Soil Oxidative Enzyme Activity Exhibits Clear Seasonal Variations across a Simulated Gradient of Nitrogen Deposition in the Gurbantunggut Desert

In recent years, Nitrogen (N) deposition has become one of the most heated global issues, and an increasing number of studies have focused on soil microbial activity and its function in soil nutrient transformation.

Desert soil usually has high oxidative enzyme activity and suffers from influences of increasing N deposition. The high oxidative enzyme activity in desert soil may increase the decomposition potential and limit organic matter accumulation. In addition, the N deposition in recent years has been high in some desert areas. It could negatively or positively affect microbial activity, increase dominance of alien annuals and decrease the diversity of native annual plants. Therefore, the ways by which oxidative enzymes in desert soil respond to N addition should be examined in terms of their essential role in nutrient transformation. Furthermore, the exploration of how oxidative enzymes respond to N addition is critical to evaluate the carbon transformation in desert ecosystems. The Gurbantunggut Desert is an optimal area for studying oxidative enzyme activity in response to N addition.

From 2009 to 2010, Prof. ZHANG Yuanming and his research team conducted a continuous field experiment and laboratory analysis to determine the seasonal responses of oxidative enzyme activity to N addition and their major regulating factors, analyze the effects of different N application rates on oxidative enzyme activity in desert soil, and evaluate the effects of N addition on oxidative activity between the topsoil (0–5 cm) and the subsoil (5–10 cm) of the Gurbantunggut Desert. Two oxidative enzymes (polyphenol oxidase (PPO) and peroxidase) and six N treatments were used in this study.

The results showed that 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 (0.5–3 g N m^-2 yr^-1), but the activity decreased in response to high N addition (6–24 g N m^-2 yr^-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. The study was published in Geoderma in January 2014.