Abstract As a kind of biological fertilizer sources， green manure can improve soil fertility and the quality of agricultural products. This paper introduced the germplasm resource of green manures in the major provinces in China， mainly summarized the characteristics of decomposition and nutrients release of returning green manures to soils， as well as the influence on soil fertility and succeeding crops， with the aim to provide references for rational utilization of green manures and the scientific management of farmland nutrients.
　　Key words Green manure； Decomposition characteristics； Nutrients release characteristics； Fertility improvement
　　Green manure is the essence of Chinas traditional agriculture. As a source of biofertilizer， it has the effect of improving soil fertility and the quality of agricultural products. In the 1970s， the sown area of green manure crops in China reached 1.3×107 hm2. However， with the promotion of fertilizer application， the cultivation area of green manure crops has declined rapidly since the 1980s[1]. In recent years， with the deterioration of soil environment， peoples demand for green agricultural products has been increasing[2]. Modern agriculture calls for the return of the essence of traditional agriculture. In 2015， the Ministry of Agriculture proposed to achieve the goal of "1 control， 2 decreases and 3 bases" （controlling agricultural water consumption， decreasing the application amounts of chemical fertilizers and pesticides， adopting measures to solve problems of livestock wastes， mulching recycling and straw burning）， and formulated the Action Plan for Zero Growth of Chemical Fertilizer Use by 2020. The cultivation and use of green manure crops is beneficial to the reduction of fertilizer use， which is of important significance to ensure the safety of agricultural products， protect the farmland ecological environment， and promote sustainable agriculture. In this paper， we summarized the green manure germplasm resources in the major provinces of China， stated the characteristics of decomposition and nutrient release after returning green manure crops to fields， as well as the effects on soil fertility and succeeding crops after returning， with the aim to provide referential bases for the reasonable use of green manure resources and scientific management of farmland nutrients.
　　Green Manure Germplasm Resources in China
　　China has a vast territory and strong regional differences. There are great differences in green manure varieties in different regions， mainly including legumes， grasses， crucifers， and asteraceae. The main green manure crops in different provinces are shown in Table 1. 　　Decomposition Characteristics of Green Manure Crops
　　Decomposition characteristics of different green manure crops after returning to fields
　　After ploughing and incorporating green manure crops into fields， the decomposition speed is different under soil environment and climatic conditions[19]. However， the decomposing process generally includes rapid decomposition period and slow decomposition period. Pan et al.[20]studied the decomposing and nutrient release characteristics of 3 kinds of leguminous green manure crops （V. sativa， V. dasycarpa and L. quinquenervius） under dryland conditions， and the results showed that the 3 green manure crops were decomposed fast in the first 15 d after returning to fields， and the average decomposition rates were 0.34， 0.30， and 0.35 g/d， respectively. After 15-70 d of mulching， the average decomposition rates were only 0.023， 0.026 and 0.021 g/d， significantly less than those of the first 15 d. Li et al.[21]planted oilseed rape between rows of rice fields to study its decomposition characteristics. The results showed that during the decomposing process of rape stalks， the mechanic tissue damage was observed within the first 10 d when the vascular cambium， phloem fiber， cortical parenchyma and cuticle above the secondary xylem were broken. The reason for the fast decomposition of green manure crops in early stage and slow in late is that there are a lot of soluble organic matters and inorganic nutrients in the straws in the early stage of decomposition， which provide a large amounts of carbon sources and nutrients for microorganisms， and thus the number of microorganisms increases and the activity enhances. However， as the decomposition progresses， the soluble organic matters in the straw gradually decreases， and the remaining part is mainly composed of hardtodecompose organic matters， which leads to a decrease in the activity of the microorganisms， and the decomposition of the straw also becomes slower[22-23]. Similar rules have also been found in the decomposition of leguminous green manures （soybean， mung bean， Changwuhuai bean， milk vetch and M. sativa）[24-26]， cruciferous green manures （Orychophragmus violaceus）[27]， gramineous manures （L. perenne， Festuca arundinacea）[26]， asteraceae green manures （chicory， Tithonia diversifolia）[26， 28]. Different green manure crops have different decomposition rates， which may be related to the ratio of carbon to nitrogen. In general， a C/N ratio of （25-30）∶1 is most conducive to microbial activity[29]， which contributes to the decomposition of organic matters. Natanael et al.[30]studied the decomposition rates of 6 leguminous green manure species （Crotalaria juncea， Canavalia ensiformes， Cajanus cajan， C. spectabilis， Dolichos lablab and Mucuna deeringiana）， finding that the decomposition rate of C. juncea was the slowest， which was related to its high carbon to nitrogen ratio， while the decomposition of C. ensiformes was the fastest， which was related to its low carbon to nitrogen ratio. 　　Nutrient release characteristics of different green manure crops after returning to field
