Tropical land use alters functional diversity of soil food webs and leads to monopolization of the detrital energy channel

A global meta-analysis reveals effects of heavy metals on soil microorganisms

Heavy metal (HM) contamination disrupts soil ecosystem functions. Microorganisms are pivotal for sustaining soil health, but accurately assessing the ecological risks of HM contamination to microorganisms remains challenging. Here, we conducted a meta-analysis synthesizing 914 datasets from 72 studies to quantify and evaluate the impacts of HMs on microorganisms. The overall effect value results indicate that HM negatively impacts most microbiological indicators, with bacterial abundance (-38 %), fungal abundance (-18 %), microbial biomass carbon (-42 %), microbial biomass nitrogen (-44 %), arylsulfatase (-45 %) and dehydrogenase activity (-66 %) were significantly reduced (p < 0.01), suggesting they can act as sensitivity indicators for assessing ecological risk of microorganisms. Compared to bacteria, fungal indicators (e.g., fungal community structure and Shannon index) are less responsive to HM contamination. At low potential ecological risk index (RI < 150), HM contamination positively impacted certain microbial indicators, such as fungal abundance, fungal Shannon index, and β-glucosidase activity. With increasing RI levels, the negative effects of HMs on microorganisms became more pronounced. Microbiological indicators in acidic soils (pH < 6.5), coarse textured soils, and mining soils were more negatively affected by HMs. Random forest and structural equation modeling analysis also identified RI levels and pH as crucial factors in determining the microbial response to HMs. Adjusted RI (adRI) were calculated using adjusted toxicity factors (adTF). The adRI demonstrated stronger correlations with microbial indicators and lower root-mean-square error (RMSE) in the random forest model than the RI, indicating that adTF is a more effective method for evaluating the effects of HMs on microorganisms. This study enhances the accuracy of quantifying and assessing HM impacts on microorganisms, offering crucial scientific basis for environmental protection and soil remediation.

Different Land Use Systems in the Brazilian Cerrado and Their Effects on Soil Bacterial Communities

The effect of agricultural practices on soil bacterial communities is not constant and depends a lot on the climatic context, changes in the soil characteristics, land use, and agricultural strategy. Thus, knowledge about how different land use systems in the Cerrado influence the diversity and taxonomic structure of microbial communities under the same soil type remains limited. In this context, the objective of this work was to analyze and compare the bacterial communities of Cerrado soil under two different land use systems (cover crop and potato cultivation) and in a neighboring native Cerrado area. For this, we used high-throughput amplicon sequencing of 16S rRNA genes (metabarcoding) to characterize the bacterial community at different taxonomic levels in a native Cerrado area, in a potato crop area, and in an area with cover crops. Our data indicated significant impacts on soil physicochemical properties and enzymatic activity, which directly reflect the dynamics of bacterial communities. The three bacterial phyla with the highest relative abundance in the three areas were Proteobacteria, Actinobacteriota, and Acidobacteriota. At the taxonomic class level, small variations were observed among areas, while at the amplicon sequence variant (ASV) level, these variations were more pronounced. The alpha diversity indices showed that the bacterial communities among the areas are rich and diverse. Bray-Curtis and Jaccard distance-based PCoA demonstrated an overlap of bacterial communities present in the cover crop area with the native Cerrado area and separation from the potato cultivation area. The in silico prediction demonstrated that the native Cerrado area presented the highest values of functional diversity of the soil bacterial community compared to the others. Thus, our results provide a holistic view of how different land use systems in the Cerrado can influence the taxonomic and functional diversity of soil bacterial communities.

High litter quality enhances plant energy channeling by soil macro-detritivores and lowers their trophic position

Detritus-based resources, that is, plant litter, are a major energy source for many living organisms and are considered to be a key determinant of primary production and nutrient cycling. Earthworms are among the most important macro-detritivores in terrestrial food webs and play a crucial role in facilitating these processes in terrestrial ecosystems. Yet, the influence of litter quality on earthworm nutrition, and consequently on soil food web dynamics, has remained largely underexplored, mainly for methodological reasons. Here, we combined bulk and compound-specific stable isotope analysis of amino acids to investigate the dietary contribution of different quality litter resources to earthworm species of different ecological groups. Our findings show that earthworms acquired most essential amino acids from bacterial (~60%) and plant (~30%) resources, with the latter increasing in importance with higher litter quality, resulting in lower trophic positions across earthworm species. The high bacterial contribution to earthworms corresponds to the dominance of bacteria in the experimental soil, suggesting that bacteria served as an important intermediate link in transferring detritus-based resources to earthworms. Bacterial contributions were notably higher in the soil-feeding earthworm species than in the litter-feeding earthworm species, likely due to more pronounced ingestion of soil by soil-feeding earthworms. Overall, our study indicates that a major group of soil macro-detritivores, earthworms, receive detrital resources via the bacterial energy channel. Further, it underscores the important role of litter quality in shaping the trophic niches of detritivores, thereby influencing the overall structure of soil food webs.

The productivity-stability trade-off in global food systems

Historically, humans have managed food systems to maximize productivity. This pursuit has drastically modified terrestrial and aquatic ecosystems globally by reducing species diversity and body size while creating very productive, yet homogenized, environments. Such changes alter the structure and function of ecosystems in ways that ultimately erode their stability. This productivity-stability trade-off has largely been ignored in discussions around global food security. Here, we synthesize empirical and theoretical literature to demonstrate the existence of the productivity-stability trade-off and argue the need for its explicit incorporation in the sustainable management of food systems. We first explore the history of human management of food systems, its impacts on average body size within and across species and food web stability. We then demonstrate how reductions in body size are symptomatic of a broader biotic homogenization and rewiring of food webs. We show how this biotic homogenization decompartmentalizes interactions among energy channels and increases energy flux within the food web in ways that threaten their stability. We end by synthesizing large-scale ecological studies to demonstrate the prevalence of the productivity-stability trade-off. We conclude that management strategies promoting landscape heterogeneity and maintenance of key food web structures are critical to sustainable food production.

