Impacts of simulated nitrogen deposition on soil enzyme activity in a northern temperate forest ecosystem depend on the form and level of added nitrogen

Abstract

Extracellular enzymes in the soil are pivotal to nutrient cycling in the ecosystem. Simulated nitrogen (N) deposition has resulted in positive, negative, and neutral effects of added N on soil extracellular enzyme activity (EEA).A field experiment involving two levels (50 and 150 kg ha−1 yr−1) and three forms (NaNO3, NH4NO3, (NH4)2SO4) of added N was conducted in a northern temperate forest. Over 8 years beginning in 2011, we measured the extracellular enzyme activity of six soil-based enzymes involved in C- (β-glucosidase, BG; polyphenol oxidase, PPO; catalase, CAT), N- (urease, UR) and P- (acid phosphatase, ACP; alkaline phosphatase, AKP) cycling during the growing season (March to September).High-level N addition produced a more obvious effect in promoting the activity of C-cycling enzymes and ACP, while the activities of UR and AKP under high-level N addition decreased significantly at rates of 10.0 and 26.8%, respectively. NH4+-N had a stronger effect on soil EEAs than NO3−-N, except UR activity. Furthermore, N addition induced a time-lag effect on the activity of BG, PPO, and ACP. BG activity achieved a maximum value in 2016 after increasing at an average rate of 21.2%. Furthermore, additional N produced nonlinear effects in the inter-annual variation of BG and CAT activity, which emphasized the importance of long-term monitoring.The effects of N addition on soil EEAs depended on the forms and levels of N added. The stimulatory effects of N addition on the activity of C-cycling enzymes may lead to instability in the soil C pool.