Influence of Crop Residues and Organic Manures on the Hydrolysis of Urea in a Typic Haplustept

Abstract

The emphasis on integrated use of organic amendments and fertilizers has made it imperative to study the effect of crop residues and organic manures on urea hydrolysis, which governs the release of N from urea fertilizer. Urea hydrolysis in soils is an enzymatic decomposition process facilitated by the extracellular enzyme urease. Our aim was to assess the influence of crop residues (rice straw, wheat straw) and organic manures (poultry manure, farm yard manure, cowpea, and sesbania) on urea hydrolysis by conducting a laboratory incubation experiment employing a Hasanpur silt loam soil (coarse loamy, mixed, hypothermic Typic Haplustept). The study was conducted at a constant temperature of 33 ± 1°C at two moisture levels (1:1 soil:water ratio and field capacity). The results showed that the rate of urea hydrolysis decreased with an increase in time of incubation (12 to 96 h), while the disappearance of urea N was associated with the corresponding increase in ( + )-N content in soils treated with crop residues and organic manures. In the nonamended soil, the time required for the complete hydrolysis of the applied urea (200 μg urea N g−1 soil) was more than 96 h at both the moisture levels, whereas in amended soils, 60 to 72 h of incubation periods were required for urea hydrolysis. The rate of urea hydrolysis in the amended soil was of the following order: sesbania > cowpea > poultry manure > farmyard manure > rice straw and wheat straw, irrespective of the moisture regime. Urea hydrolysis followed first-order kinetics in all the treatments at both moisture levels. The rate constant value for urease activity was higher at field capacity, indicating that urea hydrolysis proceeded rapidly under the condition of field capacity rather than saturated conditions (1:1 soil:water ratio).