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Short-term competition between crop plants and soil microbes for inorganic N fertilizer |
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| Authors: | Erich Inselsbacher Nina Hinko-Najera Umana Markus Gorfer Katrin Ripka Rebecca Hood-Novotny Wolfgang Wanek |
| Institution: | a Department of Chemical Ecology and Ecosystem Research, Faculty Centre for Ecology, Faculty of Life Sciences, University of Vienna, Althanstraβe 14, 1090 Vienna, Austria b Department of Forest Ecology and Soils, Unit of Soil Biology, Federal Research and Training Centre for Forests, Natural Hazards and Landscape (BFW), Vienna, Austria c UFZ Department Soil Physics, Helmholtz Centre for Environmental Research-UFZ, Halle, Germany d Institute for Applied Genetics and Cell Biology, Fungal Genomics Unit, Austrian Institute of Technology and BOKU University Vienna, Vienna, Austria e Austrian Institute of Technology, Department of Health and Environment, A-2444 Seibersdorf, Austria |
| Abstract: | 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. |
| Keywords: | Agriculture Ammonium monooxygenase Archaea Bacteria Fungi Gross N transformations Inorganic N fertilizer qPCR 15N tracer |
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