Nonlinear response of soil microbial necromass nitrogen to nitrogen addition gradient in an alpine grassland

Microbial necromass nitrogen (N) constitutes a major component of soil N pools, and elucidating its response to N enrichment is essential for optimizing fertilizer management in grassland ecosystems. However, the effects of varying N addition levels on microbial necromass N and the underlying mechanisms remain elusive. Here, we leveraged a decade-long field experiment in an alpine meadow on the Qinghai-Tibetan Plateau (QTP), applying six N addition levels (0, 2, 4, 8, 16, 32 g N m−2 yr−1) to investigate the response of microbial necromass N and its key drivers. Microbial necromass N exhibited a nonlinear response to N enrichment, remaining stable under low N inputs (2, 4, and 8 g N m−2 yr−1) but increasing significantly at higher N addition levels (16 and 32 g N m−2 yr−1). Across all treatments, microbial necromass N accounted for 67–76% of total soil N. Nitrogen-induced changes in plant, soil, and microbial factors collectively explained 60% of the variation in microbial necromass N. Among these, soil factors were the dominant predictors, accounting for 34% of the total variance, with mineral protection (Feo + Alo) and soil inorganic nitrogen (SIN) identified as the primary drivers. These findings highlight the pivotal role of microbial necromass N in soil N storage and its nonlinear response to N enrichment, underscoring the importance of effective N management for enhancing soil N retention and stability in alpine grasslands.