Effects Of N And Padditions And Density On Leaf Decomposition And Soil Characteristics In Acacia Auriculaeformis Stands

Forest litter decomposition is a key factor in the ecosystem functions,which is very important for the nutrient to return to soil through leaves in order to remain the sustainable productivity of the forests.Acacia auriculiformis A.Cunn.,a fast-growing leguminous species native to Australia and New Zealand,was introduced to China in the early 1960s,and recently has become one of the major silviculture tree species in southern China.It is widely used to help improve soil fertility via nitrogen fixation.Differences in stand density may affect light,water and nutrient availability,which may influence leaf decomposition.However,few studies have examined the relationship between stand density and leaf decomposition,and the both relationship remains poorly understood.Moreover,nitrogen(N)deposition is increasingly serious in China,and phosphorous(P)fertility is widely applied in the soils of some subtropical forests suffering from P deficiency,but there are relatively few researches of N and P addition on leaf decomposition and soil characteristics in subtropical forests.To date,no studies have compared the decomposition?of green leaves and leaf litter treated with N,P?and N plus P additions from stands differing in density.In this study,decomposition dynamics of green leaves and leaf litter in A.auriculaeformis stands with four densities were studied using litter bag method.Five subsample plots were established in the four density stands(1,667,2,500,4,444 and10,000 trees ha^-1)and treated with N addition,P addition,N and P addition and control.Litter bags with green leaves and leaf litter were put on the surface of each subsample plot in April 2013.Five bags of each treatment were taken back at interval of 3 months,and remaining mass and carbon(C),N and P contents were determined,and soil treated with N,P?and N plus P additions were collected and analyzed,which can provide scientific basis for the management of different densities of A.auriculaeformis stands.The main results were as follows:(1)Green leaves and leaf litter initially decomposed fast and then became slow over time in different density stands,and green leaves decayed faster than leaf litter.Remaining mass of green leaves was significantly correlated to C/N and N/P,whereas that of leaf litter was significantly correlated to C/N and C/P.(2)Mass loss significantly accelerated as the stand density increased.N and N+P additions had a negative effect on leaf decomposition,whereas P addition had a positive effect.The N concentration in green leaves increased rapidly during the first 3 months,followed by fluctuations.The N concentration in residual leaf litter tended to increase during the first 18 months,followed by decreases.The P concentration decreased in residual green leaves during the experimental period and decreased in residual leaf litter during the first 3 months,followed by an increasing trend.(3)Initially,remaining amounts of N and P in green leaves rapidly decreased and later gradually decreased,whereas N remaining amounts gradually decreased and P remaining amounts fluctuated in leaf litter during the experimental period.Remaining amounts of N and P in both leaf types decreased with increasing stand density 12 months after the experiment.(4)Soil p H value significantly decreased after N and N+P additions in the four density stands,whereas it significantly increased after P addition;contents of soil organic matter,total P and available P significantly increased after P and N+P additions;soil total N content significantly increased after N addition and significantly decreased after P addition;soil alkalized N content significantly increased after P and N+P additions,and soil available potassium(K)content significantly decreased after N addition.(5)AWCD value of soil microbial community significantly increased after P addition and significantly decreased after N and N+P additions;Mc Intosh index,Simpson index of soil microbial community in each density stand and Shannon-Wiener index in most density stands significantly decreased after N addition,and significantly increased after P addition.(6)The activities of soil urease and catalase significantly decreased after N and N+P additions,soil acid phosphatase significantly decreased after P and N+P additions,and soil catalase activity of significantly increased after P addition.The above findings suggest that stand density can significantly impact litter decomposition,and elemental stoichiometric ratios are important determinants of the decomposition rate in A.auriculaeformis stands.