Soil resource supply influences faunal size–specific distributions in natural food webs

The large range of body-mass values of soil organisms provides a tool to assess the ecological organization of soil communities. The goal of this paper is to identify graphical and quantitative indicators of soil community composition and ecosystem functioning, and to illustrate their application to real soil food webs. The relationships between log-transformed mass and abundance of soil organisms in 20 Dutch meadows and heathlands were investigated. Using principles of allometry, maximal use can be made of ecological theory to build and explain food webs. The aggregate contribution of small invertebrates such as nematodes to the entire community is high under low soil phosphorus content and causes shifts in the mass–abundance relationships and in the trophic structures. We show for the first time that the average of the trophic link lengths is a reliable predictor for assessing soil fertility responses. Ordered trophic link pairs suggest a self-organizing structure of food webs according to resource availability and can predict environmental shifts in ecologically meaningful ways

Changes in soil microbial and nematode communities during ecosystem decline across a long-term chronosequence

Abstract Following the creation of new land surfaces, there is an initial build-up phase, but in the prolonged absence of catastrophic disturbance an ecosystem decline phase has often been observed. While a number of studies have investigated the changes in soil biota that occur during the build-up phase, few studies have investigated how the soil food web changes during the ecosystem decline phase, even though such studies may assist our understanding of biotic factors that contribute to long-term ecosystem changes. We investigated the response of soil microbial and nematode communities to ecosystem decline by studying each of four stages of a long-term (280,000 year) forested chronosequence caused by uplift of marine terraces in the Waitutu region of Fiordland National Park, New Zealand. With increasing chronosequence age there were large increases in ratios of C to N, C to P, and N to P in both the organic layer and mineral soil layer, indicative of greater nutrient (notably P) limitation over time. Variables related to soil microbial biomass and activity were lower on the older terraces when expressed on a per unit soil C basis, reflecting that the quality of the soil organic matter, which is the resource that supports microbial metabolism, declined over time. This in turn had important consequences for population densities of soil nematodes and enchytraeids, including both microbefeeding and predatory groups. There were significant increases in the fungal:bacterial biomass ratio and in the fungal-feeding:bacterialfeeding nematode densities. Taken collectively, our results suggest a decline in soil microbial activity and soil fauna, and an increase in the relative importance of the fungal-based (vs. bacterial-based) energy channel during long-term ecosystem development on terraces of marine origin. This corroborates the hypothesis that the studied sites represent a retrogressive shift in soil organic matter quality over a long-term chronosequence.

The influence of nematodes on below-ground processes in grassland ecosystems.

This review summarises recent information on beneficial roles that soil nematodes play in the cycling of carbon and other plant nutrients in grassland ecosystems. In particular, we focus on the role of the two dominant functional groups of nematodes, namely the microbial- and root-feeders, and how their activities may enhance soil ecosystem-level processes of nutrient cycling and, ultimately, plant productivity in managed and unmanaged grassland ecosystems. We report recent experiments which show that low amounts of root herbivory by nematodes can increase the allocation of photoassimilate carbon to roots, leading to increased root exudation and microbial activity in the rhizosphere. The effects of these interactions on soil nutrient cycling and plant productivity are discussed. Evidence is presented to show that the feeding activities of microbial-feeding nematodes can enhance nutrient mineralization and plant nutrient uptake in grasslands, but that these responses are highly species-specific and appear to be strongly regulated by higher trophic groups of fauna (top-down regulation). We recommend that future studies of the roles of nematodes in grasslands ecosystems should consider these more complex trophic interactions and also the effects of species diversity of nematodes on soil ecosystem-level processes

Community- and trophic-level responses of soil nematodes to removal of a non-native tree at different stages of invasion.

Success of invasive non-native plant species management is usually measured as changes in the abundance of the invasive plant species or native plant species following invader management, but more complex trophic responses to invader removal are often ignored or assumed. Moreover, the effects of invader removal at different stages of the invasion process is rarely evaluated, despite a growing recognition that invader impacts are density or stage-dependent. Therefore, the effectiveness of invasive species management for restoring community structure and function across trophic levels remains poorly understood. We determined how soil nematode diversity and community composition respond to removal of the globally invasive tree species Pinus contorta at different stages of invasion by reanalysing and expanding an earlier study including uninvaded vegetation (seedlings removed continuously), early invader removal (saplings removed), late removal (trees removed), and no removal (invaded). These treatments allowed us to evaluate the stage-dependent belowground trophic responses to biological invasion and removal. We found that invaded plots had half the nematode taxa richness compared to uninvaded plots, and that tree invasion altered the overall composition of the nematode community. Differences in nematode community composition between uninvaded nematode communities and those under the tree removal strategy tended to dilute higher up the food chain, whereas the composition of uninvaded vs. sapling removal strategies did not differ significantly. Conversely, the composition of invaded compared to uninvaded nematode communities differed across all trophic levels, altering the community structure and function. Specifically, invaded communities were structurally simplified compared to uninvaded communities, and had a higher proportion of short life cycle nematodes, characteristic of disturbed environments. We demonstrate that a shift in management strategies for a globally invasive tree species from removing trees to earlier removal of saplings is needed for maintaining the composition and structure of soil nematode communities to resemble uninvaded conditions