Significance of microbial urea turnover in N cycling of three Danish agricultural soils

Abstract

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-NH+<inf>4</inf> dilution technique and urea turnover determined by a new 14C-urea tracer technique. Average urea turnover rates were 1.5 to 4.2 μg N g−1 d−1 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 (<it>r</it>= 0.79, <it>P</it><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 NH+<inf>4</inf> pool. Our study suggests that microbial urea turnover, by-passing the conventional mineralization-immobilization pathway involving a free NH+<inf>4</inf> pool, has a significant role in N cycling of agricultural soils.