Despite numerous future promises, there is a multitude of concerns about the impact of GM crops on the environment. Key issues in the environmental assessment of GM crops are putative invasiveness, vertical or horizontal gene flow, other ecological impacts, effects on biodiversity and the impact of presence of GM material in other products. These are all highly interdisciplinary and complex issues. A crucial component for a proper assessment is defining the appropriate baseline for comparison and decision. For GM crops, the best and most appropriately defined reference point is the impact of plants developed by traditional breeding. The latter is an integral and accepted part of agriculture. In many instances, the putative impacts identified for GM crops are very similar to the impacts of new cultivars derived from traditional breeding. When assessing GM crops relative to existing cultivars, the increased knowledge base underpinning the development of GM crops will provide greater confidence in the assurances plant science can give on the risks of releasing such crops.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.0960-7412.2003.01602.x

The release of genetically modified crops into the environment. Part II. Overview of ecological risk assessment.

The fungi-to-bacteria ratio in soil ecological concepts and its application to explain the effects of land use changes have gained increasing attention over the past decade. Four different main approaches for quantifying the fungal and bacterial contribution to microbial tissue can be distinguished: (1) microscopic methods, (2) selective inhibition, (3) specific cell membrane components and (4) specific cell wall components. In this review, the different methods were compared and we hypothesized that all these approaches result in similar values for the fungal and bacterial contribution to total microbial biomass, activity, and residues (dead microbial tissue) if these methods are evenly reliable for the estimation of fungal biomass. The fungal contribution to the microbial biomass or respiration varied widely between 2 and 95% in different data sets published over the past three decades. However, the majority of the literature data indicated that fungi dominated microbial biomass, respiration or non-biomass microbial residues, with mean percentages obtained by the different methodological approaches varying between 35 and 76% in different soil groups, i.e. arable, grassland, and forest soils and litter layers. Microscopic methods generally gave the lowest average values, especially in arable and grasslands soils. Very low ratios in fungal biomass C-to-ergosterol obtained by microscopic methods suggest a severe underestimation of fungal biomass by certain stains. Relatively consistent ratios of ergosterol to linoleic acid (18:2ω6,9) indicate that both cell membrane components are useful indicators for saprotrophic and ectomycorrhizal fungi. More quantitative information on the PLFA content of soil bacteria and the 16:1ω5 content of arbuscular mycorrhizal fungi is urgently required to fully exploit the great potential of PLFA measurements. The most consistent results have been obtained from the analysis of fungal glucosamine and bacterial muramic acid in microbial residues. Component-specific δ 13 C analyses of PLFA and amino sugars are a promising prospect for the near future.

Quantitative assessment of the fungal contribution to microbial tissue in soil

This microcosm study is concerned with understanding those factors which regulate ecosystem processes of nutrient cycling and plant productivity in a montane grassland ecosystem. We examined the effects of different groups of soil fauna, namely bacterial-feeding nematodes and Collembola, on nutrient mineralization (N and P) in an acid, organic soil taken from a montane grassland in the Peak District National Park, United Kingdom. We also examined whether faunal influences on nutrient release, a measure of nutrient mineralization, resulted in changes in nutrient uptake and biomass production of an indigenous montane grass species (Nardus stricta (L.)). We found that in the presence of Collembola, and when nematodes and Collembola were combined, N mineralization, nutrient leaching and shoot N contents of N. stricta was significantly increased relative to a defaunated control. We also found that net P mineralization and leaching increased (although not significantly) in the presence of both nematodes and Collembola, resulting in a significant increase in shoot P content of N. stricta. The presence of nematodes alone, which were largely bacterial-feeders, had no effect on the mineralization of N or P, or shoot nutrient content. We suggest that differences in the effect of faunal treatments on nutrient mineralization are related to the feeding strategies of the added fauna, and to their consequent effect on the size of the soil microbial biomass. The treatments that increased N mineralization and plant nutrient content (N and P) also significantly reduced plant growth (shoot and root). We suggest that high NH4+–N concentrations in the soil solution of Collembola treatments inhibited the growth of N. stricta and that the growth of other grassland species may benefit from this improvement in nutrient availability.

Experimental evidence that soil fauna enhance nutrient mineralization and plant nutrient uptake in montane grassland ecosystems

This review focuses on literature pertaining to the interactions of soil yeasts with biotic and abiotic factors in their environment. Soil yeasts not only affect microbial and plant growth, but may also play a role in soil aggregate formation and maintenance of soil structure. Serving as a nutrient source for bacterial, faunal and protistan predators, soil yeasts contribute to essential ecological processes such as the mineralization of organic material and dissipation of carbon and energy through the soil ecosystem. Some soil yeasts may also play a role in both the nitrogen and sulphur cycles and have the ability to solubilize insoluble phosphates making it more readily available for plants. Recently, the potential of soil yeasts as plant growth promoters and soil conditioners has been studied with the goal of using them in the field of sustainable agriculture.

The importance and ecology of yeasts in soil

Trophic interactions play a vital role in soil functioning and are increasingly considered as important drivers of the soil microbiome and biogeochemical cycles. In the last decade, novel tools to decipher the structure of soil food webs have provided unprecedent advance in describing complex trophic interactions. Yet, the major challenge remains to understand the drivers of the trophic interactions. Evidence suggests that small scale soil physical structure may offer a unifying framework for understanding the nature and patterns of trophic interactions in soils. Here, we review the current knowledge of how restrictions on soil organisms’ ability to sense and access food resources/prey inherent to soil physical structure essentially shape trophic interactions. We focus primarily on organisms unable to deform the soil and create pores themselves, such as bacteria, fungi, protists, nematodes and microarthropods, and consider pore geometry, connectivity and hydration status as main descriptors of the soil physical structure. We point that the soil physical structure appears to mostly limit the sensing and accessibility to food resources/prey, with negative effects on bottom up controls. The main mechanisms are (i) the reduced transport of sensing molecules, notably volatiles, through the soil matrix and (ii) the wide presence of refuges leading to pore size segregation of consumer/predators and food sources/prey in pores of contrasting size. In addition, variations in the connectivity of the soil pores and the water film is suggested as a central aspect driving encounter probability between consumers/predator and food source/prey and hence locally decrease or increase top-down controls. Constraints imposed by the soil physical structure on trophic interactions are thought to be major drivers of the soil diversity and local community assemblage, notably by favoring a variety of adaptations to feed in this dark labyrinth (food specialists/flexible/generalists) and by limiting competitive exclusion through limited encounter probability of consumers. We conclude with possible future ways for an interdisciplinary and more quantitative research merging soil physics and soil food web ecology.

The physical structure of soil: Determinant and consequence of trophic interactions

Ecology and biology of soil protozoa, nematodes, and microarthropods.

Interactions between microbe-feeding invertebrates and soil microorganisms.

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Ecology and biology of soil protozoa, nematodes, and microarthropods.

Interactions between microbe-feeding invertebrates and soil microorganisms.