Short-term nitrogen fluxes in grassland soils under different long-term nitrogen management regimes

Short-term N fluxes were measured during Spring in perennial grasslands in S.W. England to examine processes controlling plant N availability. Three long-term management regimes were used, being grass plus white 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 enrichment calculations. Plant uptake was the dominant process of removal of inorganic N from soil, with short-term rates being similar for all three treatments. Rapid and large N fluxes through plant uptake and mineralization-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 intermediate between the 0 and 200 N treatments. Thus, treatment differences in N fluxes reflected differences in their long-term N inputs. Measurements of net N mineralization using several techniques showed a 2-fold difference between the 0 and 200 N treatments, with the clover treatment being intermediate. However, isotope dilution calculations revealed that rates of gross N mineralization were 3-12 times larger than net N mineralization rates, and that treatments differed in gross N mineralization rate by only 35% (3.0-4.1 kg N ha(-1) d(-1)). An important 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 Science Ltd. All rights reserved.