Study on the rate of slow-release nitrogen fertilizer for super hybrid late rice[J]. Phosphate & Compound Fertilizer， 2009， 24（1）： 80-82.
[12] XU JK， LI XH， LI W， et al. Effects of mixture of slow-release urea and conventional urea on ammonia volatilization and soil nitrogen dynamic change[J]. Soil and fertilizer in China， 2015， （6）： 23-27.
[13] CHEN XY， WU LH， HAN KF， et al. Effects of different mixture rates of coated urea and prilled urea on Rice Grain Yield and nitrogen use efficiency[J]. plant nutrition and Fertilizer Science， 2010， 16（4）： 918-923.
[14] WANG XY， TIAN XH， WANG XL， et al. Effects of bulk blending urea on rice yield and nitrogen fertilizer agronomic efficiency[J]. Hubei Agricultural Sciences， 2010， 40（10）： 2386-2389.
[15] WU SF， HAN KF， WU LH， et al. Effects of application of controlled-release bulk blending urea on yield， quality and nitrogen utilization efficiency of rice[J]. Journal of Zhejiang agriculture， 2011， 23（5）： 983-987.
[16] SHAVIV A. Plant response and environmental aspects as affected by rate and pattern of nitrogen release from controlled release N fertilizers[J]. Springer Netherlands， 1996， 68：285-291.
[17] LI Y， LI YH， ZHAO JH， et al. Effects of slow-and cortolled-release nitrigen fertilizer on nitrogen utilization characteristics and yield of machine-transplanted rice[J]. Journal of Zhejiang University Agric.& Life Sci.， 2015，41（6）：672-684.
[18] DU J， SUN KG， ZHANG YH， et al. Effects of controlled release urea on physiological characteristics and nitrogen use efficiency of rice and NO-3-N contents in soil[J]. Journal of Agricultural Resources and Environment， 2016， 33（2）： 134-141.
[19] DAI PA ，NIE J ，ZHENG SX， et al. Efficiency of nutrient utilization of controlled-release nitrogen fertilizer for rice at different soil fertility levels[J]. 2003， 34（2）： 115-119.
[20] SUN KG， DU J， HE AL， et al. Effect of controlled release urea fertilizer on rice yield and nitrogen utilization efficiency[J]. Journal of Henan Agricultural Sciences， 2015， 44（12）：57- 60.
[21] TIAN XH， MA GH，YANG LZ， et al. Effects of slow release nitrogen fertilizer on growth and physiological function in the late stage of super hybrid rice[J]. Hybrid Rice， 2010， 25（5）： 64-69.
[22] MA GH， LONG JR， DAI QM， et al. Studies on the optimized allocation of contrlled-release nitrogen fertilizer application and planting densith for medium super hybrid rice combination Y Liang You1[J]. Hybrid Rice， 2008， 23（6）：73-77.
[23] MA GH， ZHOU J， LONG JR， et al. Effect of slow-released nitrogen fertilizer on growth and yield of super hybrid early rice[J]. Journal of Hunan Agricultural University （Natural Sciences）， 2008， 34（1）： 95-99.
[24] LI DP， WU ZJ， LIANG CH， et al. Effect of slow/controlled release nitrogen fertilizers on soil biological activities[J]. Journal ofAgro-Environment Science， 2006， 25（3）： 664-669.
[25] HUANG P， ZHANG JB， ZHU AN， et al. Coupled water and nitrogen （N） management as a key strategy for the mitigation of gaseous N losses in the Huang-Huai-Hai Plain[J]. Biol Fertil Soils， 2015， 51： 333-342.
[26] TING LAN， YONG HAN， MARCO ROELCKE， et al. Temperature dependence of gross N transformation rates in two Chinese paddy soils under aerobic condition[J]. Biol Fertil Soils， 2014， 50： 949-959.