Response of soil-microbe-extracellular enzyme stoichiometric characteristics to nitrogen deposition in a forest in central Yunnan Province, Southwest China.

To understand the effects of nitrogen deposition on element cycling and nutrient limitation status in forest ecosystems, we examined the effects of nitrogen deposition on the stoichiometric characteristics of forest soil-microbial-extracellular enzymes in forest. We conducted a field experiment with control (CK, 0 g N·m·a), low nitrogen (LN, 10 g N·m·a), medium nitrogen (MN, 20 g N·m·a) and high nitrogen (HN, 25 g N·m·a) since 2019. We collected soil samples (0-5 cm, 5-10 cm and 10-20 cm) at September 2022, and measured the contents of soil organic, total nitrogen, total phosphorus, microbial biomass carbon, nitrogen and phosphorus (MBC, MBN, MBP) and the activities of C, N, and P acquisition enzymes. The results showed that nitrogen deposition significantly reduced soil organic content, C:N and C:P by 6.9%-29.8%, 7.6%-45.2% and 6.5%-28.6%, and increased soil total N content and N:P by 10.0%-45.0% and 19.0%-46.0%, respectively. Nitrogen addition did not affect soil total P content. Except for soil C:N and C:P, soil nutrient content and stoichiometric ratio were highest in 0-5 cm soil layer. MN and HN treatments significantly decreased MBN by 11.0%-12.7%. MBC, MBP, and their stoichiometry did not change significantly under nitrogen deposition. Soil microbial nutrient content in 0-5 cm soil layer was significantly higher than that in other soil layers. Nitrogen deposition significantly decreased the activities of cellobiose hydrolase and leucine aminopeptidase (decreased by 14.5%-16.2% and 48.7%-66.3%). HN treatment promoted β-1,4-glucosidase activity (increased by 68.0%), but inhibited soil enzyme stoichiometric carbon to nitrogen ratio and nitrogen to phosphorus ratio (decreased by 95.4% and 88.4%). LN and MN treatment promoted β-1,4-N-acetylglucosaminidase activity (increased by 68.3%-116.6%), but inhibited enzyme stoichiometric carbon to phosphorus ratio (decreased by 14.9%-29.4%). Alkaline phosphatase activity had no significant change. Soil enzyme activities were significantly decreased with increasing soil depth. Soil total N and total P and microbial nutrients were negatively correlated with vector angle (representing microbial nitrogen or phosphorus limitation), while vector length (representing microbial carbon limitation) was consistently significantly positively correlated with vector angle, suggesting the synergistic promotion between microbial carbon limitation and phosphorus limitation. Nitrogen deposition gradually shifted to phosphorus limitation while alleviating microbial nitrogen limitation in forest. In addition, microbial activities in this region was limited by C availability, and the relationship between microbial C and P limitation was proportional.