　　The release of N， P， and K nutrients from plants after returning the green manure crops to fields has an impact on the growth of succeeding crops. It is of great significance for the scientific and reasonable utilization of green manure crops to clarify release rules of nutrients after the green manure crop is ploughed and incorporated into fields. According to the mineralization rate of nutrients， the release rate of K is the largest in the general case， followed by P， C， N. The main reason is that K in the stalk does not exist in chemical combination but K+ in the cells or plant tissues， which can be easily released by water extraction， so it releases fast. On the other hand， P， N and C are mainly in the organic state， which is difficult to decompose under physical effect， so the releases are slow[31]. Previous studies have found that after returning milk vetch， V. sativa， V. dasycarpa， L. quinquenervius， Chuangwuhuai bean， soy bean， mung bean， O. violaceus， wheat and rape straws to field， the nutrient release rates are all in the order of K > P > N[20-21， 24， 27， 32]. Lv et al.[33]studied the characteristics of soybean and peanut decomposition under the mulching method， and the results showed that nutrients release rates of soybean and peanut straws were all in the order of K > P > C > N. Zou et al.[34-35]and Wu et al.[36]studied the decomposition characteristics of cassava stalks， banana stalks， and wheat stalks in different field returning ways， and the decomposition rates were in the order of K > P > N ≈ C.
　　Decomposition characteristics of green manure in different returning modes
　　The decomposition of green manures in soil is a complicated process. Different field returning methods have some influences on the decomposition characteristics of green manures. Soil microorganisms are fond of high temperature and humidity environment. Under the conditions of earth covering， soil has a good ventilation condition， and the soil temperature increases rapidly， which is conducive to the improvement of soil microorganisms and enzyme activities. In flooded conditions， oxygen is blocked， and some aerobic microorganisms lose their activity due to insufficient oxygen， so that the decomposition rate of the residue under flooding conditions is slowed down and the decomposition amount is reduced[37]. Wu et al.[38]studied the decomposition characteristics of rapeseed and wheat straws under different rice cultivation patterns and straw mulching methods. The results showed that under conventional cultivation mode （conventional rice cultivation means that the soil surface layer keeps at the shallow water layer state during the growth stages except soil drying period）， the straws that were mulched decomposed faster than the straws incorporated into soil； under watersaving cultivation mode （rice watersaving irrigation cultivation refers to the use of waterfree layer irrigation technology， that is， after the rice turns green， field surface has no water layer）， the straws incorporated into soil decomposed faster than the straws mulched. Wang et al.[39]explored the decomposing and nutrient releasing characteristics of crop straws under 3 different incorporation approaches of crop straws into fields， namely soil coverage， water submergence and openair treatment， and the results showed that under soil coverage treatment， the decomposition rates of wheat and rapeseed straws were faster than those of openair and water submergence treatments， while the releasing rates of N， P and K were in the order of water submergence treatment > openair treatment > soil coverage treatment. When returning green manures to fields， different strawreturning amounts and strawreturning depth also have a certain influences on the decomposition characteristics. Hu et al.[40]found that the decomposition rate was in the order of entire amount of straw returning to soil < 2/3 amount of straw returning < 1/2 amount of straw returning < 1/3 amount of straw returning. Moreover， when planting rice， the decomposition rate of returning rapeseed straw was in the order of surface layer > 20 cm deep > 10 cm deep. The reason is that in summer， the surface of paddy field not only has high temperature， but also has active microorganisms. When the rapeseed straw is exposed to the surface soil， it will be exposed to many active microorganisms， and the decomposition rate will be faster. In the case of 10 cm depth， the combination of water and heat in the soil layer is general， and the microbial activity is weak， while at the depth of 20 cm， rice root oxygenation， root aeration， and microbial activity are strong[41-42]， contributing to microbial activities. 　　Effects of decomposing inoculants on decomposition of green manures
