SHORT-TERM NITROGEN FLUXES IN GRASSLAND SOILS UNDER DIFFERENT LONG-TERM NITROGEN MANAGEMENT REGIMES

Short-term N fluxes were measured during Spring in perennial grassland s in S.W. England to examine processes controlling plant N availabilit y. Three long-term management regimes were used, being grass plus whit e clover or grass receiving N fertilizer at 0 or 200 kg N ha(-1) y(-1) . N-15-labelled NH; solution was injected into microplot cores in the field and after 1 and 3 d the cores were removed for analysis of total N and N-15 in soil inorganic N fractions, microbial biomass and plant shoots and roots. Gross N mineralization, gross N immobilization and N-2 fixation were estimated using 15N isotope dilution or pool enrichm ent calculations. Plant uptake was the dominant process of removal of inorganic N from soil, with short-term rates being similar for all thr ee treatments. Rapid and large N fluxes through plant uptake and miner alization-immobilization reactions maintained a small soil inorganic N pool which turned over approximately daily. In the clover treatment, all fluxes of N (except N2 fixation at 2.4 kg N ha(-1) d(-1)) were int ermediate between the 0 and 200 N treatments. Thus, treatment differen ces in N fluxes reflected differences in their long-term N inputs. Mea surements of net N mineralization using several techniques showed a 2- fold difference between the 0 and 200 N treatments, with the clover tr eatment being intermediate. However, isotope dilution calculations rev ealed that rates of gross N mineralization were 3-12 times larger than net N mineralization rates, and that treatments differed in gross N m ineralization rate by only 35% (3.0-4.1 kg N ha(-1) d(-1)). An importa nt Factor influencing net N fluxes was the marked differences in gross N immobilization rate, which varied between 1.0 and 0.1 kg N ha(-1) d (-1) in the 0 and 200 N treatments, respectively. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.