Short-term nitrogen fluxes in grassland soils under different long-term nitrogen management regimes
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文摘
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. 15N-labelled NH4+ 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 15N in soil inorganic N fractions, microbial biomass and plant shoots and roots. Gross N mineralization, gross N immobilization and N2 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.