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Faculty for Biology, Chemistry and Earth Sciences

Department Soil Ecology - Prof. Dr. Eva Lehndorff

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Fuhrmann, I; He, Y; Lehndorff, E; Brüggemann, N; Amelung, W; Wassmann, R; Siemens, J: Nitrogen fertilizer fate after introducing maize and upland-rice into continuous paddy rice cropping systems, Agriculture, Ecosystems & Environment, 258, 162-171 (2018), doi:10.1016/j.agee.2018.02.021
Water scarcity and economic incentives favor the introduction of upland crops into permanent paddy rice sys-tems during dry seasons. However, introducing upland crops into permanently flooded cropping systems tem-porarily changes soil conditions from anaerobic to aerobic, affecting nitrogen (N) dynamics profoundly. Wehypothesized that under maize and dry rice, total fertilizer15N recovery in soil as well as the immobilization offertilizer15N in microbial residues is reduced compared with continuous paddy rice cropping. Furthermore, weexpected enhanced emissions of fertilizer15N in form of nitrous oxide (N2O) under maize and dry rice. To testthese hypotheses, we traced the fate of a15N-urea pulse in afield experiment in the Philippines with threedifferent crop rotations: continuous paddy rice, paddy rice–dry rice, and paddy rice–maize for two years.Indeed, the15N recovery in thefirst 5 cm of bulk soil was lowest in the paddy rice–maize rotation (arithmeticmean with standard error: 19.2 ± 1.8% of applied15N), while twice as much was recovered in thefirst 5 cm ofbulk soil of the continuous paddy rice cropping systems (37.8 ± 2.2% of applied15N) during thefirst dryseason. The15N recovery in the plant biomass (shoots and roots) in the continuous paddy rice cropping was 13%larger than in the dry rice plant biomass and 5% larger than in the maize plant biomass during thefirst dryseason. Fertilizer15N remained longest in paddy rice–maize (mean residence time = 90 ± 25 days) and incontinuous paddy rice (mean residence time = 77 ± 30 days), compared with dry rice–paddy rice rotation(mean residence time = 16 ± 5 days). After 2 years, 10% (paddy rice–dry rice, paddy rice–maize) to 23%(continuous paddy rice) of the applied fertilizer15N were still stored in soil. The largest fraction of this15N wasimmobilized by soil microbes, which stored 3–4% of applied15N in the form of amino sugars as specific cell wallconstituents, in all cropping systems. Nevertheless, introducing upland crops into continuous paddy rice systemslikely increased N leaching losses and resulted in initial losses of urea-15NtoN2O, which thus has to be con-sidered in climate smart mitigation strategies.
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