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Nutrient enrichment is widespread throughout grassland systems and expected to increase during the Anthropocene. Trophic interactions, like aboveground herbivory, have been shown to mitigate its effect on plant diversity. Belowground herbivory may also impact these habitats' response to nutrient enrichment, but its influence is much less understood, and likely to depend on factors such as the herbivores' preference for dominant species and the symmetry of belowground competition. If preferential toward the dominant, fastest growing species, root herbivores may reduce these species' relative fitness and support diversity during nutrient enrichment. However, as plant competition belowground is commonly considered to be symmetric, root herbivores may be less impactful than shoot herbivores because they do not reduce any competitive asymmetry between the dominant and subordinate plants. To better understand this system, we used an established, two‐layer, grassland community model to run a full‐factorially designed simulation experiment, crossing the complete removal of aboveground herbivores and belowground herbivores with nutrient enrichment. After 100 yr of simulation, we analyzed communities' diversity, competition on the individual level, as well as their resistance and recovery. The model reproduced both observed general effects of nutrient enrichment in grasslands and the short‐term trends of specific experiments. We found that belowground herbivores exacerbate the negative influence of nutrient enrichment on Shannon diversity within our model grasslands, while aboveground herbivores mitigate its effect. Indeed, data on individuals' above‐ and belowground resource uptake reveals that root herbivory reduces resource limitation belowground. As with nutrient enrichment, this shifts competition aboveground. Since shoot competition is asymmetric, with larger, taller individuals gathering disproportionate resources compared to their smaller, shorter counterparts, this shift promotes the exclusion of the smallest species. While increasing the root herbivores' preferences toward dominant species lessens their negative impact, at best they are only mildly advantageous, and they do very little reduce the negative consequences of nutrient enrichment. Because our model's belowground competition is symmetric, we hypothesize that root herbivores may be beneficial when root competition is asymmetric. Future research into belowground herbivory should account for the nature of competition belowground to better understand the herbivores' true influence. [ABSTRACT FROM AUTHOR]

Aims Soil organisms can influence the plant diversity-productivity relationship at species level; however, little is known about their role in the relationship at an intraspecific level. This study aimed to investigate the interaction between the effects of plant intraspecific diversity and a root-knot nematode on primary production and community evenness. Methods A greenhouse experiment was conducted by using intraspecific diversity of a clover-grass community through sowing 2-, 4-, 8- and 16-cultivar mixtures of Trifolium pratense L. and Lolium perenne L. and by adding the nematode Meloidogyne hapla. We investigated the effects of plant intraspecific diversity and M. hapla on biomass of plant community, biomass of cultivar individuals and community evenness. Important Findings We found that L. perenne dominated the communities, yet its biomass was not affected by the treatments. Meloidogyne hapla influenced the effect of plant intraspecific diversity on aboveground biomass of T. pratense at the four-cultivar mixture level of plant intraspecific diversity. Community evenness increased linearly with increasing plant intraspecific diversity, and M. hapla affected community evenness at the four-cultivar mixture level. Both T. pratense and L. perenne cultivars differed in aboveground biomass. Depending on plant intraspecific diversity and cultivar identity, nematode addition positively affected the aboveground biomass of T. pratense individuals. The response of T. pratense cultivars to M. hapla and to various plant intraspecific diversity levels influenced the intraspecific diversity-productivity relationship. This study highlights the influence of M. hapla on the effect of plant intraspecific diversity on aboveground biomass of T. pratense, suggesting that future research on the impact of intraspecific plant diversity on ecosystem functions should consider the effects of soil organisms. [ABSTRACT FROM AUTHOR]

