Linking land use intensity, biodiversity, soil microbial processes and organo-mineral interactions for a mechanistic understanding of Nitrogen turnover in grassland ecosystems (BE_BioMON)

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Increase in land use intensity (LUI) is a strong driver for environmental nitrogen (N) losses and decline of biodiversity and multifunctionality in grassland ecosystems. Still, interactions between LUI, above- and belowground biodiversity and N cycling in grassland ecosystems lack a mechanistic understanding, which prevents targeted mitigation measures. Through an interdisciplinary integration of soil microbial ecology, short-term processes of biotic N partitioning in the plant-soil-microbe system, and long-term physicochemical soil organic nitrogen (SON) retention pathways, BE_BioMon aims to fill this knowledge gap by linking measurements and modelling. We hypothesize that with decreasing LUI and more diverse microbiome, N partitioning shifts from nitrification/denitrification to biotic assimilation. This promotes N retention via necromass stabilization in particulate and mineral-associated SON depending on aggregate turnover and microscale soil architecture. Furthermore, we expect that increasing LUI will result in a shift from symbiotic to associative N fixation, which will increase the N2:N2O emission ratio due to truncated denitrification pathways in symbiotic diazotrophs. Moreover, we hypothesize that soil δ15N is fingerprinting LUI effects on N cycling, and thus can serve as a process integrating benchmark quantity for ecosystem models. This is expected to advance modeling-based spatiotemporal scaling of LUI effects on grassland N cycling. To test the hypotheses, we will perform field 15N fertilizer tracing experiments at selected plots of different LUI at all 3 Biodiversity Exploratories (BE) (WP1). In WP2, we will conduct mesocosm experiments with 15N2 exposure to disentangle LUI effects on the fates of BNF-N. WP3 is designed to fingerprint N cycling within the BE soil sampling campaign at all grassland EPs through vertical δ15N soil profiles, metagenomics, and SON fractionation. The gained knowledge from experimental work of WP1-3 and already available BExIS data will be used for testing and advancing N routines of the process-based ecosystem model LandscapeDNDC. This model, expanded by a dynamic vegetation model CoSMo, is used to evaluate differences in N cycling across the entirety of BE grassland plots (WP4). In WP5 (synthesis) we will mechanistically link effects of LUI and biodiversity with measured and modeled biogeochemical N transformations, N retention through organo-mineral interactions and full N balances at scales from days to years. We expect that this synthesis finally allows for an advanced mechanistic and functional understanding of ecosystem N cycling as influenced by LUI, above- and belowground biodiversity and site-specific properties.