Effects of Simulated Nitrogen Deposition on Soil Nutrients, Microbial Biomass and Enzyme Activities in Abies georgei var. smithii Forest

Nitrogen deposition has emerged as a significant threat to alpine forest ecosystems on the Qinghai–Xizang Plateau. However, the specific responses of subalpine fir soil to increasing nitrogen input remain relatively unknown. To investigate the effects of simulated nitrogen (N) deposition on the soil ecosystem of Abies georgei var. smithii forests in the Sejila Mountains of southeastern Xizang, China, this study established four treatments: control (CK), low nitrogen (LN, 10 kg ha−1 yr−1), medium nitrogen (MN, 15 kg ha−1 yr−1), and high nitrogen (HN, 20 kg ha−1 yr−1). A two-year field experiment was conducted to examine the responses of soil physicochemical properties, microbial biomass, and enzyme activities to nitrogen addition. The results showed that nitrogen addition significantly decreased soil pH, with reductions of 6.1%, 9.4%, and 9.8% under LN, MN, and HN treatments, respectively, compared with CK. Under MN treatment, soil organic carbon (SOC) and readily oxidizable organic carbon (ROC) increased by 14.7% and 89.3%, respectively, compared with CK. Available phosphorus (AP) and available potassium (AK) reached their highest concentrations under the MN treatment. Total nitrogen (TN) and ammonium nitrogen (NH4+-N) contents increased significantly, whereas exchangeable calcium (EX-Ca) and magnesium (EX-Mg) contents decreased significantly under N addition treatments. Microbial biomass carbon (MBC) increased by 58.7% under LN treatment, whereas microbial biomass nitrogen (MBN) decreased significantly under all nitrogen addition treatments. Soil enzyme activities showed divergent responses to nitrogen addition: cellulase activity increased most strongly under HN treatment; amylase and urease activities were significantly enhanced under LN and MN treatments; and β-glucosidase and polyphenol oxidase activities increased significantly under all nitrogen addition treatments. These results indicate that nitrogen addition induces differentiated responses in alpine forest soils. MN input may temporarily enhance soil nutrient availability, but it is accompanied by potential risks of soil acidification and microbial inhibition, whereas high nitrogen input may intensify base cation loss and disturb soil ecological functions. These findings provide a scientific basis for the conservation of alpine forest ecosystems and the management of nitrogen deposition on the Qinghai–Xizang Plateau.