Plant species richness and functional groups have different effects on soil water content in a decade‐long grassland experiment

The temporal and spatial dynamics of soil waterare closely interlinked with terrestrial ecosystems functioning. The interaction between plant community propertiessuch as species composition and richness and soil watermirrors fundamental ecological processes determining above-ground–below-ground feedbacks. Plant–water relations and water stress have attracted considerable attention in biodiversity experiments. Yet, although soil scientific research suggests an influence of ecosystem productivity on soil hydraulic properties, temporal changes of the soil watercontent and soil hydraulic properties remain largely understudied in biodiversity experiments. Thus, insights on how plant diversity—productivity relationships affect soil waterare lacking. Here, we determine which factors related to plant community composition (species and functional group richness, presence of plant functional groups) and soil (organic carbon concentration) affect soil waterin a long-term grassland biodiversity experiment (The Jena Experiment). Both plant species richnessand the presence of particular functional groups affected soil watercontent, while functional group richness played no role. The effect of species richnesschanged from positive to negative and expanded to deeper soil with time. Shortly after establishment, increased topsoilwater content was related to higher leaf area index in species-richplots, which enhanced shading. In later years, higher species richnessincreased topsoilorganic carbon, likely improving soil aggregation. Improved aggregation, in turn, dried topsoilsin species-richplots due to faster drainage of rainwater. Functional groups affected soil waterdistribution, likely due to plant traits affecting root water uptake depths, shading, or water-use efficiency. For instance, topsoilsin plots containing grasses were generally drier, while plots with legumes were moister. Synthesis. Our decade-long experiment reveals that the maturation of grasslands changes the effects of plant richness from influencing soil watercontent through shading effects to altering soil physicalcharacteristics in addition to modification of water uptake depth. Functional groups affected the soil waterdistribution by characteristic shifts of root water uptake depth, but did not enhance exploitation of the overall soil waterstorage. Our results reconcile previous seemingly contradictory results on the relation between grassland species diversity and soil moisture and highlight the role of vegetation composition for soil processes.