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 xperiment 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$${}^{+}$ and NO3${}^{-}$ in soil solution, and exchangeable NH${}_4$${}^{+}$) and eight N transformations (gross N mineralization from labile and recalcitrant organic N, NH${}_4$${}^{+}$ immobilization into labile and recalcitrant organic N, autotrophic nitrification, heterotrophic nitrification, NO${}_3$${}^{-}$ immobilization, adsorption of NH${}_4$${}^{+}$) expected in aerobic soils with the help of the N-cycle model Ntrace. We used grassland soil of the Jena Experiment, which includes plant mixtures with 1 to 60 species and 1 to 4 functional groups (legumes, grasses, tall herbs, small herbs). The 19 soil samples of one block of the Jena Experiment were labeled with either 15NH${}_4$${}^{+}$ or 15NO3- 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. ... mehrSimilarly, 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.