Abstract:In order to reveal the response mechanism of nutrient cycling and water cycling to nitrogen deposition in forest ecosystem, we conducted a study on a situ decomposition of leaf and twig litters of Pinus armandii forest and Pinus yunnanensis forest in the middle Yunnan Plateau. Four treatments were set up: control (N 0 g/ (m2·a), CK), low nitrogen (N 10 g/ (m2·a), LN), medium nitrogen (N 20 g/ (m2·a), MN) and high nitrogen (N 25 g/ (m2·a), HN). Nylon mesh bag method and laboratory soaking method were used. The response of nutrient element residual rate, water holding capacity, water holding rate and water absorption rate of leaf and twig litters to nitrogen addition were explored. The results showed that: (1) With the continuation of decomposition time, carbon, nitrogen and phosphorus content of leaf and twig litters in two types of forests showed the trends of release, enrichment-release and enrichment, respectively, and the residue rates of C, N and P in leaf litters were significantly lower than those in twig litters (p < 0.05). (2) The maximum water holding capacity and water absorption rate of leaf litters were significantly higher than that of twig litters (p < 0.05). After 24 months of decomposition, compared with CK, the C residual rates of leaf and twig litters in two types of forests, and the N residual rate of leaf litters in P. armandii forest under LN treatment were reduced by 1.98%~7.27% and 10.79%, respectively. The C residual rates of leaf and twig litters in two types of forests, and the N residual rate of leaf and twig litters in P. armandii forest and twig litters in P. yunnanensis forest, the P residual rate of twig litters in P. armandii forest under HN treatment increased by 4.26%~9.08%, 11.94%~44.51% and 42.42%, respectively. (3) After 24 months of decomposition, compared with CK, the maximum water holding capacity and water absorption rate of leaf and twig litters in P. armandii forest and leaf litters in P. yunnanensis forest under LN, MN and HN treatments decreased by 11.44%~25.24% and 5.81%~32.23%, respectively; while twig litters in P. yunnanensis forest increased by 15.48%~24.26%, 17.97%~23.74%, respectively. (4) The water capacity of leaf and twig litters in two types of forests increased with the extension of soaking time, but the water absorption rate decreased. The relationship between water capacity and soaking time was logarithmic function (m=a+bln t), and the relationship between water absorption rate and soaking time was power function (v=at-b). (5) C content was positively correlated with the water holding capacity of twig litters in P. yunnanensis forest (p < 0.05), N content was negatively correlated with the water holding capacity of leaf litters in two types of forests (p < 0.05). In conclusion, nitrogen addition affects the water holding capacity of litters by changing the residual characteristics of C, N and P nutrient elements during the decomposition process.