Small scale distribution of insect root herbivores may promote plant species diversity by creating patches of different herbivore pressure. However, determinants of small scale distribution of insect root herbivores, and impact of land use intensity on their small scale distribution are largely unknown. We sampled insect root herbivores and measured vegetation parameters and soil water content along transects in grasslands of different management intensity in three regions in Germany. We calculated community-weighted mean plant traits to test whether the functional plant community composition determines the small scale distribution of insect root herbivores. To analyze spatial patterns in plant species and trait composition and insect root herbivore abundance we computed Mantel correlograms. Insect root herbivores mainly comprised click beetle (Coleoptera, Elateridae) larvae (43%) in the investigated grasslands. Total insect root herbivore numbers were positively related to community-weighted mean traits indicating high plant growth rates and biomass (specific leaf area, reproductive- and vegetative plant height), and negatively related to plant traits indicating poor tissue quality (leaf C/N ratio). Generalist Elaterid larvae, when analyzed independently, were also positively related to high plant growth rates and furthermore to root dry mass, but were not related to tissue quality. Insect root herbivore numbers were not related to plant cover, plant species richness and soil water content. Plant species composition and to a lesser extent plant trait composition displayed spatial autocorrelation, which was not influenced by land use intensity. Insect root herbivore abundance was not spatially autocorrelated. We conclude that in semi-natural grasslands with a high share of generalist insect root herbivores, insect root herbivores affiliate with large, fast growing plants, presumably because of availability of high quantities of food. Affiliation of insect root herbivores with large, fast growing plants may counteract dominance of those species, thus promoting plant diversity. [ABSTRACT FROM AUTHOR]

Abstract: Biodiversity, a multidimensional property of natural systems, is difficult to quantify partly because of the multitude of indices proposed for this purpose. Indices aim to describe general properties of communities that allow us to compare different regions, taxa, and trophic levels. Therefore, they are of fundamental importance for environmental monitoring and conservation, although there is no consensus about which indices are more appropriate and informative. We tested several common diversity indices in a range of simple to complex statistical analyses in order to determine whether some were better suited for certain analyses than others. We used data collected around the focal plant Plantago lanceolata on 60 temperate grassland plots embedded in an agricultural landscape to explore relationships between the common diversity indices of species richness (S), Shannon's diversity (H'), Simpson's diversity (D1), Simpson's dominance (D2), Simpson's evenness (E), and Berger–Parker dominance (BP). We calculated each of these indices for herbaceous plants, arbuscular mycorrhizal fungi, aboveground arthropods, belowground insect larvae, and P. lanceolata molecular and chemical diversity. Including these trait‐based measures of diversity allowed us to test whether or not they behaved similarly to the better studied species diversity. We used path analysis to determine whether compound indices detected more relationships between diversities of different organisms and traits than more basic indices. In the path models, more paths were significant when using H', even though all models except that with E were equally reliable. This demonstrates that while common diversity indices may appear interchangeable in simple analyses, when considering complex interactions, the choice of index can profoundly alter the interpretation of results. Data mining in order to identify the index producing the most significant results should be avoided, but simultaneously considering analyses using multiple indices can provide greater insight into the interactions in a system. [ABSTRACT FROM AUTHOR]

A frequent pattern emerging from biodiversity-ecosystem function studies is that functional group richness enhances ecosystem functions such as primary productivity. However, the manipulation of functional group richness goes along with major disadvantages like the transformation of functional trait data into categories or the exclusion of functional differences between organisms in the same group. In a mesocosm study we manipulated plant functional diversity based on the multi-trait Functional Diversity (FD)-approach of Petchey and Gaston by using database data of seven functional traits and information on the origin of the species in terms of being native or exotic. Along a gradient ranging from low to high FD we planted 40 randomly selected eight-species mixtures under controlled conditions. We found a significant positive linear correlation of FD with aboveground productivity and a negative correlation with invasibility of the plant communities. Based on community-weighted mean calculations for each functional trait, we figured out that the traits N-fixation and species origin, i.e. being native or exotic, played the most important role for community productivity. Our results suggest that the identification of the impact of functional trait diversity and the relative contributions of relevant traits is essential for a mechanistic understanding of the role of biodiversity for ecosystem functions such as aboveground biomass production and resistance against invasion. [ABSTRACT FROM AUTHOR]

