Spatial patterns and ecological drivers of soil nematode β-diversity in natural grasslands vary among vegetation types and trophic position

Understanding biogeographic patterns of community assemblages is a core objective in ecology, but for soil communities these patterns are poorly understood. To understand the spatial patterns and underlying mechanisms of β-diversity in soil communities, we investigated the β-diversity of soil nematode communities along a 3,200-km transect across semi-arid and arid grasslands. Spatial turnover and nested-resultant are the two fundamental components of β-diversity, which have been attributed to various processes of community assembly. We calculated the spatial turnover and nested-resultant components of soil nematode β-diversity based on the β-partitioning framework. Distance matrices for the dissimilarity of soil nematode communities were computed using the 'Sørensen' method. We fitted negative exponential models to compare the distance decay patterns in nematode community similarity with geographic distance and plant community distance in three vegetation types (desert, desert steppe and typical steppe) and along the whole transect. Variation partitioning was used to distinguish the contribution of geographic distance and environmental variables to β-diversity and the partitioned components. Geographic distance and environmental filtering jointly drove the β-diversity patterns of nematode community, but environmental filtering explained more of the variation in β-diversity in the desert and typical steppe, whereas geographic distance was important in the desert steppe. Nematode community assembly was explained more by the spatial turnover component than by the nested-resultant component. For nematode feeding groups, the β-diversity in different vegetation types increased with geographic distance and plant community distance, but the nested-resultant component of bacterial feeders in the desert ecosystem decreased with geographic distance and plant community distance. Our findings show that spatial variation in soil nematode communities is regulated by environmental processes at the vegetation type scale, while spatial processes mainly work on the regional scale, and emphasize that the spatial patterns and drivers of nematode β-diversity differ among trophic levels. Our study provides insight into the ecological processes that maintain soil biodiversity and biogeographic patterns of soil community assemblage at large spatial scales.

Globally, plant-soil feedbacks are weak predictors of plant abundance

Plant-soil feedbacks (PSFs) have been shown to strongly affect plant performance under controlled conditions, and PSFs are thought to have far reaching consequences for plant population dynamics and the structuring of plant communities. However, thus far the relationship between PSF and plant species abundance in the field is not consistent. Here, we synthesize PSF experiments from tropical forests to semiarid grasslands, and test for a positive relationship between plant abundance in the field and PSFs estimated from controlled bioassays. We meta-analyzed results from 22 PSF experiments and found an overall positive correlation (0.12 ≤  r ¯  ≤ 0.32) between plant abundance in the field and PSFs across plant functional types (herbaceous and woody plants) but also variation by plant functional type. Thus, our analysis provides quantitative support that plant abundance has a general albeit weak positive relationship with PSFs across ecosystems. Overall, our results suggest that harmful soil biota tend to accumulate around and disproportionately impact species that are rare. However, data for the herbaceous species, which are most common in the literature, had no significant abundance-PSFs relationship. Therefore, we conclude that further work is needed within and across biomes, succession stages and plant types, both under controlled and field conditions, while separating PSF effects from other drivers (e.g., herbivory, competition, disturbance) of plant abundance to tease apart the role of soil biota in causing patterns of plant rarity versus commonness.

Empirical and theoretical challenges in aboveground-belowground ecology

A growing body of evidence shows that aboveground and belowground communities and processes are intrinsically linked, and that feedbacks between these subsystems have important implications for community structure and ecosystem functioning. Almost all studies on this topic have been carried out from an empirical perspective and in specific ecological settings or contexts. Belowground interactions operate at different spatial and temporal scales. Due to the relatively low mobility and high survival of organisms in the soil, plants have longer lasting legacy effects belowground than aboveground. Our current challenge is to understand how aboveground-belowground biotic interactions operate across spatial and temporal scales, and how they depend on, as well as influence, the abiotic environment. Because empirical capacities are too limited to explore all possible combinations of interactions and environmental settings, we explore where and how they can be supported by theoretical approaches to develop testable predictions and to generalise empirical results. We review four key areas where a combined aboveground-belowground approach offers perspectives for enhancing ecological understanding, namely succession, agro-ecosystems, biological invasions and global change impacts on ecosystems. In plant succession, differences in scales between aboveground and belowground biota, as well as between species interactions and ecosystem processes, have important implications for the rate and direction of community change. Aboveground as well as belowground interactions either enhance or reduce rates of plant species replacement. Moreover, the outcomes of the interactions depend on abiotic conditions and plant life history characteristics, which may vary with successional position. We exemplify where translation of the current conceptual succession models into more predictive models can help targeting empirical studies and generalising their results. Then, we discuss how understanding succession may help to enhance managing arable crops, grasslands and invasive plants, as well as provide insights into the effects of global change on community re-organisation and ecosystem processes.

