Contrasting responses of phosphatase kinetic parameters to nitrogen and phosphorus additions in forest soils

Global changes include increasing nitrogen (N) and phosphorus (P) deposition, which affect microbial nutrient demand and biogeochemical cycles. The responses of P-mineralizing enzymes to these global change components are poorly defined and are not specified in forest soils differing in P content. We chose one site in a P-rich and two sites in P-poor forests and established sixteen 20x20m plots at each site. Control, either N only, P only, or combined N and P, were randomly distributed through each forest site with four replicates. We investigated the effects of N and P additions over 4 years on the phosphomonoesterase potential activity (V-max), its half-saturation constant (K-m) and its catalytic efficiency (V-max/K-m). Without N and P additions, the enzyme kinetic parameters V-max, K-m and V-max/K-m were higher in P-rich than in P-poor forest soils. These parameters increased with soil pH, SOC, TN and TP contents increased. Remarkably, P additions caused the V-max and K-m to increase in P-rich soils, but had no effect on V-max/K-m. P additions to P-poor soils resulted in a decrease in the V-max/K-m via the inhibitory effects of inorganic P on the V-max. N additions had no effect on the V-max/K-m in P-rich and P-poor soils because of the similar increases in the V-max and K-m. The effects of combined N and P and P only additions to P-poor soils on the V-max and K-m were similar, but were stronger than the effects of N only or P only additions on the P-rich soils. Phosphatase kinetic parameters were positively related to the availability of N and P in P-rich soils, but inorganic P inhibited phosphatase activity and caused a decrease in the catalytic efficiency in P-poor soils. More microbial community groups could contribute to the secretion of a broader spectrum of iso-enzymes under combined additions of N and P in P-rich soils. We conclude contrast responses of phosphatase kinetics to P and N inputs in P-rich and P-poor forest soils, while long-term N deposition might mitigate P limitation by increasing phosphatase secretion. A is available for this article.