Nutrient supplementation changes chemical composition of soil organic matter density fractions in desert steppe soil in northern China

The influence of reactive nitrogen (N) and phosphorus (P) supplementation on ecosystem dynamics of soil organic matter (SOM) shows extensive variations. The underlying mechanisms of regulation of the chemical composition and dynamics of different SOM density fractions (DFs) in aggregates in a desert steppe by N and P supplementation are unclear. Here, the chemical composition and organic carbon (C) and N dynamics of various SOM DFs under N and P addition were assessed. Surface soil samples taken at 0–10 cm depth were obtained from 4-year (2017–2021) experiments under four conditions, namely N treatment, P treatment, combined N and P treatment (NP), and no nutrient supplementation (CK), in a desert steppe located in northern China. Nutrient supplementation significantly increased (P < 0.05) SOM content by 8.89–35.4%; however, the total N content remained constant. N addition mainly increased SOM content in the macroaggregates (>0.25 mm), while P enhanced SOM in microaggregates (0.25–0.053 mm) and silt-clay (