Elevation gradients regulate microbial necromass and lignin phenols in alpine peatlands: Evidence from the Qinghai–Tibetan Plateau

Abstract

Microbial necromass carbon (MNC) has recently been recognized as a critical contributor to soil organic C (SOC) sequestration and stabilization. Peatlands, which serve as important C sinks, are developed through the gradual accumulation of partially decomposed plant residues. However, the relative importance of and the key factors determining plant − and microbial − derived C for SOC accumulation remain poorly understood in peatlands. Here, we use amino sugars and lignin phenols as biomarkers to investigate the relative contributions of MNC and plant lignins to the SOC in the topsoil (0 − 15 cm) and subsoil (15 − 30 cm) layers of four alpine peatlands on the Qinghai − Tibetan Plateau. The results revealed that the contributions of MNC to SOC decreased significantly from an average of 283 mg g⁻¹ SOC (213 − 335 mg g⁻¹ SOC) in the topsoil to 229 mg g⁻¹ SOC (142 − 277 mg g⁻¹ SOC) in the subsoil. Conversely, lignin phenols exhibited the opposite trend, with contributions increasing from an average of 21.5 mg g⁻¹ SOC (19.6 − 25.5 mg g⁻¹ SOC) in the topsoil to 24.6 mg g⁻¹ SOC (18.7 − 26.9 mg g⁻¹ SOC) in the subsoil. Among the peatlands, the contributions of MNC to SOC at both depths and the contribution of lignin phenols in the subsoil varied significantly and were significantly positively correlated with the elevation gradient. This pattern was partly attributed to the decline in the activities of C − oxidizing enzymes with increasing elevation, which in turn slowed the decomposition of microbial necromass and plant lignins. Moreover, stoichiometric imbalances, particularly the C-to-phosphorus (P) imbalances, likely stimulate the decomposition of labile organic components to alleviate microbial nutrient limitations, thereby increasing the proportions of MNC and lignin phenols in the SOC pools. Our findings provide important insights into the mechanism of soil C sequestration and stabilization in peatland ecosystems, as well as their feedback to climate change.