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 respond to the direction and intensity of soil nitrogen conversion. The study of the enzymatic reaction process of soil urease after the application of different forms of organic nitrogen fertilizers has a positive significance for the maintenance of nitrogen supply in farmland soils. A 60-day 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 using formulae, and to analyze their relationship with soil carbon and nitrogen fractions. The results showed that nitrogen fertilizers significantly affected both soil urease kinetic and thermodynamic parameters; whereas warming and its interaction with nitrogen fertilizers only significantly affected soil URE activity, soil urease kinetic parameters, and thermodynamic parameter ΔG. 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℃. At each temperature, the Michaelis constant (K_m) of soil urease increased significantly after UR application, no significant change was observed after BM application, and PM did not change regularly at different temperatures; the ratio of the maximum speed of urease reaction (V_max) to K_m (V_max/K_m ) of soil urease remained unchanged or decreased significantly after the three nitrogen fertilizers were applied; except for urea at 35℃, pig manure at 35℃ and 45℃, and P. brevitarsis manure at 5℃ and 45℃. Soil urease V_max, activation energy of reaction (E_a) and thermodynamic parameter ΔH were significantly decreased and thermodynamic parameter ΔS was significantly increased by UR and BM, while no significant change was observed in PM. Statistical analysis showed a correlation between soil URE activity, its kinetic and thermodynamic parameters, and soil carbon and nitrogen fractions after nitrogen fertilizer application. The kinetic and thermodynamic parameters of urease were the main factors affecting 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 could be used to indicate the changes in soil nitrogen after nitrogen fertilizer 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 spontaneity of the reaction, which is more conducive to the conversion of soil nitrogen.