Phosphorus supply suppressed microbial necromass but stimulated plant lignin phenols accumulation in soils of alpine grassland on the Tibetan Plateau

Increased nitrogen (N) and phosphorus (P) inputs have fundamental effects on the soil organic carbon (SOC) composition and dynamics. However, the responses of plant- and microbial-derived SOC components to N and P addition in alpine grasslands are poorly understood. Based on a 10-year N and P addition experiment conducted in the alpine grassland of the Tibetan Plateau, we used amino sugars and lignin phenols as tracers for microbial necromass and plant lignin components, respectively, and explored their accumulation with the addition of N and P. We found that P and N + P addition (P supply) decreased microbial necromass, whereas N addition did not have a significant effect. In comparison, the P supply increased lignin phenols in the topsoil, but N addition decreased them in the subsoil. Among these factors, soil total P played a non-negligible role in controlling the accumulation of amino sugars and lignin phenols in soils. In addition, decreased ratios of fungi-to-bacteria necromass carbon and amino sugars to lignin phenols were observed with P supply. This implies that although the 10-year P supply did not change the SOC significantly, it may have eventually increased the SOC loss potential. Collectively, we attempted to elucidate the underlying mechanisms of long-term SOC sequestration, which has important implications for plant- and microbial-mediated carbon processes in the context of increasing N and P inputs.