Response of the soil urease enzymatic reaction to the application of three organic nitrogen fertilizers

Soil urease (URE) is an important enzyme that catalyzes the conversion of soil nitrogen, and the kinetic and thermodynamic properties of its enzymatic reaction can respond to the direction and intensity of soil nitrogen conversion. The study of the enzymatic reaction process of soil urease after different forms of organic nitrogen fertilizers were applied has a positive significance for the maintenance of nitrogen supply in farmland soils. A 60−d wheat pot experiment was conducted to determine the changes in soil urease activity at different temperatures and substrate concentrations after the application of urea (UR), pig manure (PM) and Potosa brevitarsis manure (BM), to calculate enzyme kinetic and thermodynamic parameters by formulae, and to analyze their relationship with soil carbon and nitrogen fractions. The results showed that: Nitrogen fertilizers significantly (P<0.05) affected both soil urease kinetic and thermodynamic parameters; whereas warming and its interaction with nitrogen fertilizers only significantly (P<0.05) affected soil URE activity, soil urease kinetic parameters, and ΔG. Soil URE activity varied in the range of 9.62~234.26 mg·100g⁻¹·24h⁻¹ after the three nitrogen fertilizer treatments at different temperatures, the soil urease reaction properties showed significant increase in V_max, significant decrease in ΔG, and no significant change in V_max /K_m; among them, K_m was significantly increased by UR and PM, but did not change significantly by BM; and E_a, ΔH, and ΔS were significantly decreased by UR and BM, but did not change significantly by PM. The kinetic and thermodynamic parameters of soil urease in the three nitrogen fertilizer treatments increased significantly with increasing temperature, and peaked at 35 or 45℃. Statistical analysis showed that there was a certain correlation between soil URE activity, its kinetic and thermodynamic parameters and soil carbon and nitrogen fractions after nitrogen fertilizers application, and that the kinetic and thermodynamic parameters of urease were the main factors of soil nitrogen transformation. In summary, the effects of the three nitrogen fertilizer treatments on the enzymatic reaction of soil urease differed, and the parameters of the enzymatic reaction of soil urease can be used to indicate the changes of soil nitrogen after nitrogen fertilizers application; Compared with UR and PM, BM was able to maintain the binding capacity of soil urease and substrate, reduce the energy required for the reaction, and improve the order and spontaneity of the reaction, which was more conducive to the conversion of soil nitrogen.