Carbon and Nitrogen Dynamics in Soil Aggregates under Long-Term Nitrogen and Water Addition in a Temperate Steppe

Anthropogenic-driven changes in N and water availability are two of the most important factors determining soil C and N turnover in temperate grassland ecosystems. To gain insight into changes in soil aggregation and C and N dynamics in response to N and water addition, we collected soil samples from a field study conducted for 9 yr in a semiarid steppe grassland in Inner Mongolia, China. Three aggregate size classes (microaggregates, <250 μm; small macroaggregates, 250–2000 μm; macroaggregates, >2000 μm) were isolated and analyzed for their mass proportions, soil organic C (SOC), total N (TN), total extractable inorganic N (TIN), and stable isotope ratio of 13C relative to 12C (δ13C) and stable isotope ratio of 15N relative to 14N (δ15N) values. Water addition on average increased large macroaggregates by 33% and decreased microaggregates by 42%. Nitrogen and water addition interacted significantly and increased TIN concentration but had no impact on SOC or TN. Soil organic C was negatively correlated with δ13C values of large and small macroaggregates under ambient precipitation and within all soil aggregates under water addition. Significant positive correlations between TIN and δ15N values were detected for large macroaggregates and microaggregates under ambient precipitation. Our results suggest that water addition accelerated plant residue incorporation into soil organic matter (SOM) (indicated by depleted 13C values) while N addition potentially increased gaseous N losses (suggested by 15N enrichment without changing soil C and N concentrations). Water, but not N, improved soil structure in this semiarid grassland. Our study provides new insights for using natural abundance 13C and 15N to better understand the sensitivity of SOM within different soil particles to coupled N–water changes under global change scenarios.