Rainforest transformation reallocates energy from green to brown food webs

Terrestrial animal biodiversity is increasingly being lost because of land-use change1,2. However, functional and energetic consequences aboveground and belowground and across trophic levels in megadiverse tropical ecosystems remain largely unknown. To fill this gap, we assessed changes in energy fluxes across 'green' aboveground (canopy arthropods and birds) and 'brown' belowground (soil arthropods and earthworms) animal food webs in tropical rainforests and plantations in Sumatra, Indonesia. Our results showed that most of the energy in rainforests is channelled to the belowground animal food web. Oil palm and rubber plantations had similar or, in the case of rubber agroforest, higher total animal energy fluxes compared to rainforest but the key energetic nodes were distinctly different: in rainforest more than 90% of the total animal energy flux was channelled by arthropods in soil and canopy, whereas in plantations more than 50% of the energy was allocated to annelids (earthworms). Land-use change led to a consistent decline in multitrophic energy flux aboveground, whereas belowground food webs responded with reduced energy flux to higher trophic levels, down to -90%, and with shifts from slow (fungal) to fast (bacterial) energy channels and from faeces production towards consumption of soil organic matter. This coincides with previously reported soil carbon stock depletion3. Here we show that well-documented animal biodiversity declines with tropical land-use change4-6 are associated with vast energetic and functional restructuring in food webs across aboveground and belowground ecosystem compartments.

Isopod mouthpart traits respond to a tropical forest recovery gradient

Functional trait ecology has the potential to provide generalizable and mechanistic predictions of ecosystem function from data of species distributions and traits. The traits that are selected should both respond to environmental factors and influence ecosystem functioning. Invertebrate mouthpart traits fulfill these criteria, but are seldom collected, lack standardized measurement protocols, and have infrequently been investigated in response to environmental factors. We surveyed isopod species that consume plant detritus, and tree communities in 58 plots across primary and secondary forests in Singapore. We measured body dimensions (body size traits), pereopod and antennae lengths (locomotory traits), dimensions of mandible structures (morphological mouthpart traits), and mechanical advantages generated by mandible shape (mechanical mouthpart traits) for six isopod species found in these plots and investigated if these traits respond to changes in tree community composition, tree diversity, and forest structure. Morphological mouthpart traits responded to a tree compositional gradient reflecting forest recovery degree. Mouthpart features associated with greater consumption of litter (broader but less serrated/rugose lacinia mobilis [an important cutting and chewing structure on the mandible]) were most prevalent in abandoned plantation and young secondary forests containing disturbance-associated tree species. Feeding strategies associated with fungi grazing (narrower and more serrated/rugose lacinia mobilis) were most prevalent in late secondary forests containing later successional tree species. Since morphological mouthpart traits likely also predict consumption and excretion rates of isopods, these traits advance our understanding of environment-trait-ecosystem functioning relationships across contrasting tropical forest plots that vary in composition, disturbance history, and post-disturbance recovery.

Heavy metal pollution triggers a shift from bacteria-based to fungi-based soil micro-food web: Evidence from an abandoned mining-smelting area

Heavy metals pose significant threats to soil biota, ultimately disrupting soil micro-food web. However, no studies have yet elucidated the impact of heavy metals on soil micro-food web. In this study, we explored the response of bacteria, fungi, nematodes, and soil micro-food web along a gradient of heavy metals in an abandoned smelting-mining area. We found that bacteria responded strongly to heavy metals, whereas fungi showed greater resistance and tolerance. Nematodes responses were less apparent. With the increasing levels of heavy metal pollution, the importance of heavy metal-tolerant organisms in micro-food webs increased significantly. For instance, the keystone bacteria in soil micro-food web shifted from copiotrophic to oligotrophic types, while the keystone nematodes shifted from to bacterial-feeding (e.g., Eucephalobus) to fungal-feeding species (e.g., Ditylenchus). Additionally, elevated heavy metal concentrations increased the proportion of fungi (e.g., Mortierellomycota), intensifying their interactions with bacteria and nematodes and causing a shift from bacteria-based to fungi-based soil micro-food web. Furthermore, heavy metal contamination induced a more complex and stable soil micro-food web. Overall, we highlight the changes in soil micro-food web as a mechanism for coping with heavy metal stress. Our study provides valuable insights into how heavy metal pollution can cause shifts in soil micro-food webs and has critical implications for enhancing our understanding of the ecological consequences of environmental pollution at the ecosystem level.

Plant roots fuel tropical soil animal communities

Belowground life relies on plant litter, while its linkage to living roots had long been understudied, and remains unknown in the tropics. Here, we analysed the response of 30 soil animal groups to root trenching and litter removal in rainforest and plantations in Sumatra, and found that roots are similarly important to soil fauna as litter. Trenching effects were stronger in soil than in litter, with an overall decrease in animal abundance in rainforest by 42% and in plantations by 30%. Litter removal little affected animals in soil, but decreased the total abundance by 60% in rainforest and rubber plantations but not in oil palm plantations. Litter and root effects on animal group abundances were explained by body size or vertical distribution. Our study quantifies principle carbon pathways in soil food webs under tropical land use, providing the basis for mechanistic modelling and ecosystem-friendly management of tropical soils.