Incorporation of mineral nitrogen into the soil food web as affected by plant community composition.

Although nitrogen (N) deposition is increasing globally, N availability still limits many organisms, such as microorganisms and mesofauna. However, little is known to which extent soil organisms rely on mineral-derived N and whether plant community composition modifies its incorporation into soil food webs. More diverse plant communities more effectively compete with microorganisms for mineral N likely reducing the incorporation of mineral-derived N into soil food webs. We set up a field experiment in experimental grasslands with different levels of plant species and functional group richness. We labeled soil with 15NH4 15NO3 and analyzed the incorporation of mineral-derived 15N into soil microorganisms and mesofauna over 3 months. Mineral-derived N incorporation decreased over time in all investigated organisms. Plant species richness and presence of legumes reduced the uptake of mineral-derived N into microorganisms. In parallel, the incorporation of mineral-derived 15N into mesofauna species declined with time and decreased with increasing plant species richness in the secondary decomposer springtail Ceratophysella sp. Effects of both plant species richness and functional group richness on other mesofauna species varied with time. The presence of grasses increased the 15N incorporation into Ceratophysella sp., but decreased it in the primary decomposer oribatid mite Tectocepheus velatus sarekensis. The results highlight that mineral N is quickly channeled into soil animal food webs via microorganisms irrespective of plant diversity. The amount of mineral-derived N incorporated into soil animals, and the plant community properties affecting this incorporation, differed markedly between soil animal taxa, reflecting species-specific use of food resources. Our results highlight that plant diversity and community composition alter the competition for N in soil and change the transfer of N across trophic levels in soil food webs, potentially leading to changes in soil animal population dynamics and community composition. Sustaining high plant diversity may buffer detrimental effects of elevated N deposition on soil biota.

Biodiversity increases multitrophic energy use efficiency, flow and storage in grasslands.

The continuing loss of global biodiversity has raised questions about the risk that species extinctions pose for the functioning of natural ecosystems and the services that they provide for human wellbeing. There is consensus that, on single trophic levels, biodiversity sustains functions; however, to understand the full range of biodiversity effects, a holistic and multitrophic perspective is needed. Here, we apply methods from ecosystem ecology that quantify the structure and dynamics of the trophic network using ecosystem energetics to data from a large grassland biodiversity experiment. We show that higher plant diversity leads to more energy stored, greater energy flow and higher community-energy-use efficiency across the entire trophic network. These effects of biodiversity on energy dynamics were not restricted to only plants but were also expressed by other trophic groups and, to a similar degree, in aboveground and belowground parts of the ecosystem, even though plants are by far the dominating group in the system. The positive effects of biodiversity on one trophic level were not counteracted by the negative effects on adjacent levels. Trophic levels jointly increased the performance of the community, indicating ecosystem-wide multitrophic complementarity, which is potentially an important prerequisite for the provisioning of ecosystem services.

Plants are less negatively affected by flooding when growing in species-rich plant communities.

Flooding is expected to increase in frequency and severity in the future. The ecological consequences of flooding are the combined result of species-specific plant traits and ecological context. However, the majority of past flooding research has focused on individual model species under highly controlled conditions. An early summer flooding event in a grassland biodiversity experiment in Jena, Germany, provided the opportunity to assess flooding responses of 60 grassland species in monocultures and 16-species mixtures. We examined plant biomass, species-specific traits (plant height, specific leaf area (SLA), root aerenchyma, starch content) and soil porosity. We found that, on average, plant species were less negatively affected by the flood when grown in higher-diversity plots in July 2013. By September 2013, grasses were unaffected by the flood regardless of plant diversity, and legumes were severely negatively affected regardless of plant diversity. Plants with greater SLA and more root aerenchyma performed better in September. Soil porosity was higher in higher-diversity plots and had a positive effect on plant performance. As floods become more frequent and severe in the future, growing flood-sensitive plants in higher-diversity communities and in soil with greater soil aeration may attenuate the most negative effects of flooding.

Plant diversity shapes microbe-rhizosphere effects on P mobilisation from organic matter in soil.

Plant species richness (PSR) increases nutrient uptake which depletes bioavailable nutrient pools in soil. No such relationship between plant uptake and availability in soil was found for phosphorus (P). We explored PSR effects on P mobilisation [phosphatase activity (PA)] in soil. PA increased with PSR. The positive PSR effect was not solely due to an increase in Corg concentrations because PSR remained significant if related to PA:Corg . An increase in PA per unit Corg increases the probability of the temporal and spatial match between substrate, enzyme and microorganism potentially serving as an adaption to competition. Carbon use efficiency of microorganisms (Cmic :Corg ) increased with increasing PSR while enzyme exudation efficiency (PA:Cmic ) remained constant. These findings suggest the need for efficient C rather than P cycling underlying the relationship between PSR and PA. Our results indicate that the coupling between C and P cycling in soil becomes tighter with increasing PSR.

Spatial heterogeneity of plant-soil feedback affects root interactions and interspecific competition.

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.

No evidence of complementary water use along a plant species richness gradient in temperate experimental grasslands.

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.