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Niche complementarity in resource use has been proposed as a key mechanism to explain the positive effects of increasing plant species richness on ecosystem processes, in particular on primary productivity. Since hardly any information is available for niche complementarity in water use, we tested the effects of plant diversity on spatial and temporal complementarity in water uptake in experimental grasslands by using stable water isotopes. We hypothesized that water uptake from deeper soil depths increases in more diverse compared to low diverse plant species mixtures. We labeled soil water in 8 cm (with 18O) and 28 cm depth (with ²H) three times during the 2011 growing season in 40 temperate grassland communities of varying species richness (2, 4, 8 and 16 species) and functional group number and composition (legumes, grasses, tall herbs, small herbs). Stable isotope analyses of xylem and soil water allowed identifying the preferential depth of water uptake. Higher enrichment in 18O of xylem water than in ²H suggested that the main water uptake was in the upper soil layer. Furthermore, our results revealed no differences in root water uptake among communities with different species richness, different number of functional groups or with time. Thus, our results do not support the hypothesis of increased complementarity in water use in more diverse than in less diverse communities of temperate grassland species.

Plant–soil feedback is receiving increasing interest as a factor influencing plant competition and species coexistence in grasslands. However, we do not know how spatial distribution of plant–soil feedback affects plant below-ground interactions. We investigated the way in which spatial heterogeneity of soil biota affects competitive interactions in grassland plant species. We performed a pairwise competition experiment combined with heterogeneous distribution of soil biota using four grassland plant species and their soil biota. Patches were applied as quadrants of ‘own’ and ‘foreign’ soils from all plant species in all pairwise combinations. To evaluate interspecific root responses, species-specific root biomass was quantified using real-time PCR. All plant species suffered negative soil feedback, but strength was species-specific, reflected by a decrease in root growth in own compared with foreign soil. Reduction in root growth in own patches by the superior plant competitor provided opportunities for inferior competitors to increase root biomass in these patches. These patterns did not cascade into above-ground effects during our experiment. We show that root distributions can be determined by spatial heterogeneity of soil biota, affecting plant below-ground competitive interactions. Thus, spatial heterogeneity of soil biota may contribute to plant species coexistence in species-rich grasslands.

Human‐caused declines in biodiversity have stimulated intensive research on the consequences of biodiversity loss for ecosystem services and policy initiatives to preserve the functioning of ecosystems. Short‐term biodiversity experiments have documented positive effects of plant species richness on many ecosystem functions, and longer‐term studies indicate, for some ecosystem functions, that biodiversity effects can become stronger over time. Theoretically, a biodiversity effect can strengthen over time by an increasing performance of high‐diversity communities, by a decreasing performance of low‐diversity communities, or a combination of both processes. Which of these two mechanisms prevail, and whether the increase in the biodiversity effect over time is a general property of many functions remains currently unclear. These questions are an important knowledge gap as a continuing decline in the performance of low‐diversity communities would indicate an ecosystem‐service debt resulting from delayed effects of species loss on ecosystem functioning. Conversely, an increased performance of high‐diversity communities over time would indicate that the benefits of biodiversity are generally underestimated in short‐term studies. Analyzing 50 ecosystem variables over 11 years in the world's largest grassland biodiversity experiment, we show that overall plant diversity effects strengthened over time. Strengthening biodiversity effects were independent of the considered compartment (above‐ or belowground), organizational level (ecosystem variables associated with the abiotic habitat, primary producers, or higher trophic levels such as herbivores and pollinators), and variable type (measurements of pools or rates). We found evidence that biodiversity effects strengthened because of both a progressive decrease in functioning in species‐poor and a progressive increase in functioning in species‐rich communities. Our findings provide evidence that negative feedback effects at low biodiversity are as important for biodiversity effects as complementarity among species at high biodiversity. Finally, our results indicate that a current loss of species will result in a future impairment of ecosystem functioning, potentially decades beyond the moment of species extinction.

Biodiversity experiments generally report a positive effect of plant biodiversity on aboveground biomass (overyielding), which typically increases with time. Various studies also found overyielding for belowground plant biomass, but this has never been measured over time. Also, potential underlying mechanisms have remained unclear. Differentiation in rooting patterns among plant species and plant functional groups has been proposed as a main driver of the observed biodiversity effect on belowground biomass, leading to more efficient belowground resource use with increasing diversity, but so far there is little evidence to support this. We analyzed standing root biomass and its distribution over the soil profile, along a 1–16 species richness gradient over eight years in the Jena Experiment in Germany, and compared belowground to aboveground overyielding. In our long-term dataset, total root biomass increased with increasing species richness but this effect was only apparent after four years. The increasingly positive relationship between species richness and root biomass, explaining 12% of overall variation and up to 28% in the last year of our study, was mainly due to decreasing root biomass at low diversity over time. Functional group composition strongly affected total standing root biomass, explaining 44% of variation, with grasses and legumes having strong overall positive and negative effects, respectively. Functional group richness or interactions between functional group presences did not strongly contribute to overyielding. We found no support for the hypothesis that vertical root differentiation increases with species richness, with functional group richness or composition. Other explanations, such as stronger negative plant–soil feedbacks in low-diverse plant communities on standing root biomass and vertical distribution should be considered.

