Short-term competition between crop plants and soil microbes for inorganic N fertilizer

摘要

Agricultural systems that receive high amounts of inorganic nitrogen (N) fertilizer in the form of either ammonium (NH sub(4) super(+)), nitrate (NO sub(3) super(-)) or a combination thereof are expected to differ in soil N transformation rates and fates of NH sub(4) super(+) and NO sub(3) super(-). Using super(15)N tracer techniques this study examines how crop plants and soil microbes vary in their ability to take up and compete for fertilizer N on a short time scale (hours to days). Single plants of barley (Hordeum vulgare L. cv. Morex) were grown on two agricultural soils in microcosms which received either NH sub(4) super(+), NO sub(3) super(-) or NH sub(4)NO sub(3). Within each fertilizer treatment traces of super(15)NH sub(4) super(+) and super(15)NO sub(3) super(-) were added separately. During 8 days of fertilization the fate of fertilizer super(15)N into plants, microbial biomass and inorganic soil N pools as well as changes in gross N transformation rates were investigated. One week after fertilization 45-80% of initially applied super(15)N was recovered in crop plants compared to only 1-10% in soil microbes, proving that plants were the strongest competitors for fertilizer N. In terms of N uptake soil microbes out-competed plants only during the first 4 h of N application independent of soil and fertilizer N form. Within one day microbial N uptake declined substantially, probably due to carbon limitation. In both soils, plants and soil microbes took up more NO sub(3) super(-) than NH sub(4) super(+) independent of initially applied N form. Surprisingly, no inhibitory effect of NH sub(4) super(+) on the uptake and assimilation of nitrate in both, plants and microbes, was observed, probably because fast nitrification rates led to a swift depletion of the ammonium pool. Compared to plant and microbial NH sub(4) super(+) uptake rates, gross nitrification rates were 3-75-fold higher, indicating that nitrifiers were the strongest competitors for NH sub(4) super(+) in both soils. The rapid conversion of NH sub(4) super(+) to NO sub(3) super(-) and preferential use of NO sub(3) super(-) by soil microbes suggest that in agricultural systems with high inorganic N fertilizer inputs the soil microbial community could adapt to high concentrations of NO sub(3) super(-) and shift towards enhanced reliance on NO sub(3) super(-) for their N supply.