Depth‐dependent mechanisms regulate accumulation of plant‐ and microbial‐derived residues under long‐term nitrogen addition in a semiarid grassland

Abstract Plant‐ and microbial‐derived residues constitute the primary sources of soil organic carbon (SOC) in grassland ecosystems. However, their differential responses to chronic nitrogen (N) enrichment and the depth‐dependent mechanisms governing their accumulation remain poorly characterized, particularly for water‐limited grassland systems. Based on a 13‐year field experiment in a semiarid grassland, we quantified the effects of long‐term N addition on the accumulation of plant‐ (lignin phenols) and microbial‐derived (amino sugars) residues. We found that N addition significantly increased lignin phenol content and its contribution to SOC in the topsoil, whereas lignin phenols exhibited a hump‐shaped response peaking under moderate N levels in the subsoil. Amino sugar concentrations and their relative contribution to SOC increased in both soil layers under N addition but declined at the highest N input. The dominant factors regulating residue accumulation varied with soil depth: in the topsoil, microbial K −/ r ‐traits and community composition primarily explained lignin phenol and amino sugar dynamics, while in the subsoil, mineral‐associated protection and microbial composition were the key drivers. These findings underscore the depth‐dependent nature of SOC formation pathways and highlight the importance of incorporating both plant‐ and microbial‐derived residues into Earth System Models to improve projections of carbon‐climate feedback under changing nitrogen regimes. Read the free Plain Language Summary for this article on the Journal blog.