Short-term immobilization of ammonium and nitrate added to a grassland soil

Short-term immobilization of added (NH4+)-N-15-N and (15) NO3-N (with or without added glucose) was determined at time intervals in a preconditioned grassland soil over laboratory incubation periods of up to 15 days at 25 C. Exchangeable N was removed from soil sub-samples by repeated 2 m KCl extraction and centrifugation. The solid residues were immediately blast frozen, freeze-dried and analysed for total N and N isotope ratio. Sub-samples without glucose amendment showed very little immobilization of added N (5 and <2% of added NH4+-N and NO3-N respectively after 15 days). In contrast, when glucose was added, 16% of added NH4+ -N and 8% of added NO3--N appeared to be immobilized, and most immobilization occurred within about 3 days of N application. This preliminary experiment indicates that this simple method could be used routinely to compare the N immobilization potentials of different soils. (C) 2001 Elsevier Science Ltd. All rights reserved.