Effects Of Nitrogen And Phosphorus Addition On Soil Carbon Processes In A Tropical Forest

Terrestrial biosphere–atmosphere carbon(C)exchangeis is dominated by tropical forests,where the ecosystem functions are thought to be phosphorus(P)limited.Human activities are altering natural patterns of tropical nitrogen(N)availability through N deposition,which may interact with P to affect tropical C cycling.We conducted an experiment in a tropical forest in Xiaoliang Research Station for Tropical Costal Ecosystem.The experiment site in the field established in 2009 included six blocks.There are four plots in a block.The treatment of N addition,P addition,N plus P addition and control were randomly distributed to the plots in each block.In the present study,the effects of nutrients addition on litter production,forest floor decomposition,soil heterotrophic respiration,soil organic carbon(SOC)stability and soil microbial community were examined.The aim of this study is to reveal the underlying mechanism regulating the effect of N deposition and its interaction with P addition on soil C balance in tropical forests,which can improve the management of tropical forest C sink.The main results and conclusions are listed as follow:1.NP addition influences C input and output in soil surface: N addition did not significantly affect aboveground litter production,soil heterotrophic respiration and litter pool on the forest floor.P addition did not significantly affect aboveground litter production,but significantly increased soil heterotrophic respiration and decreased litter pool by 29.7%.N plus P addition increased aboveground litter production by 36.3%,decreased litter pool by 23.2%,but did not affect soil heterotrophic respiration.The results indicate that in the tropical forest,N deposition would not affect C input and output in soil surface;P addition would increase C loss by accelerating soil heterotrophic respiration;N plus P addition would increase C input by increasing litter production,but increase carbon output by accelerating litter decomposition.2.NP addition changes SOC fractions: N addition significantly increased recalcitrant SOC.P addition significantly decreased active SOC and light fraction of SOC.IN contrast,N plus P addition significantly increased slow SOC,and decreased active SOC and light fraction of SOC.3.NP addition regulates composition of soil microbial community: N addition significantly decreased soil microbial carbon in wet and dry season respectively.P addition significantly increased soil microbial nitrogen,and significantly increased arbuscular mycorrhizal fungi in wet season.The response pattern of soil microbes is consistent with that of SOC loss,which indicates that the effect of NP addition on soil C balance is predominately regulated by soil microbes.The present study suggests that N deposition cannot affect carbon input and output.However,it could increase recalcitrant SOC thus enhancing the soil C sequestration.P addition can accelerate decomposition of soil organic matter and litter in forest floor,and decrease total SOC and active SOC,which indicate that P addition decreases C sequestration in soil and forest floor.N plus P addition can increase aboveground litter input,but increase C loss from forest floor litter.The result of SOC fraction showed that N plus P can increase slow SOC.The effect of N plus P addition on soil and forest floor C sequestration needs more study.