Temporal dynamics of soil oxidative enzyme activity across a simulated gradient of nitrogen deposition in the Gurbantunggut Desert, Northwestern China

Abstract

Increasing nitrogen (N) deposition has been reported to result in positive, negative, and neutral impacts on soil oxidative enzyme activities, which are associated with lignin degradation. The oxidative enzyme activities are usually high and stable in desert ecosystems. Therefore, the exploration of how these enzymes respond to N addition is critical to evaluate the carbon transformation in desert ecosystems. We present the seasonal variations in oxidative enzyme activity and their responses to simulated N deposition in the topsoil (0–5cm) and in subsoil (5–10cm) of the Gurbantunggut Desert in Northwestern China. Polyphenol oxidase (PPO) and peroxidase activity exhibited clear seasonal variations over the 2-year study, with relatively higher values in spring than in the other seasons. The seasonal changes in oxidative enzyme activity were mainly driven by the seasonal patterns in soil moisture and temperature. Shifts in pH, organic carbon, and electrical conductivity were not closely correlated with the seasonal changes in oxidative enzyme activity. In general, PPO and peroxidase activity responded minimally to low N addition (0.5–3gNm−2yr−1), but the activity decreased in response to high N addition (6–24gNm−2yr−1). Although higher organic carbon concentrations were observed in the topsoil than in the subsoil, few significant differences in PPO and peroxidase activity and in their sensitivity to N addition were observed between the two layers. These results suggest that the carbon transformation related to oxidative enzyme activity is regulated by the seasonal patterns of soil moisture, soil temperature, and the N concentration in desert soil.