The biodiversity-N cycle relationship: a(15)N tracer experiment with soil from plant mixtures of varying diversity to model N pool sizes and transformation rates

We conducted a(15)N tracer experiment in laboratory microcosms with field-fresh soil samples from a biodiversity experiment to evaluate the relationship between grassland biodiversity and N cycling. To embrace the complexity of the N cycle, we determined N exchange between five soil N pools (labile and recalcitrant organic N, dissolved NH(4)(+)and NO(3)(-)in soil solution, and exchangeable NH4+) and eight N transformations (gross N mineralization from labile and recalcitrant organic N, NH(4)(+)immobilization into labile and recalcitrant organic N, autotrophic nitrification, heterotrophic nitrification, NO(3)(-)immobilization, adsorption of NH4+) expected in aerobic soils with the help of the N-cycle modelNtrace. We used grassland soil of the Jena Experiment, which includes plant mixtures with 1 to 60 species and 1 to 4 functional groups (legumes, grasses, tall herbs, small herbs). The 19 soil samples of one block of the Jena Experiment were labeled with either(15)NH(4)(+)or(15)NO(3)(-)or both. In the presence of legumes, gross N mineralization and autotrophic nitrification increased significantly because of higher soil N concentrations in legume-containing plots and high microbial activity. Similarly, the presence of grasses significantly increased the soil NH(4)(+)pool, gross N mineralization, and NH(4)(+)immobilization, likely because of enhanced microbial biomass and activity by providing large amounts of rhizodeposits through their dense root systems. In our experiment, previously reported plant species richness effects on the N cycle, observed in a larger-scale field experiment within the Jena Experiment, were not seen. However, specific plant functional groups had a significant positive impact on the N cycling in the incubated soil samples.