Nitrogen deposition in low-phosphorus tropical forests benefits soil C sequestration but not stabilization

The stability of soil organic carbon (SOC) plays a vital role in C sequestration, and largely depends on the availability of soil nitrogen (N) and phosphorus (P). Understanding how different fractions of SOC respond to N and P availability and the underlying microbial mechanism is crucial for mitigating climate changes. Here, we assessed how soil N and P availability modifies different SOC fractions and the soil microbial communities in a tropical forest. We measured soil chemical properties, SOC fractions, microbial PLFA abundance, fungal rDNA and its predicted gene abundance, and extracellular enzyme activities within a field N and P addition experiment. P addition decreased the concentration of recalcitrant SOC and greatly increased the soil oxidative extracellular enzyme activities, while N addition increased active SOC, mainly light fractions, and decreased soil phenol oxidase activity. P addition also induced the greatest abundance of oxidoreductases. Additionally, the transferases, lyases, hydrolases, isomerases, and ligases were also expressed at higher levels after P addition. The results indicate that enhanced soil microbial activities after P addition accelerated recalcitrant SOC decomposition by higher oxidative enzyme activities. Given the increasing N deposition, tropical forests that characterized by a low P have a great potential to sequester more SOC which will mitigate climate change. However, the increase in SOC might be vulnerable to disturbance, because most of the increased C is the active SOC.