1. Below-ground resource partitioning is often proposed as the underlying mechanism for the positive relationship between plant species richness and productivity. For example, if species have different root distributions, a mixture of plant species may be able to use the available resources more completely than the individual species in a monoculture. However, there is little experimental evidence for differentiation in vertical root distributions among species and its contribution to biodiversity effects. 2. We determined species-specific root standing biomass over depth using molecular techniques (real-time qPCR) in a large grassland biodiversity experiment (one to eight plant species mixtures), in 2 years. Species-specific root biomass data were used to disentangle the effects of positive interactions between species (complementarity effects) and effects due to dominance of productive species (selection effects) on root biomass in mixtures. In a next step, these biodiversity effects were linked to the diversity of rooting depths and the averaged rooting depth of the community. 3. Root biomass increased with species richness. This was mainly due to positive interactions (the complementarity effect), which increased with species richness below-ground. In contrast, the selection effect decreased with species richness. Although there was considerable variation in vertical root distribution between species in monocultures, the diversity of rooting strategies did not explain the complementarity effect. Rather, the abudance of deep-rooting species in mixtures (i.e. high community-weighted mean) was significantly related to the complementarity effect. Comparing the \"predicted\" root distribution (based on monocultures) to the actual distribution in mixtures, we found that mixtures rooted deeper than expected, but this did not better explain the complementarity effect. 4. Synthesis. This study demonstrates that vertical root distributions of species provide only subtle evidence for resource partitioning. W e found no evidence that functional diversity in vertical rooting patterns was important for the complementarity effect, in contrast to our expectation that the enhancement of productivity was due to resource partitioning. Alternatively, we found significant but weak relationships between the complementarity effect and deep-rooting communities, based on the communityweighted mean root distribution. This suggests that factors other than below-ground resource partitioning alone may drive the biodiversity-productivity relationship.

Background and aims Competition is an important force shaping plant communities. Here we test the hypothesis that high overall root length density and selective root placement in nutrient patches, as two alternative strategies, confer competitive advantage in species mixtures. Methods We performed a full-factorial pairwise competition experiment with eight grassland species in soil with homogeneously distributed nutrients, or with nutrients concentrated in a single patch. We measured species-specific relative growth rate, root length density, selective root placement, and ion uptake rates of all species in monocultures and in mixtures. Results Grasses showed higher specific root length overall and forbs a higher selective root placement in the nutrient patch. However, relative growth rate and root length density were more strongly related to competitive ability (measured as relative yield per plant), with little distinction between grasses and forbs. Conclusions Our results suggest that short-term competitive success was related to fast growth and high root densities, irrespective of nutrient heterogeneity. Developing a large root mass quickly may overwhelm the importance of other traits in the establishment phase of plants, although these other traits may prove to be important in the long run.

1. Below-ground resource partitioning is among the most prominent hypotheses for driving the positive biodiversity-ecosystem function relationship. However, experimental tests of this hypothesis in biodiversity experiments are scarce, and the available evidence is not consistent. 2. We tested the hypothesis that resource partitioning in space, in time or in both space and time combined drives the positive effect of diversity on both plant productivity and total community resource uptake. At the community level, we predicted that total community resource uptake and biomass production above- and below-ground will increase with increased species richness or functional group richness. We predicted that, at the species level, resource partition breadth will become narrower, and that overlap between the resource partitions of different species will become smaller with increasing species richness or functional group richness. 3. We applied multiple resource tracers (Li and Rb as potassium analogues, the water isotopologues—H₂¹⁸O and ²H₂O, and ¹⁵N) in three seasons at two depths across a species and functional group richness gradient at a grassland biodiversity experiment. We used this multidimensional resource tracer study to test if plant species partition resources with increasing plant diversity across space, time or both simultaneously. 4. At the community level, total community resource uptake of nitrogen and potassium and above- and below-ground biomass increased significantly with increasing species richness but not with increasing functional group richness. However, we found no evidence that resource partition breadth or resource partition overlap decreased with increasing species richness for any resource in space, time or both space and time combined. 5. Synthesis. These findings indicate that below-ground resource partitioning may not drive the enhanced resource uptake or biomass production found here. Instead, other mechanisms such as facilitation, species-specific biotic feedback or above-ground resource partitioning are likely necessary for enhanced overall ecosystem function.

Competition is an important force shaping plant communities. Here we test the hypothesis that high overall root length density and selective root placement in nutrient patches, as two alternative strategies, confer competitive advantage in species mixtures. We performed a full-factorial pairwise competition experiment with eight grassland species in soil with homogeneously distributed nutrients, or with nutrients concentrated in a single patch. We measured species-specific relative growth rate, root length density, selective root placement, and ion uptake rates of all species in monocultures and in mixtures. Grasses showed higher specific root length overall and forbs a higher selective root placement in the nutrient patch. However, relative growth rate and root length density were more strongly related to competitive ability (measured as relative yield per plant), with little distinction between grasses and forbs. Our results suggest that short-term competitive success was related to fast growth and high root densities, irrespective of nutrient heterogeneity. Developing a large root mass quickly may overwhelm the importance of other traits in the establishment phase of plants, although these other traits may prove to be important in the long run.