Different forms and rates of nitrogen addition show variable effects on the soil hydrolytic enzyme activities in a meadow steppe

The effects of mixed inorganic and organic nitrogen (N) addition on soil enzyme activities and the underlying mechanism remain unclear, especially in complex field conditions. We conducted a mesocosm experiment with two rates of N addition (10 and 20 g N m(-2) year(-1)) and four ratios of N addition (inorganic N : organic N = 10 : 0, 7 : 3, 3 : 7 and 1 : 9) and measured enzyme activities, soil physicochemical properties, microbial biomass and vegetation indicators. Generally, soil enzyme activities involved in carbon (C), N and phosphorus cycling increased with the increase of N addition rate. Compared to the single inorganic N addition treatment, enzyme activities were highest under mixed N addition treatments, especially medium organic N addition. The variations in soil enzyme activities across different treatments were tightly linked to the soil microbial biomass C, dissolved organic C and soil pH. These findings provide a good understanding of the response trends of soil hydrolytic enzyme activities in a meadow steppe to changes in N deposition rate and form.