Effect of different factors on hydrolysis and nitrification of urea in soils

We studied the effect of soil type, water content, salinity and sodicity on hydrolysis and nitrification of urea in laboratory at constant temperature (28°C ± 1°C). Hydrolysis and nitrification of urea were studied separately in two experiments using six soils maintained at −0.03 MPa water content after addition of 150 mg urea-N kg−1 soil. In the second set of experiments, the urea hydrolysis and nitrification were studied in normal, saline (6 dS m−1) and sodic (30 ESP) soils each maintained at water matric potential of −0.03, −0.5 and −1.5 MPa. The nitrogen was applied at the rate of 150 mg N kg−1 soil in both the experiments. Soil and urea-N mixtures were incubated for 48 h for urea hydrolysis and up to 60 days for nitrification. The rate of urea hydrolysis in different soils followed the order: Uchani silty clay loam > Kaithal clay loam > Hisar loam > Kurukshetra clay loam > Sonepat loamy sand > Bawal loamy sand. The values of first-order rate constant (k) varied from 0.039 – 0.069 h−1. The rate of urea hydrolysis slowed down with the decrease in water content from −0.03 MPa to −1.5 MPa. The amount of urea hydrolyzed was higher in normal as compared to saline or sodic soil. At −1.5 MPa water content, 31.9 – 37.6% of the added urea remained unhydrolyzed at 48 h in sodic and saline soils, respectively. The content of NO3[sbnd]N formation differed markedly among texturally different soils being highest in Kurukshetra clay loam and lowest in Bawal loamy sand. The NO2[sbnd]N appeared during 5 – 30 d and most of the NO2[sbnd]N was detected between 10 and 20 d in different soils. Decreasing the water content from −0.03 to −1.5 MPa decreased the NO3[sbnd]N in normal, saline and sodic soils.