Inhibition of extracellular enzyme activities in a forest soil by additions of inorganic nitrogen

Forest ecosystems are known to be nitrogen (N) limited, and productivity often increases with N additions. However, recent studies have suggested that additions of excessive N through atmospheric deposition may affect soil processes adversely. In this study, we conducted a laboratory-based manipulation experiment to reveal the effects of N additions on extracellular enzyme activities in a temperate forest soil. Three different levels of N additions (water-only control, low N addition, and high N addition) were applied to soil columns (2.5-cm diameter x 12 cm height) over 4 weeks. Overall, the additions of N decreased specific activities (enzyme activity per log bacterial cell number) of urease, glutaminase, phosphatase, and arylsulfatase significantly. However, dehydrogenase activity was increased with the high N addition. The results suggest that N deposition may impede nutrient cycling, particularly mineralization of organic nutrients. In addition, this enzyme inhibition may be one of the mechanisms for N retention observed in forest floors.