Divergent composition and turnover of soil organic nitrogen along a climate gradient in arid and semiarid grasslands

Greater aridity predicted with climate change in drylands worldwide will affect soil nitrogen (N) cycling and the associated ecosystem functions. Despite >90% of soil N occurring in organic forms, the pathways of soil organic N (SON) turnover remain largely unknown in drylands, where biological activity is typically limited by water availability. Here we examined patterns of SON fractions and soil N-hydrolyzing enzyme activities across a 3700 km aridity gradient in arid and semiarid grasslands of northern China. We found that both the concentrations of all SON fractions and the proportion of more stable SON increased with increasing aridity index (AI, defined as mean annual precipitation/potential evapotranspiration). The largest SON fraction was hydrolysable NH4+ in arid sites, but amino acid-N in semiarid sites. The activities of enzymes that hydrolyze relatively stable SON polymers (protease, peptidase, and N-acetyl-β-glucosaminidase) were negligible in arid sites (AI < 0.2), but increased significantly as AI increased in semiarid sites (AI > 0.2). Structural equation modeling indicated that the direct effect of microbial biomass on soil amidase was insignificant in arid sites, indicating that microbial SON turnover via enzymes is relatively weak. In semiarid sites, however, microbial biomass exerted significant direct positive effects on all soil N-hydrolyzing enzymes, suggesting strong microbial regulation of SON turnover via enzymatic mineralization. Altogether, our findings provide empirical evidence for divergent patterns of storage and turnover of SON between arid and semiarid grasslands.