Mixed Inorganic and Organic Nitrogen Addition Enhanced Extracellular Enzymatic Activities in a Subtropical Forest Soil in East China

To date, numerous studies have employed single type nitrogen (N) addition methods in reporting influences of N deposition on soil extracellular enzymatic activities (EEA) during litter decomposition in forest ecosystems. As natural atmospheric N deposition is a set of complex compounds including inorganic N and organic N, it is essential for investigating responses of soil EEA to various mixed N fertilization. In a subtropical forest stand in Zijin Mountain, East China, various N fertilizers with different inorganic N and organic N ratios were added to soils monthly from 2008 to 2009. Samples were harvested from N fertilized and control plots every 4 months. Subsequently, six EEA were assayed. A laboratory experiment was also conducted simultaneously. Both field and laboratory experiments showed that various mixed N fertilizations revealed different influences on soil EEA. Acceleration of most soil EEA by mixed N fertilization was greater than that of single N fertilization. The majority of soil extracellular enzymes exhibited the highest activities under mixed N fertilization, with the ratio of inorganic N to organic N at 3:7. These results suggested that N type and ratio of inorganic N and organic N were important factors controlling soil EEA, and the 3:7 ratio of inorganic N and organic N may be the optimum for soil EEA.