Effects of decadal nitrogen addition on carbon and nitrogen stocks in different organic matter fractions of typical steppe soils

Abstract

Recent frameworks have proposed that division of soil organic matter (SOM) into particulate and mineral-associated organic matter (POM and MAOM) can help us better understand SOM cycling and its responses to increasing atmospheric nitrogen (N) deposition. However, responses of these factions to N deposition with combination of their relative distribution across soil profile remain unclear. Here we determined total N and soil organic carbon (SOC) as POM and MAOM separately in soils at depths of 0–10, 30–40 and 70–100 cm after 10-year N addition (at rates of 50, 10, 2 and 0 g m−2 yr−1) in a typical steppe. We further calculated their stocks in POM, MAOM and bulk soil and detected their relationships with both physicochemical features and microbial properties. Nitrogen addition increased the stocks of SOC (POM: +23 %; MAOM: +11 %) and total N (POM: +27 %; MAOM: +10 %) in both POM and MAOM fractions in topsoil (0–10 cm), but increased only in MAOM in 30–40 cm (SOC: +24 %; total N: +24 %) and 70–100 cm (SOC: +15 %; total N: +13 %) soils. Moreover, the increasing effects were strengthened with increasing N addition rates. We found that the share of SOC and total N in the MAOM was slightly decreased by N addition in topsoil, but significantly increased in deeper soils. Soil physicochemical features exerted stronger controls than microbial properties in the distribution of SOC and total N in the two fractions regardless of soil depth. SOC and total N contents of MAOM were correlated negatively with soil pH across the soil profile, and were correlated positively with bulk soil total N, dissolved organic N and inorganic N. Our findings imply that more soil C would be stabilized as MAOM under increasing atmospheric N deposition, and therefore the C saturation level of MAOM should be a target for further studies and be considered in predicting SOM dynamics, especially in N-limited grassland ecosystems.