关键词: 农田生态系统, 土壤养分平衡, 循环再利用

Abstract: We carried out an experiment to understand the effects of nitrogen deposition on soil C∶N∶P stoichiometry, extracellular enzyme activities associated with C, N and P cycles in a Cupressus lusitanica Mill. plantation in Nanjing, Jiangsu Province, China. There were six levels of nitrogen addition, including 0, 24, 48, 72, 96, and 120 kg·hm^-2·a^-1, designated as N0, N1 N2, N3, N4, and N5 respectively. The results showed that: (1) The concentration of soil organic carbon (SOC) was significantly elevated with increased N and peaked in N4 treatment. The concentration of soil dissolved organic carbon (DOC) was significantly higher in N2 and N3 treatments than that in the control. The soil total nitrogen (TN) concentration and N∶P ratio were significantly higher in N2 treatment than those in the control. No significant differences were found in other treatments. Soil C∶N and C∶P were significantly higher in N4 and N5 treatments than those in the control. Nitrogen addition had no significant effects on soil total phosphorus concentration (TP). (2) Activities of soil urease, phosphatase (acid phosphatase, alkaline phosphatase, neutral phosphatase), polyphenol oxidase, cellulose, invertase, and catalase in low and middlelevel nitrogen treatments were higher than those in the control. In the high-level Naddition treatment, activities of soil urease and cellulose showed a decreasing trend, whereasactivities of phosphatase, lignin oxidase and invertase were significantly increased. (3) Results from both cluster analysis and factor analysis showed that it was reliable to regard the level of 72 kg·hm^-2·a^-1 (N3) as the optimal rate for the C. lusitanica plantation. This result would provide a reference for fertilization in the C. lusitanica plantation. Results from redundancy analysis showed that concentrations of TP, SOC, and DOC were significantly positively correlated with the activities of soil phosphatase, urease, polyphenol oxidase, and catalase. The results showed that the variation in soil enzyme activities greatly accounted for concentrations of TP, SOC, and DOC. (4) Factor analysis and redundancy analysis showed that dissolved organic carbon greatlyaccounted for the variation of soil enzyme activities. Our results provide a basis for assessing the effects of N deposition on soil nutrient cycling in the subtropical forests of China and a reference to accurately predicting regional soil nutrient balance under continued N deposition in the future.

Key words: Farmland ecosystem, Soil nutrient balance, Cycling and reutilization