Urease activity of microbial biomass in soils as affected by cropping systems

The impacts of crop rotations and N fertilization on different pools of urease activity were studied in soils of two long-term field experiments in Iowa; at the Northeast Research Center (NERC) and the Clarion-Webster Research Center (CWRC). Surface soil samples (0-15 cm) were taken in 1996 and 1997 in corn, soybeans, oats, or meadow (alfalfa) plots that received 0 or 180 kg N ha-1, applied as urea before corn and an annual application of 20 kg P and 56 kg K ha-1. The urease activity in the soils was assayed at optimal pH (THAM buffer, pH 9.0), with and without toluene treatment, in a chloroform-fumigated sample and its nonfumigated counterpart. The microbial biomass C (C(mic)) and N (N(mic)) were determined by chloroform fumigation methods. The total, intracellular, extracellular and specific urease activities in the soils of the NERC site were significantly affected by crop rotation, but not by N fertilization. Generally, the highest total urease activities were obtained in soils under 4-year oats-meadow rotations and the lowest under continuous corn. The higher total activities under multicropping systems were caused by a higher activity of both the intracellular and extracellular urease fractions. In contrast, the highest values for the specific urease activity, i.e. of urease activity of the microbial biomass, were found in soils under continuous soybean and the least under the 4-year rotations. Total and extracellular urease activities were significantly correlated with C(mic) (r > 0.30* and > 0.40**) and N(mic) (r > 0.39** and > 0.44**) in soils of the NERC and CWRC sites, respectively. Total urease activity was significantly correlated with the intracellular activity (r > 0.73***). About 46% of the total urease activity of the soils was associated with the microbial biomass, and 54% was extracellular in nature.