Long-term nitrogen application decreased mineral-associated organic carbon while increasing particulate organic carbon in purple soil in southwest China

In recent years, anthropogenic activities have increased nitrogen (N) input into terrestrial ecosystems, profoundly impacting soil organic carbon (SOC) sequestration. However, the potential mechanisms through which N affects mineral-associated organic carbon (MAOC) and particulate organic carbon (POC) remain unclear. To address this gap, we conducted a 12-year field trial applying continuous N application (0, 90, 180, 270, and 360 kg N·ha⁻¹) in a maize agro-ecosystem. We assessed plant biomass (yield, straw, and root biomass), microbial properties (enzyme activity, biomass, and diversity), soil chemistry (pH, N availability, and base ions), mineralogy (oxides and silicates), and SOC fractions to elucidate the primary control mechanisms influencing MAOC and POC. Our findings showed that N application increased SOC and POC by 6.56%–10.4% and 43.1%–54.0%, respectively, but decreased MAOC by 7.31%–17.1%. And N application increased plant biomass, but decreased soil pH (pH from 6.7 to 5.6), base ion concentrations (K⁺, Na⁺, Ca²⁺, Mg²⁺), amorphous oxides, and illite content. Partial least squares path model (PLS-PM) and correlation analyses indicated that N application enhances root biomass while increasing microbial decomposition, and ultimately their combined effect increased POC. The decline in MAOC is primarily attributed to soil acidification decreasing the C input from microbial residues, altering mineral composition and diminishing the minerals' capacity to protect SOC. Thus, our study demonstrates that N addition predominantly increases POC through enhanced root biomass, while reducing MAOC by decreasing microbial biomass and weakening mineral protection. These insights provide a deeper understanding of the mechanisms governing SOC fraction dynamics in answer to N inputs in agroecosystems.