The fate of 15N-labeled nitrogen inputs to pot cultured beech seedlings

The partitioning of nitrogen deposition among forest soil (including forest floor), leachate and above- and belowground biomass of pot cultured beech seedlings in comparison to non-cultured treatments were investigated by adding 1.92 g·m-2 15N tracer in throughfall for two successive growing seasons at a greenhouse experiment. Ammonium and nitrate depositions were simulated on four treatments (cultured and non-cultured) and each treatment was labeled with either 15N-NH4 + or sN-NO3⎯. Total recovery rates of the applied 15N in the whole system accounted for 74.9% to 67.3% after 15N-NH4 + and 85.3% to 88.1% after 15N-NO3⎯ in cultured and non-cultured treatments, respectively. The main sink for both 15N tracers was the forest soil (including forest floor), where 34.6% to 33.7% of 15N-NH4 + and 13.1% to 9.0% of 15N-NO3⎯ were found in cultured and non-cultured treatments, respectively, suggesting strong immobilization of both N forms by heterotrophic microorganisms. Nitrogen immobilization by microorganisms in the forest soil (including forest floor) was three times higher when 15N-NH4 + was applied compared to 15N-NO3⎯. The preferential heterotrophic use of ammonium resulted in a two times higher retention of deposited 15N-NH4 + in the forest soil as compared to plants. In contrast, nitrate immobilization in the forest soil was lower compared to plants, although statistically it was not significantly different. In total the immobilization of ammonium in the plant-soil system was about 60% higher than nitrate, indicating the importance of the N-forms deposition for retention in forest ecosystems.