• Plant diversity and productivity are involved in explaining δ15N plant , δ15N soil, and δ15N soil-to-plant in North Tibet. • Precipitation is critical for driving spatial variability of δ15N plant , δ15N soil and δ15N soil-to-plant in North Tibet. • The δ15N plant and δ15N soil-to-plant indicate growth limitation shifts from P in meadows to N in desert-steppes. Nitrogen (N) cycling is a critical pathway by which producer, consumer, and decomposer interact with each other and with environmental circumstances simultaneously. The natural abundance composition of 15N/14N in plants and soils (termed as δ15N plant and δ15N soil), as well as the difference between them (δ15N soil-to-plant = δ15N plant −δ15N soil), is a useful tool for better understanding ecosystem N cycling. However, the drivers and mechanisms of ecosystem N cycling in alpine grasslands on the Tibetan Plateau are mostly unknown, especially across different grassland types at a regional scale. To fill this knowledge gap, we measured δ15N plant (200 samples of top-dominant species) and δ15N soil (85 samples of top-layer soils, 0–20 cm) at nine sites that represent zonal communities of alpine deserts, steppes, and meadows in North Tibet, and calculated the corresponding δ15N soil-to-plant. Our results showed that δ15N plant, δ15N soil, and δ15N soil-to-plant were significantly different among the three zonal grassland types (analysis of differences with non-parametric Kruskal Test, P < 0.05), with the lowest values in meadows and the highest values in deserts. Regression analyses showed that the δ15N plant , δ15N soil, and δ15N soil-to-plant decreased with the increases of growing season precipitation (GSP) and habitat aridity index (Aridity), soil organic carbon (SOC) and soil total nitrogen (STN), plant species richness, Shannon diversity index, and plant community productivity, whereas increased with the increases of accumulated active temperature (AccT) and soil total phosphorus (STP) across alpine grassland types at the regional scale. Multiple linear models with analysis of covariance (ANCOVA) confirmed GSP to be the most critical driver, which alone explained most variances of δ15N plant (56%), δ15N soil (62%), and δ15N soil-to-plant (35%). However, structural equation modeling performed better than multiple linear modeling in predicting δ15N plant (76% vs. 66%) and worse in predicting δ15N soil (79% vs. 84%) and δ15N soil-to-plant (31% vs. 46%), likely due to the exclusion of collinear predictors and the removal of non-significant influencing paths. Overall, this study has highlighted the importance to uncover the complexity of climate, soil nutrients, and vegetation properties in networking to drive the different components of ecosystem N cycling in alpine grasslands on the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Intensive land use is a major cause of biodiversity loss, but most studies comparing the response of multiple taxa rely on simple diversity measures while analyses of other community attributes are only recently gaining attention. Species-abundance distributions (SADs) are a community attribute that can be used to study changes in the overall abundance structure of species groups, and whether these changes are driven by abundant or rare species. We evaluated the effect of grassland management intensity for three land-use modes (fertilization, mowing, grazing) and their combination on species richness and SADs for three belowground (arbuscular mycorrhizal fungi, prokaryotes and insect larvae) and seven aboveground groups (vascular plants, bryophytes and lichens; arthropod herbivores; arthropod pollinators; bats and birds). Three descriptors of SADs were evaluated: general shape (abundance decay rate), proportion of rare species (rarity) and proportional abundance of the commonest species (dominance). Across groups, taxonomic richness was largely unaffected by land-use intensity and only decreased with increasing mowing intensity. Of the three SAD descriptors, abundance decay rate became steeper with increasing combined land-use intensity across groups. This reflected a decrease in rarity among plants, herbivores and vertebrates. Effects of fertilization on the three descriptors were similar to the combined land-use intensity effects. Mowing intensity only affected the SAD descriptors of insect larvae and vertebrates, while grazing intensity produced a range of effects on different descriptors in distinct groups. Overall, belowground groups had more even abundance distributions than aboveground groups. Strong differences among aboveground groups and between above- and belowground groups indicate that no single taxonomic group can serve as an indicator for effects in other groups. In the past, the use of SADs has been hampered by concerns over theoretical models underlying specific forms of SADs. Our study shows that SAD descriptors that are not connected to a particular model are suitable to assess the effect of land use on community structure. [ABSTRACT FROM AUTHOR]

