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Tropical forests, where the soils are nitrogen (N) rich but phosphorus (P) poor, have a disproportionate influence on global carbon (C) and N cycling. While N deposition substantially alters soil C and N retention in tropical forests, whether P input can alleviate these N-induced effects by regulating soil microbial functions remains unclear. We investigated soil microbial taxonomy and functional traits 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 N losses, thereby increasing soil CO 2 and N 2 O emissions by 86% and 110%, respectively. Phosphorus and NP additions elevated C-mining enzymes activity mainly due to intensified C limitation, causing 82% increase in CO 2 emission. In secondary forest, P and NP additions reduced phosphatase activity, enriched fungal copiotrophs and increased microbial biomass, suggesting removal of nutrient deficiencies and stimulation of fungal growth. Meanwhile, soil CO 2 emission decreased by 25% and N 2 O emission declined by 52–82% due to alleviated P acquisition from organic matter decomposition and increased microbial C and N immobilization. Overall, N addition accelerates most microbial processes for C and N release in tropical forests. Long-term P addition increases C and N retention via reducing soil CO 2 and N 2 O emissions in the secondary but not primary forest because of strong C limitation to microbial N immobilization. Further, the seasonal and annual variations in CO 2 and N 2 O emissions should be considered in future studies to test the generalization of these findings and predict and model dynamics in greenhouse gas emissions and C and N cycling. [Display omitted] • N and P inputs shift soil fungi towards copiotrophs and bacteria towards oligotrophs in tropical forest. • Soil microbial responses to nutrient inputs differs between primary and secondary forests. • P boosts fungal growth and N retention in secondary forest, reducing soil CO 2 and N 2 O efflux. • N enhances microbial P excavation and denitrification in primary forest, raising CO 2 and N 2 O efflux. • Soil N:P ratio regulates microbial C and N retention and release capacity under N and P inputs. [ABSTRACT FROM AUTHOR]

Aims Tropical forest plays a key role in global C cycle; however, there are few studies on the C budget in the tropical rainforests in Asia. This study aims to (i) reveal the seasonal patterns of total soil respiration (RT), litter respiration (RL) and soil respiration without surface organic litter (RNL) in the primary and secondary Asian tropical mountain rainforests and (ii) quantify the effects of soil temperature, soil moisture and substrate availability on soil respiration. Methods The seasonal dynamics of soil CO2 efflux was measured by an automatic chamber system (Li-8100), within the primary and secondary tropical mountain rainforests located at the Jianfengling National Reserve in Hainan Island, China. The litter removal treatment was used to assess the contribution of litter to belowground CO2 production. Important Findings The annual RT was higher in the primary forest (16.73±0.87 Mg C ha−1) than in the secondary forest (15.10±0.26 Mg C ha−1). The rates of RT, RNL and RL were all significantly higher in the hot and wet season (May–October) than those in the cool and dry season (November–April). Soil temperature at 5cm depth could explain 55–61% of the seasonal variation in RT, and the temperature sensitivity index (Q10) ranked by RL (Q10 = 3.39) > RT (2.17) > RNL (1.76) in the primary forest and by RL (4.31) > RT (1.86) > RNL (1.58) in the secondary forest. The contribution of RL to RT was 22–23%, while litter input and RT had 1 month time lag. In addition, the seasonal variation of RT was mainly determined by soil temperature and substrate availability. Our findings suggested that global warming and increased substrate availability are likely to cause considerable losses of soil C in the tropical forests. [ABSTRACT FROM AUTHOR]