Nitrogen and phosphorus addition exerted different influences on litter and soil carbon release in a tropical forest

Terrestrial soils release large amount of carbon dioxide (CO2) each year, which are mainly derived from litter and soil carbon (C) decomposition. Nutrient availability, especially nitrogen (N) and phosphorus (P), plays an important role in both litter and soil C decomposition. Therefore, understanding the underlying mechanism is crucial for mitigating CO2 emission and climate changes. Here, we assessed patterns of litter and soil C decomposition after 11 yrs. in-situ N and P addition in a tropical forest where corn leaves or corn roots were added as litter C. The total CO2 efflux was quantified and partitioned using C-13 isotope signatures to determine the sources (litter or soil C) every three months. In addition, Changes in C-degrading enzyme activities: beta-1,4-glucosidase (BG), phenol oxidase (PHO) and peroxidase (PER), and microbial biomarkers were assessed to interpret the underlying mechanism. Total C-release was enhanced up to17% by the long-term N addition but inhibited up to 15% by P addition. Precisely, N addition only accelerated the litter decomposition and increased about 42% and 6% of the litter C release at 0-5 cm and 5-10 cm soil depths, respectively; while P addition only impeded the soil C decomposition and decreased about 9% and 11% of the soil C release at 0-5 cm and 5-10 cm, respectively. The enhanced C release under N addition might be attributed to the enhanced microbial biomass, the ratio of fungi to bacteria and C-degrading enzyme activities. However, P addition resulted in the reverse result in microbial properties and C-degrading enzyme activities, associated with a decreased C release. Our study suggests that the long-term N and P addition selectively affected the litter and soil C decomposition because of their different physiochemical properties and this tendency might be more pronounced in tropical forests exposed to increasing atmospheric N deposition in the future. The study indicates that the different patterns of litter and soil C decomposition under climate change should be taken account in the future C management strategies.