Long-Term Phosphorus Addition Alleviates Co2 and N2o Emissions Via Altering Soil Microbial Functions in Secondary Rather Primary Tropical Forests

Tropical forests, where the soils are nitrogen (N) rich but phosphorus (P) poor, have a disproportionate influence on global carbon (C) and N cycling. However, whether P addition can accelerate soil microbial C and N sequestrations and reduce greenhouse gas emissions (CO 2 and N 2 O) under increasing N deposition remains unclear. We investigated soil microbial taxonomy and functional traits involved in C and N cycling in response to 10-year independent and interactive effects of N and P additions in a primary and a secondary tropical forest in Hainan Island. In the primary forest, N addition boosted oligotrophic bacteria and phosphatase and enriched genes responsible for C-, P-mineralization, nitrification and denitrification, suggesting aggravated P limitation while N excess. This might stimulate P excavation via organic matter mineralization, and enhance gaseous N release, leading to 86-110% increases in soil CO 2 and N 2 O emissions. Phosphorus and NP additions reduced phosphatase activity and microbial network keystone species capable of P excavation in both forests, suggesting mitigation of P limitation. This further stimulated fungal growth in the secondary rather than primary forest mainly associated with lower C availability in primary forest. Soil CO 2 and N 2 O emissions declined by 25-82% following P and NP additions in the secondary forest, which could be ascribed to the alleviated P-mining from organic matter and increased microbial C and N immobilization with fungal growth. Overall, N addition accelerates all microbial processes for C and N release in tropical forests. Long-term P addition alleviates C and N release and reduces soil CO 2 and N 2 O emissions via reducing P mining from organic matter and promoting microbial C and N immobilizations in the secondary rather primary forest, mainly because of strong C limitation to microbial growth in primary forest.