Increasing atmospheric CO₂ concentration and related climate change have stimulated much interest in the potential of soils to sequester carbon. In 'The Jena Experiment', a managed grassland experiment on a former agricultural field, we investigated the link between plant diversity and soil carbon storage. The biodiversity gradient ranged from one to 60 species belonging to four functional groups. Stratified soil samples were taken to 30 cm depth from 86 plots in 2002, 2004 and 2006, and organic carbon contents were determined. Soil organic carbon stocks in 0-30 cm decreased from 7.3 kg C m(-2) in 2002 to 6.9 kg C m(-2) in 2004, but had recovered to 7.8 kg C m(-2) by 2006. During the first 2 years, carbon storage was limited to the top 5 cm of soil while below 10 cm depth, carbon was lost probably as short-term effect of the land use change. After 4 years, carbon stocks significantly increased within the top 20 cm. More importantly, carbon storage significantly increased with sown species richness (log-transformed) in all depth segments and even carbon losses were significantly smaller with higher species richness. Although increasing species diversity increased root biomass production, statistical analyses revealed that species diversity per se was more important than biomass production for changes in soil carbon. Below 20 cm depth, the presence of one functional group, tall herbs, significantly reduced carbon losses in the beginning of the experiment. Our analysis indicates that plant species richness and certain plant functional traits accelerate the build-up of new carbon pools within 4 years. Additionally, higher plant diversity mitigated soil carbon losses in deeper horizons. This suggests that higher biodiversity might lead to higher soil carbon sequestration in the long-term and therefore the conservation of biodiversity might play a role in greenhouse gas mitigation. [References: 52

In the literature, contrasting effects of plant species richness on the soil water balance are reported. Our objective was to assess the effects of plant species and functional richness and functional identity on soil water contents and water fluxes in the experimental grassland of the Jena Experiment. The Jena Experiment comprises 86 plots on which plant species richness (0, 1, 2, 4, 8, 16, and 60) and functional group composition (zero to four functional groups: legumes, grasses, tall herbs, and small herbs) were manipulated in a factorial design. We recorded meteorological data and soil water contents of the 0·0–0·3 and 0·3–0·7msoil layers and calculated actual evapotranspiration (ETa), downward flux (DF), and capillary rise with a soil water balance model for the period 2003–2007. Missing water contents were estimated with a Bayesian hierarchical model. Species richness decreased water contents in subsoil during wet soil conditions. Presence of tall herbs increased soil water contents in topsoil during dry conditions and decreased soilwater contents in subsoil duringwet conditions. Presence of grasses generally decreasedwater contents in topsoil, particularly during dry phases; increased ETa and decreased DF from topsoil; and decreased ETa from subsoil. Presence of legumes, in contrast, decreased ETa and increased DF from topsoil and increased ETa from subsoil. Species richness probably resulted in complementary water use. Specific functional groups likely affected the water balance via specific root traits (e.g. shallow dense roots of grasses and deep taproots of tall herbs) or specific shading intensity caused by functional group effects on vegetation cover

The temporal and spatial dynamics of soil water are closely interlinked with terrestrial ecosystems functioning. The interaction between plant community properties such as species composition and richness and soil water mirrors 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 water content and soil hydraulic properties remain largely understudied in biodiversity experiments. Thus, insights on how plant diversity—productivity relationships affect soil water are 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 water in a long‐term grassland biodiversity experiment (The Jena Experiment). Both plant species richness and the presence of particular functional groups affected soil water content, while functional group richness played no role. The effect of species richness changed from positive to negative and expanded to deeper soil with time. Shortly after establishment, increased topsoil water content was related to higher leaf area index in species‐rich plots, which enhanced shading. In later years, higher species richness increased topsoil organic carbon, likely improving soil aggregation. Improved aggregation, in turn, dried topsoils in species‐rich plots due to faster drainage of rainwater. Functional groups affected soil water distribution, likely due to plant traits affecting root water uptake depths, shading, or water‐use efficiency. For instance, topsoils in 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 water content through shading effects to altering soil physical characteristics in addition to modification of water uptake depth. Functional groups affected the soil water distribution by characteristic shifts of root water uptake depth, but did not enhance exploitation of the overall soil water storage. 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.

Previous research has shown that plant diversity influences N and P cycles. However, the effect of plant diversity on complete ecosystem N and P budgets has not yet been assessed. For 20 plots of artificially established grassland mixtures differing in plant diversity, we determined N and P inputs by bulk and dry deposition and N and P losses by mowing (and subsequent removal of the biomass) and leaching from April 2003 to March 2004. Total deposition of N and P was 2.3 +/- 0.1 and 0.2 +/- 0.01 g m(-2) yr(-1), respectively. Mowing was the main N and P loss. The net N and P budgets were negative (-6.3 +/- 1.1 g N and -1.9 +/- 0.2 g P m(-2) yr(-1)). For N, this included a conservative estimate of atmospheric N(2) fixation. Nitrogen losses as N(2)O were expected to be small at our study site (<0.05 g m(-2) yr(-1)). Legumes increased the removal of N with the harvest and decreased leaching of NH(4)-N and dissolved organic nitrogen (DON) from the canopy. Reduced roughness of grass-containing mixtures decreased dry deposition of N and P. Total dissolved P and NO(3)-N leaching from the canopy increased in the presence of grasses attributable to the decreased N and P demand of grass-containing mixtures. Species richness did not have an effect on any of the studied fluxes. Our results demonstrate that the N and P fluxes in managed grassland are modified by the presence or absence of particular functional plant groups and are mainly driven by the management.

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