Empirical and theoretical challenges in aboveground-belowground ecology

A growing body of evidence shows that aboveground and belowground communities and processes are intrinsically linked, and that feedbacks between these subsystems have important implications for community structure and ecosystem functioning. Almost all studies on this topic have been carried out from an empirical perspective and in specific ecological settings or contexts. Belowground interactions operate at different spatial and temporal scales. Due to the relatively low mobility and high survival of organisms in the soil, plants have longer lasting legacy effects belowground than aboveground. Our current challenge is to understand how aboveground-belowground biotic interactions operate across spatial and temporal scales, and how they depend on, as well as influence, the abiotic environment. Because empirical capacities are too limited to explore all possible combinations of interactions and environmental settings, we explore where and how they can be supported by theoretical approaches to develop testable predictions and to generalise empirical results. We review four key areas where a combined aboveground-belowground approach offers perspectives for enhancing ecological understanding, namely succession, agro-ecosystems, biological invasions and global change impacts on ecosystems. In plant succession, differences in scales between aboveground and belowground biota, as well as between species interactions and ecosystem processes, have important implications for the rate and direction of community change. Aboveground as well as belowground interactions either enhance or reduce rates of plant species replacement. Moreover, the outcomes of the interactions depend on abiotic conditions and plant life history characteristics, which may vary with successional position. We exemplify where translation of the current conceptual succession models into more predictive models can help targeting empirical studies and generalising their results. Then, we discuss how understanding succession may help to enhance managing arable crops, grasslands and invasive plants, as well as provide insights into the effects of global change on community re-organisation and ecosystem processes.

Above- and below-ground terpenoid aldehyde induction in cotton, Gossypium herbaceum, following root and leaf injury

Studies on induced defenses have predominantly focused on foliar induction by above-ground herbivores and pathogens. However, roots are attacked by as many if not more phytophages than shoots, so in reality plants are exposed to above- and below-ground attack. Here, we report effects of foliar and/or root damage on terpenoid aldehyde accumulation in cotton (Gossypium herbaceum). Using HPLC, we analyzed concentrations of individual terpenoid aldehydes in foliage and root tissue. In undamaged plants, terpenoid aldehydes were concentrated in young immature main leaves. Concentrations in side leaves, branching from the main leaves, did not differ among leaf position. Above-ground feeding by Spodopterta exigua larvae on a mature leaf enhanced terpenoid concentrations in immature leaves but not in the damaged leaf. In particular, concentrations of hemigossypolone and the heliocides 1 and 4 were enhanced following herbivory. Root herbivory by wireworms (Agriotes lineatus) also resulted in an increase in terpenoid levels in the foliage. In contrast with foliar herbivory, both immature and mature leaves were induced. However, the level of induction after root herbivory was much lower compared to foliar herbivory. Plants exposed to root herbivory also had significantly higher levels of terpenoid aldehydes in root tissue, while no such effect was found following foliar herbivory. Plants exposed to both root and foliar herbivory appeared to induce primarily above-ground at the cost of below-ground defense. The implications for above- and below-ground Mutitrophic interactions are discussed.

Above- and below ground trophic interactions on creeping thistle (Cirsium arvense) in high- and low-diversity plant communities: potential for biotic resistance?

The capacity of local communities to control introduced plants is called biotic resistance. Biotic resistance has been almost exclusively tested for plant competition and above ground herbivores and pathogens, while neglecting root herbivores and soil pathogens. Here, we present biotic resistance by above- and below ground herbivores in concert, and relate the abundance of the plant enemies to the species diversity of the local plant communities. The study was carried out in a 7-year-old biodiversity field experiment. We used creeping thistle (Cirsium arvense) as a model, and quantified sap-sucking herbivores: above ground aphids, their antagonists, and root-feeding nematodes. As plant diversity treatments, we used field plots sown with high (15 plant species, HSD) or low (4 plant species, LSD) diverse seed mixtures in 1996 and that were not weeded. Creeping thistle became established spontaneously at the start of the experiment. In 2002, in HSD plots, 90 % of the plant community was made up by 11 species, compared to seven species in LSD plots. No differences were found for C. arvense abundance or biomass. Above ground, three aphid species were found on C. arvense-Uroleucon cirsii, Aphis fabae, and Macrosiphum euphorbiae, but the latter was found only in low densities. Significantly more aphid species were found on individual plants in HSD plots. Moreover, in HSD plots, on average 10 % of aphids were parasitized, while no parasitism was observed in LSD plots. In the root zone of C. arvense, significantly more nematodes were found in HSD than in LSD plots, and a significantly higher proportion of those nematodes were plant parasites. The dominant plant parasitic nematode in both treatments was Paratylenchus. We conclude that biotic resistance by natural enemies may be enhanced by plant species diversity, but that above- and below ground sap-sucking herbivores do not necessarily have to respond similarly to the diversity of the surrounding plant community.

Impacts of rising atmospheric carbon dioxide on model terrestrial ecosystems

In model terrestrial ecosystems maintained for three plant generations at elevated concentrations of atmospheric carbon dioxide, increases in photosynthetically fixed carbon were allocated below ground, raising concentrations of dissolved organic carbon in soil. These effects were then transmitted up the decomposer food chain. Soil microbial biomass was unaffected, but the composition of soil fungal species changed, with increases in rates of cellulose decomposition. There were also changes in the abundance and species composition of Collembola, fungal-feeding arthropods. These results have implications for long-term feedback processes in soil ecosystems that are subject to rising global atmospheric carbon dioxide concentrations.