Soil microbial biomass, phosphatase and their relationships with phosphorus turnover under mixed inorganic and organic nitrogen addition in a Larix gmelinii plantation

Abstract

Atmospheric nitrogen (N) deposition can affect soil microbial biomass, phosphatase activity, and the concentrations of organic phosphorus (P), inorganic P and available P, but the changes in soil microbial biomass under mixed inorganic and organic N addition and their relationships with soil P turnover in forest ecosystems are not fully understood. In this study, a simulated N deposition experiment was conducted in a Larix gmelinii plantation in Northeastern China to investigate the variation of soil microbial biomass, phosphatase activity, P forms and available P concentrations as well as the relationships between these features. The experiment was arranged in a randomized block design with three replications for each treatment, including the control (no N addition, CK), total inorganic N (NH4NO3, TIN), mixed inorganic and organic N (inorganic N and organic N in the ratio of 7:3, ION), and total organic N (urea and glycine 1:1, TON). The results indicated that there was a significant difference in the response of soil biochemical characteristics to different types of N addition. Soil total microbial biomass, the biomass of bacteria and actinomycetes, alkaline phosphomonoesterase (AlP) activity, as well as oxalate-extracted aluminum (Al), cMonoesters, cDiesters and orthophosphate concentrations were significantly increased by the addition of mixed inorganic and organic N, whereas they were not affected by single inorganic or organic N addition. Compared to the CK treatment, the increase in soil microbial biomass and Al oxides concentrations under ION treatment significantly increased soil cMonoesters and cDiesters concentrations, and the increased bacterial biomass contributed to the increase of soil AlP activity. Moreover, the ION treatment also significantly increased soil total P concentration and enhanced microbial immobilization of P compared to the CK treatment. However, there was no significant difference in available P concentration between the ION treatment and other treatments, which could be attributed to increased orthophosphate concentration under the ION treatment. Overall, the findings suggest that bacteria and AlP can play important roles in maintaining soil P availability by accelerating P turnover under conditions of increased atmospheric N deposition in a Larix gmelinii plantation.