The precise analysis of isotopic compositions of nitrate(NO3-)in Antarctic snow/ice(i.e.,concentrations on the order of ng g-1)is challenging.The denitrifier method by using denitrifying bacteria(Pseudomonas aureofaciens)lacking the N2O reductase enzyme to quantitatively convert NO3-to N2O(g)offers a powerful tool for the measurement of isotopic ratios of NO3-.With the aid of this method,the δ15N-and δ18O-NO3-in different environments across the East Antarctic Ice Sheet(EAIS)were determined.At high snow accumulation sites(i.e.,near the coast),large variations in concentration and isotopic compositions of NO3-throughout the snowpack were observed,suggestive of a seasonality of these parameters.Higher NO3-concentrations were observed in warm-season snow layers while higher values of δ15N-and δ18O-NO3-were present in the cold season.The post-depositional processing of NO3-cannot explain this large variation; whereas this variation is most likely explained as presenting a seasonal NO3-source shift over different periods,i.e.,the importance of stratospheric ozone influence on the production of atmospheric NO3-in cold season compared to more tropospheric NOx source influence in the warm season.At low snow accumulation sites(i.e.,more interior area),a large extent of NO3-loss is found,accompanied by increasing δ15N-NO3-and decreasing δ18O-NO3-,suggesting the dominant role of photolysis and secondary formation of NO3-within the snowpack.At the inland sites,the imprint of post-depositional alteration is not uniform with depth,however,raising the possibility that photolytic loss cannot be assumed to operate consistently over time.This makes it hard to predict the isotopic composition of NO3-at depth based on the fractionation of near-surface NO3-alone.The changing relationship of concentration vs.δ18O of NO3-at depth(vs.the upper layer)at the inland sites possibly associated with the local snow chemistry changes in recent decades,e.g.,the photolability of NO3-.In summary,the outcomes of this investigation suggest the sensitivity of isotopic composition to post-depositional processing of NO3-across the EAIS.