Interactive Effects of Amendment Materials and Soil Salinity on Net Rates of Urea Hydrolysis and Nitrification in Salt-Affected Soil

Soil salinization greatly alters nitrogen cycling and causes many environmental problems, such as non-point source pollution and greenhouse gas emission. However, the potential influence of amendment material on the nitrogen transformation characteristics in the saline soil remains unclear. Using the potassium humate (PH)-amended (application rate 1.5 t hm−2) and biochar (BC)-amended (application rate 22.5 t hm−2) soils at four salinity levels (EC1:5 of 0.24 mS cm−1, 1.12 mS cm−1, 1.98 mS cm−1, and 2.79 mS cm−1), an incubation experiment was conducted to investigate the net rates and kinetic parameters of urea hydrolysis and nitrification, which occurred successively after the urea application. Results indicated that PH and BC additions and soil salinity shifted the temporal dynamics of NH4+-N and NO3−-N concentrations and prolonged the duration of hydrolysis and nitrification. PH and BC additions and soil salinity showed significant direct negative influences on the net hydrolysis and nitrification rates, and significant indirect negative influences through enhancing soil salinity and NH3 volatilization. The fitted kinetic parameters (including hydrolysis rate constant, potential nitrification rate, and nitrification potential) showed distinct (p < 0.01) responses to BC and PH additions and soil salinity. PH and BC additions aggravated the inhibitory impact of soil salinity on hydrolysis and nitrification, and the inhibition on hydrolysis was stronger than that on nitrification. The present study revealed the interactions of amendment materials and soil salinity on the key transformation processes of urea, which were promising for developing appropriate irrigation and fertilization practices to synergistically minimize salinization harm and nitrogen loss.