Reduced soil organic carbon sequestration driven by nitrogen deposition–induced increases in microbial carbon to phosphorus ratio in alpine grassland

Aims Effect of nitrogen deposition on soil organic carbon and the underlying mechanisms in grassland ecosystems remains a topic of debate. Moreover, previous research has primarily concentrated on interaction between carbon and nitrogen cycles in response to nitrogen deposition, with less attention paid to how nitrogen-induced phosphorus deficits may impact soil organic carbon sequestration. Methods we applied a meta-analysis to quantify how soil organic carbon and phosphorus respond to nitrogen enrichment in grassland ecosystem. Besides, we conducted an eight-year field experiment involving nitrogen and phosphorus additions. Results the meta-analysis revealed that soil organic carbon increased below 5 g·m ^− 2 but decreased above 10 g·m ^− 2 under nitrogen addition. The field experiment also indicated that soil available phosphorus did not significantly decrease with nitrogen addition of 10 g·m ^− 2 , suggesting an increase in soil available phosphorus due to nitrogen addition. The microbial biomass carbon to phosphorus (MC:MP) ratio significantly decreased under any level of nitrogen addition, indicating that nitrogen enhanced phosphorus limitation of microbes. Moreover, the significant negative correlation between MC:MP ratio and soil organic carbon indicated that microbial carbon limitation increased with microbial phosphorus limitation under nitrogen enrichment. Furthermore, both microbial carbon limitation and phosphorus limitation were significantly correlated with reduced soil organic carbon, suggesting that increases in the MC:MP ratio will reduce soil organic carbon sequestration. Conclusions soil organic carbon will decrease above 10 g·m ^− 2 under nitrogen addition, and the nitrogen deposition-induced MC:MP imbalance may lead to decreased soil organic carbon in alpine grassland ecosystems.