Signi¢cance of microbial urea turnover in N cycling of three Danish agricultural soils

The importance of microbial urea turnover in N cycling was investigated in three agricultural soils by comparison of gross N mineralization determined by the 15N-NH4 dilution technique and urea turnover determined by a new 14C-urea tracer technique. Average urea turnover rates were 1.5 to 4.2 Wg N g31 d31 indicating that the soil urea pool was turned over every 9 to 30 min. Urea turnover rates were generally lowest in set-aside soil with increasing activities in bulk and rhizosphere soil from a barley field. Gross N mineralization and urea turnover rates were correlated (r = 0.79, P6 0.005) and of similar size in the three soils. The high urea turnover rates indicated that urea-N was immobilized directly in soil microorganisms, rather than mineralized to the free NH4 pool. Our study suggests that microbial urea turnover, by-passing the conventional mineralization-immobilization pathway involving a free NH4 pool, has a significant role in N cycling of agricultural soils. z 1998 Feder