Soil urease activity in winter wheat residue management systems

Surface residue accumulation under reduced and no-till cropping systems has been shown to cause changes in the activities of several soil enzymes. The addition of crop residues containing urease increases the rate of urea hydrolysis which can increase the potential for ammonia volatilization. Our objective was to characterize the distribution of urease activity in the surface of two Oklahoma soils (Grant silt loam, fine, silty, mixed, thermic Udic Argiustoll and Norge clay loam, fine, silty, mixed, thermic Udic Paleustoll) which had been in conventional, reduced, and no-till wheat (Triticum aestivum L.) residue management for four consecutive years. Soil cores 60-mm deep were taken from residue management plots that had been previously fertilized with 168 kg urea-N ha-1 yr-1 and separated into 10-mm increments. Urease activity was determined in the soil increments and in mature undecomposed wheat straw. Urease activity was uniformly distributed in soil to a depth of 60 mm under conventional tillage. Significant increases in urease activity were measured in the surface 10 mm of both soils where accumulation of residue had occurred. Average urease activities in the surface 10 mm of soil under conventional, reduced, and no-till residue management systems were, 12.1, 41.4, and 45.2 microgram NH4-N g-1 h-1 for Norge clay loam and 9.7, 12.4, and 35.0 microgram NH4-N g-1 h-1 for Grant silt loam, respectively. Activity of urease in mature undecomposed wheat straw grown at the site was 376 microgram NH4-N g-1 h-1 which was 28 X larger than the average activity found in soil. Wheat straw was shown to be a major source of the observed increase in enzymatic activity. Other sources for increased urease activity include increased microbial colonization during decomposition of residue and increased urea concentration in microsites due to fertilization, particularly in combination with residue.