　　The decomposition rates of decomposed green manure are slow after directly returning to fields. Straw decomposing inoculants are rich in highly effective microbial bacteria， which can promote rapid decomposition of the straws[43-44]， but the effect is different under different ways of field returning. Liu et al.[45]studied the weight loss ratios of rape straws decomposed with 8 different strawdecomposing inoculants， and the results showed that the 8 strawdecomposing inoculants promoted the decomposition of rape straws in varying degrees. Wang et al.[39]also found that under the openair and soil coverage treatments of returning straws to fields， the decomposition rates of wheat and rapeseed straws added with decomposing inoculants were faster than those without， while under the water submergence treatment， the effect of adding decomposing inoculants was not significant in improving the decomposition rates of straws. Li et al.[21]found that under water submerging environment， adding decomposing inoculants did not accelerate the decomposition rates of wheat and rapeseed straws， because the selected decomposing inoculants were an aerobiotic microbial agent， which could not promote the decomposition under anaerobic submerged conditions.
　　Decomposing inoculants can not only affect the decomposition of green manures， but also improve the nutrient structure of the soil. Huang et al.[46]found that after adding the decomposed agent， the contents of soil organic matter， available N， available K and CEC increased compared with that without decomposing inoculants. Wang et al.[47]studied the influence of crop yields and soil physical and chemical properties with inoculants added to straws after mulched in fields， finding that the soil nutrients increased to varying degrees， and that the porosity of the soil increased and the bulk density decreased. Wu et al.[48]investigated the influences of adding organic matters decomposing inoculants on the effect of returning straws to the field， finding that the total K content of the soil added inoculants was slightly lower than that in the nonapplication treatment group， but the contents of organic matter， total N， and total P were increased， and the physical and chemical properties of the soil were improved.
　　Zhongyi LI et al. General Situation of Green Manure Germplasm Resources in China and Research Progress on Decomposition Characteristics and Fertility Improvement of Green Manures 　　Effect of Returning Green Manures to Fields on Soil Fertility and Succeeding Crops
　　Effect of green manure returning on soil fertility
　　Returning green manures can improve soil physical properties[49-51]， increase soil organic matter content[52-53]， conserve water and soil[54-55]， increase soil nitrogen[56-58]， activate and enrich soil phosphorus[59-61]， potassium[62-63]， increase soil microbial population and improve the functional diversity of soil microbial community structure[64-66]. In addition， green manures play an active role in increasing soil organic carbon pools and improving the ecological environment in small areas. Lan et al.[67]found that green manure rotation， especially milk vetchricericerapeseed/peanutrice could increase soil organic carbon contents and soil carbon pool management index. Sharma and Behera[68]found that in the maizewheat cropping system， leguminous green manures like cowpea， mung bean and Sesbania significantly increased soil organic carbon. Through 30 years of longterm paddy field experiment， Liu et al.[69]compared the soil characteristics in the cropping patterns of ricericerape， ricerice milk vetch， ricericeryegrass and ricericewinter fallow. The results showed that the average distribution ratio of total organic carbon in paddy soils increased significantly in different soil particulates， and significant positive linear correlations were found between the contents of total soil organic carbon， the organic carbon in coarse clay and fine clay and cultivation years in treatments with green manures. Wang et al.[70]investigated the effects of longterm green manure application on the productivity and stress resistance of a double rice cropping system in a 28year field experiment， pointing out that green manure application could enhance the resistance of the double rice cropping system to nutrient depletion. Peng et al.[71]suggested that compared with general treatment， intercropping white clover and straw mulching reinforced the ability of selfregulating and controlling of the tea plantation ecological system with the yield increasing 33.99% and 26.19%， respectively.
