A novel 15N tracer model reveals: Plant nitrate uptake governs nitrogen transformation rates in agricultural soils

One major challenge in agriculture is improving the nitrogen (N) use efficiency of crop plants and at the same time reducing the losses of fertilizer N to the environment. The use of 15N tracer studies in combination with process-based models has been proven to be a powerful tool for increasing our understanding of the dynamic interactions between soil, microbes and plants. Here we present a novel approach that includes plant uptake of fertilizer NH4+ and NO3−. We developed, evaluated and applied an analytical model allowing the simultaneous estimation of 14 processes within the N cycle using results from a previously published 15N tracer study (Inselsbacher, E., Hinko-Najera Umana, N., Stange, F.C., Gorfer, M., Schüller, E., Ripka, K., Zechmeister-Boltenstern, S., Hood-Novotny, R., Strauss, J., Wanek, W., 2010. Short-term competition between crop plants and soil microbes for inorganic N fertilizer. Soil Biology & Biochemistry 42, 360–372]. The model revealed that plant NO3− uptake governed the overall N cycle during the 8-days greenhouse study. Nitrification was the main fate of NH4+ but its kinetics differed significantly between soils. The model-based calculations proved to be a major advancement compared to the commonly used calculations based on the pool dilution technique, due to the number of estimated parameters, their respective kinetic shifts over prolonged time periods and their explanatory power. In future 15N tracer studies this analytical tool will allow accounting for the effect of plant N uptake on soil N transformations.