Aridity-driven divergence in soil microbial necromass carbon in alpine grasslands of the Tibetan Plateau

Abstract

Soil microbial necromass carbon (MNC) contributes to the long-term stability of soil organic carbon (SOC). However, the response of MNC across aridity gradients remains unclear, especially in vulnerable alpine ecosystems. Here, we examined alpine grasslands from 180 sites spanning a 3,500 km aridity gradient on the Tibetan Plateau to investigate how MNC abundance and composition (contributions of bacterial and fungal necromass carbon) vary with climate. MNC was variable, ranging from 0.55 to 26.95 g kg⁻¹ soil, with higher content observed in humid and dry-subhumid regions than in arid and semiarid regions in the Western Tibetan Plateau. Soil properties were the dominant drivers of MNC, with soil fertility (cation exchange capacity and total phosphorus) and weathering products (clay, silt and iron/aluminum oxides) facilitating MNC accumulation, while a negative correlation was observed between MNC and soil pH. A pivotal aridity threshold of 0.60 underpinned a non-linear decrease in MNC with increasing aridity across soil condition gradients; MNC was negatively correlated with aridity below this threshold and showed no correlation beyond it. Given this pivotal aridity threshold, we delineated the drivers of MNC under conditions of low (aridity < 0.6) versus high (aridity > 0.6) aridity. In low-aridity conditions, MNC accumulation was governed by aridity, soil fertility, weathering products, and pH, whereas in high-aridity conditions, the interplay between soil properties and temperature took precedence. Species richness enhanced carbon accumulation from microbial residues under low-aridity conditions more so than under high-aridity conditions, with fungal necromass carbon consistently being a higher contributor to SOC than bacterial necromass carbon, particularly in humid regions. These findings highlight aridity-driven divergence in MNC and propose that conserving plant diversity may mitigate the adverse effects of aridification on MNC under low-aridity conditions in alpine grasslands.