Abstract: With a pot experiment, we compared the effects of N addition (0, 2.8, 5.6, 11.2, 22.4 and 44.8 g N·m^-2·a^-1) on soil enzyme activities (invertase, urease and alkaline phosphatase) and microbial biomass beneath Pinus tabuliformis seedlings after 6 years consecutive treatment. The results showed that soil invertase and urease activities were improved when the N addition was below 11.2 g N·m^-2·a^-1, while soil invertase, urease and alkaline phosphatase activities did not increase, even declined with a higher rate of N addition, especially more than 22.4 g N·m^-2·a^-1. The microbial biomass C significantly increased at N addition ≥5.6 g N·m^-2·a^-1, while microbial biomass N significantly increased at N addition ≥11.2 g N·m^-2·a^-1, compared with the control. Significant correlations were found between soil invertase and urease activities and microbial biomass C and microbial biomass N. No significant correlations, however, were found among the three enzyme activities, or between the microbial biomass C and N. Soil enzyme activities and microbial biomass were significantly correlated with aboveground biomass, soil organic carbon and total nitrogen content. Additionally, the growth of P. tabuliformis seedlings reached the maximum at the N addition levels of 5.6-11.2 g N·m^-2·a^-1. The current N deposition level of 2.06 g N·m^-2·a^-1 on the Loess Plateau can be favorable to P. tabuliformis growth, and the growth of P. tabuliformis would be improved through appropriately enhanced N fertilization.

Key words: soybean., derivative spectrum, unmanned aerial vehicle, high definition digital image, nitrogen balance index, hyperspectral data, vegetation index