How plant diversity impacts the coupled water, nutrient and carbon cycles

Soils are important for ecosystem functions and services. However, soil processes are complex and changes of solid phase soil properties, such as soil organic matter contents are slow. As a consequence, a comprehensive understanding of the role of soil in the biodiversity-ecosystem functioning (BEF) relationship is still lacking. Thus, long-term observations and experiments are needed in biodiversity research in order to better understand how biodiversity influences soil properties and thus the BEF relationships. To elucidate the integrated response of soil-related functions and processes to plant diversity, we reviewed literature on the water, nutrient and carbon cycles in biodiversity research with specific focus on the Jena Experiment. Furthermore, we took advantage of the long-term observations of water, nutrient and carbon dynamics gathered in the Jena Experiment to investigate changes of the plant diversity effect over time on theses cycles and the accompanying plant-microbial interactions. We found that soil organic carbon and soil nitrogen stocks in the top 15cm constantly increased over time and that this increase was positively related to plant species richness. In contrast, the concentrations of the quantitatively most important nutrient ions nitrate and phosphate in soil solution decreased with time, likely because of the ongoing removal of nutrients by plant biomass harvest. We furthermore observed a shift in the microbial community composition, which was triggered by an increased availability of plant-derived carbon at higher plant species richness over time, suggesting that plant communities compensated for nutrient losses by stimulating the microbial nutrient cycling. In addition, water including dissolved nutrients and carbon percolated deeper in plots of higher plant diversity. Thereby, higher plant diversity spatially extended the nutrient cycling through the microbial communities to deeper soil layers from which nutrients are transferred to the topsoil by deep-rooting plants. Although microbial nutrient cycling cannot fully compensate for negative plant diversity effects on nutrient availability in soil solution, this suggests that over time the role of plant-derived inputs becomes increasingly important for ecosystem functioning. It furthermore implies that plant species richness tightens plant-microbial interactions, which in the long-term feed back on other ecosystem functions, such as productivity.