　　Effect of green manure returning on succeeding crops
　　Ploughing and incorporating green manures before planting the succeeding crops can make them decompose and release nutrients for the growth and utilization of the main crops， thereby reducing the amount of chemical fertilizers and achieving the purpose of costsaving and efficiency improvement in agricultural production[27]. However， excessive or single application of green manure can affect the yield of succeeding crops[72-73]， and the optimal substitution rate of chemical fertilizers depends on the crop type， soil type and soil fertility[74]. Xie et al.[75]investigated the effects of different substitution rates of fertilizer N by green manures （100% fertilizer N， 80% fertilizer N + 20% milk vetch and 60% fertilizer N + 40% milk vetch）， finding that soil fertility and grain yield of early and late rice were higher than that of pure fertilizer N treatments. Zhang et al.[76]found that the application of green manures （V. villosa and V. sativa） of 7 500 kg/hm2 could reduce the application of inorganic nitrogen fertilizer by about 67.5 kg/hm2， which could improve soil fertility and wheat yield. Yang et al.[77]found that after returning green manure （O. violaceus） to field， the maize grain yield and shoot N accumulation under the treatment with 15% reduction of chemical fertilizers were significantly increased by 29.0 and 0.65 g/plant， respectively from the pure chemical fertilizer treatment. in addition， ， the effect of green manure on the yield and quality of succeeding crops is significant[78]. Wan et al.[79]and Gao et al.[80]studied the green manurerice rotation system， and the results showed that applying milk vetch green manure could significantly increase rice yield. Li et al[81]found that at the blossoming and boll forming stages， SPAD values of cotton leaves of the green manure incorporated treatment were significantly increased compared with the single fertilizer treatment， and the seed cotton yield increased by 30%. Liu et al.[82]conducted a fluecured tobaccogreen manure rotation experiment for five consecutive years， and the results showed that the longterm application of green manure to fields significantly increased the K content of the fluecured tobacco leaves， improve the appearance of tobacco leaves， and enhance the smoking evaluation score. 　　Outlook
　　In 2016， the No. 1 Document of the Central Committee of the Peoples Republic of China clearly stated that the protection and efficient use of agricultural resources should be enhanced， the controlling of outstanding problems in the agricultural environment should be accelerated， the protection and rehabilitation of agricultural ecology should be strengthened， and the green development of agriculture should be promoted. In 2015， the Ministry of Agriculture of the Peoples Republic of China proposed to achieve the goal of "1 control， 2 decreases and 3 bases" by 2020， and also formulated the Action Plan for Zero Growth of Chemical Fertilizer Use by 2020. The cultivation and utilization of green manure crops is just a technical measure to protect the ecological environment of farmland and the reduction of chemical fertilizers. Therefore， the recovery and development of the green manure industry is increasingly placed in more important and prominent positions in modern agriculture.
　　Since the 1980s， with the increase of fertilizer input and the increase of the multiple cropping index of land， the development space of green manure has been objectively constrained. In addition， the national policy inputs and guidance are insufficient， and the cultivation and utilization of green manure is in a spontaneous state. The lack of germplasm resources， the degradation of varieties， the decline in yields， and the lack of labor all constrain the development of the green manure industry. Under the guidance of current policies， it is of great significance for the sustainable development of agriculture to recover and develop the green manure industry in stages and steps.
　　Government departments at all levels should formulate special support programs， set up support funds， and provide supports from the production， management and comprehensive utilization of green manures， do propaganda work well and reverse the concept of chemical fertilizeroriented fertilization concept. Scientific research departments should strengthen the collection， selection， breeding and seed stocking of green manure varieties， make a good job of elite breeding and purification and rejuvenation， improve the industrialization system of green manure seed production， systematically carry out organic manure quality evaluation of green manure crops， study the decomposing dynamics under different field returning modes， establish a corresponding effect model of green manures， and determine a reasonable field returning approach， so as to provide a technical reference for the rational use of green manure resources. As for the promotion and application， we should build demonstration bases， establish demonstration models， advocated multiple functional fertilizers like green manurecrop rotation， green manureintercropping， dual purpose crops， dualpurpose vegetables， dualpurpose feed， fruit garden green manure， take the way of comprehensive utilization to explore the optimal mode of green manure cultivation， and focus on improving the economic， social and ecological benefits of green manure production. 　　References
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　　Editor： Na LI Proofreader： Xinxiu ZHU