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Vienna, Austria

Inselsbacher E.,Faculty Center for Ecology | Inselsbacher E.,Federal Research and Training Center for Forests | Hinko-Najera Umana N.,Faculty Center for Ecology | Stange F.C.,Helmholtz Center for Environmental Research | And 7 more authors.
Soil Biology and Biochemistry

Agricultural systems that receive high amounts of inorganic nitrogen (N) fertilizer in the form of either ammonium (NH4 +), nitrate (NO3 -) or a combination thereof are expected to differ in soil N transformation rates and fates of NH4 + and NO3 -. Using 15N 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 NH4 +, NO3 - or NH4NO3. Within each fertilizer treatment traces of 15NH4 + and 15NO3 - were added separately. During 8 days of fertilization the fate of fertilizer 15N 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 15N 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 NO3 - than NH4 + independent of initially applied N form. Surprisingly, no inhibitory effect of NH4 + 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 NH4 + uptake rates, gross nitrification rates were 3-75-fold higher, indicating that nitrifiers were the strongest competitors for NH4 + in both soils. The rapid conversion of NH4 + to NO3 - and preferential use of NO3 - by soil microbes suggest that in agricultural systems with high inorganic N fertilizer inputs the soil microbial community could adapt to high concentrations of NO3 - and shift towards enhanced reliance on NO3 - for their N supply. © 2009 Elsevier Ltd. All rights reserved. Source

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