Plant diversity and soil organisms are both widely studied for their contributions to ecosystem functions. It has been recently shown that soil biota may drive the plant species diversity-function relationship in grasslands, while so far studies on plant intraspecific diversity are lacking. To improve the understanding of how ecosystem functions may be influenced by plant intraspecific diversity, soil organisms, and their interactions, we used a field setting to examine the effects of plant intraspecific diversity and functionally dissimilar soil organisms (manipulated by the addition of northern root-knot nematodes, earthworms, and arbuscular mycorrhizal fungi) on aboveground primary production, pollination and shoot herbivory. Plant intraspecific diversity was manipulated by sowing different seed mixtures of Trifolium pratense L. and Lolium perenne L. cultivars. We found a positive effect of plant intraspecific diversity on total aboveground primary production, contributed mostly by T. pratense . The plant intraspecific diversity-productivity effect on T. pratense was mediated by the nematodes, i.e. aboveground biomass was increased by intraspecific diversity only in the nematode treatment. Further, the average floral visitation length was decreased by earthworm addition, while the abundance of pollinators tended to be increased by high plant intraspecific diversity. Leaf damage of T. pratense were not influenced by the treatments. We conclude that plant intraspecific diversity and soil organisms can interactively or independently modify functions such as aboveground primary production and pollination of T. pratense in the field. Specifically, the positive plant intraspecific diversity effect appears to only emerge under conditions of stress (high nematode abundance). This result indicates that plant intraspecific diversity may be important for the maintenance of ecosystem functions, especially in an agricultural context, where pest pressure can be high and can have large detrimental consequences. [ABSTRACT FROM AUTHOR]

Plant diversity and different groups of belowground organisms have been shown to influence the characteristics of plant communities and their associated aboveground organisms; however, little is known about the interactions and combined effects of these biotic factors. Furthermore, while plant species richness has received a lot of attention, intraspecific diversity has only recently come into the focus of community ecology. The objective of this study was to determine whether the addition of beneficial soil organisms (arbuscular mycorrhizal fungi (AMF) and anecic earthworms ( Lumbricus terrestris L.) and different levels of plant cultivar diversity interactively influence productivity, weed establishment, leaf damage by herbivores and pathogens, and the diversity of aboveground herbivores in legume-grass communities. Our results indicated that the addition of AMF increased aboveground plant productivity and tended to decrease the diversity of aboveground herbivores. Earthworms attenuated the effect of AMF addition on AMF root colonization, but had no effect on plant productivity or herbivore diversity. Weed biomass was significantly lower in plant communities with high cultivar diversity compared to low cultivar diversity. We did not find interactive effects of the two soil organism groups and plant cultivar diversity. Our results demonstrate the independent roles and additive positive effects of AMF and plant cultivar diversity on functions such as productivity and resistance against weeds in the field. We suggest that AMF and plant cultivar diversity manipulations may be applied more frequently in agriculture management programs that aim for sustainable yield enhancement and biocontrol of herbivores and weeds. [ABSTRACT FROM AUTHOR]

Abstract: The productivity and diversity of plant communities are affected by soil organisms such as arbuscular mycorrhizal fungi (AMF), root herbivores and decomposers. However, it is unknown how interactions between such functionally dissimilar soil organisms affect plant communities and whether the combined effects are additive or interactive. In a greenhouse experiment we investigated the individual and combined effects of AMF (five Glomus species), root herbivores (wireworms and nematodes) and decomposers (collembolans and enchytraeids) on the productivity and nutrient content of a model grassland plant community as well as on soil microbial biomass and community structure. The effects of the soil organisms on productivity (total plant biomass), total root biomass, grass and forb biomass, and nutrient uptake of the plant community were additive. AMF decreased, decomposers increased and root herbivores had no effect on productivity, but in combination the additive effects canceled each other out. AMF reduced total root biomass by 18%, but decomposers increased it by 25%, leading to no net effect on total root biomass in the combined treatments. Total shoot biomass was reduced by 14% by root herbivores and affected by an interaction between AMF and decomposers where decomposers had a positive impact on shoot growth only in presence of AMF. AMF increased the shoot biomass of forbs, but reduced the shoot biomass of grasses, while root herbivores only reduced the shoot biomass of grasses. Interactive effects of the soil organisms were detected on the shoot biomasses of Lotus corniculatus, Plantago lanceolata, and Agrostis capillaris. The C/N ratio of the plant community was affected by AMF. In soil, AMF promoted abundances of bacterial, actinomycete, saprophytic and AMF fatty acid markers. Decomposers alone decreased bacterial and actinomycete fatty acids abundances but when decomposers were interacting with herbivores those abundances were increased. Our results suggests that at higher resolutions, i.e. on the levels of individual plant species and the microbial community, interactive effects are common but do not affect the overall productivity and nutrient uptake of a grassland plant community, which is mainly affected by additive effects of functionally dissimilar soil organisms. [Copyright &y& Elsevier]