Publisher Summary This chapter discusses the ecological implications of mycorrhizal associations in natural ecosystems and the role of soil or environmental factors, mycorrhizal fungus characteristics or host plant properties. Mycorrhizal associations are regulated by features of the host plant and mycorrhizal fungus, as well as by soil conditions and environmental factors. Factors which can influence the occurrence and effectiveness of mycorrhizal associations include (1) root properties (2) edaphic or climatic factors (3) soil organisms, (4) soil disturbance, and (5) host-fungus compatibility. Several other complex ecological topics also discussed in the chapter include (1) mycorrhizal phenology , (2) factors responsible for varying degrees of mycorrhizal dependency in host plants, (3) the role of mycorrhizal hyphae in soil (4) nutrient competition involving mycorrhizal and non-mycorrhizal plants, and (5) mycorrhizal interactions involving pollution and other stresses, the rhizosphere, soil properties and allelopathy. Finally, the chapter also discusses the population ecology of mycorrhizal fungi and the influence of their associations on plant population ecology.

Mycorrhizas in Natural Ecosystems

Farmers in mixed crop-livestock systems produce about half of the world’s food. In small holdings around the world, livestock are reared mostly on grass, browse, and nonfood biomass from maize, millet, rice, and sorghum crops and in their turn supply manure and traction for future crops. Animals act as insurance against hard times and supply farmers with a source of regular income from sales of milk, eggs, and other products. Thus, faced with population growth and climate change, small-holder farmers should be the first target for policies to intensify production by carefully managed inputs of fertilizer, water, and feed to minimize waste and environmental impact, supported by improved access to markets, new varieties, and technologies.

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Smart investments in sustainable food production: revisiting mixed crop-livestock systems.

The nature of the interaction between plants ant their soil community was investigated by testing for differential responses of four old—field perennial plant species to inocula derived from soil communities that had been grown with ("cultured by") one of these four plant species. The differentiation of the soil communities was evident in measurements of plant survival, phenology, growth, and root—shoot ratios. Effects on survival and growth suggest negative feedbacks between these species and the soil communities that they culture. Survival rates of Krigia dandelion were significantly reduced when grown with their own" soil community. Considered as a whole, the three other species (all grasses) had significantly lower growth and root—shoot ratios when grown with soil communities started with their own inocula compared to soil communities started with the inocula of other species. However, the significance of this effect on growth rate and root—shoot ratios was due primarily to the pairwise comparison of Anthoxanthum odoratum and Danthonia spicata and of Anthoxanthum and Panicum sphaerocarpon, respectively. Pairwise comparisons of Danthonia and Panicum do not suggest differential responses to each other's soils in growth rate or root—shoot ratios nor do soil communities appear to affect the relative competitive ability of these two species. While the components of the soil community responsible for these effects have not been identified, similar although less pronounced patterns were observed in experiments using inocula consisting of washed live root segments as compared to experiments using whole soil as inoculum, suggesting that root pathogens are one important agent.

https://www.jstor.org/stable/info/1941601

Feeback between Plants and Their Soil Communities in an Old Field Community

Heavy metal pollution is one of the most troublesome environmental problems faced by mankind nowadays. Copper, in particular, poses serious problems due to its widespread industrial and agricultural use. Unlike other heavy metals, such as cadmium, lead, and mercury, copper is not readily bioaccumulated and thus its toxicity to man and other mammals is relatively low. On the contrary, plants in general are very sensitive to Cu toxicity, displaying metabolic disturbances and growth inhibition at Cu contents in the tissues only slightly higher than the normal levels. The reduced mobility of Cu in soil and sediments, due to its strong binding to organic and inorganic colloids, constitutes, in a way, a barrier to Cu toxicity in land plants. In aqueous media, however, plants are directly exposed to the harmful effects of Cu and, thus, algae and some species of aquatic higher plants are more easily subjected to Cu toxicity. Excess Cu inhibits a large number of enzymes and interferes with several aspects of plant biochemistry, including photosynthesis, pigment synthesis, and membrane integrity. Perhaps its most important effect is associated with the blocking of photosynthetic electron transport, leading to the production of radicals which start peroxidative chain reactions involving membrane lipids. Copper effects on plant physiology are wide ranging, including interference with fatty acid and protein metabolism and inhibition of respiration and nitrogen fixation processes. At the whole plant level Cu is an effective inhibitor of vegetative growth and induces general symptoms of senescence. The high toxicity of Cu to plants has led to the evolution of several strategies of defense. Among the most important ones is the production of Cu-complexing compounds. Although the nature, structure, and function of these compounds is still controversial, they can be divided into two main groups: metallothionein-like compounds and phytochelatins. The latter appears to constitute the most widespread response of plants to stresses provoked by metals, including Cu.

Biochemical, physiological, and structural effects of excess copper in plants

Plant roots in natural ecosystems are typically colonized by a wide range of fungi. Some of these are pathogenic, others appear to be opportunistic and have no apparent impact, while mycorrhizal fungi are generally regarded as mutualistic. Of the various types of mycorrhizal fungi, the arbuscular mycorrhizal (AM) association is by far the most abundant and widespread. While the most widely accepted model of AM function depends upon plants benefiting from the facilitation of phosphorus uptake, recent data from field-based studies in temperate ecosystems indicate that only plant species with poorly branched root systems benefit from AM fungi in this way: species with highly branched root systems may benefit in other ways, such as by being protected against root pathogenic fungi. These two responses apparently represent extremes along a continuum of AM benefit determined by root system architecture.

Multi-functionality and biodiversity in arbuscular mycorrhizas

This guide is the first in a series on the practical application of FAO's Save and Grow model of agriculture to specific smallholder crops and farming systems. It comes as cassava production intensifies worldwide, and growers shift from traditional cultivation to monocropping, higher-yielding varieties, and increased use of irrigation and agrochemicals. Intensification carries great risks, including soil degradation and upsurges in pests and diseases. The guide shows how ecosystem-based approaches and practices can help tropical countries to avoid the risks of unsustainable intensification, while realising cassava's potential for producing higher yields, alleviating hunger and rural poverty, and contributing to national economic development.

Save and grow : cassava : a guide to sustainable production intensification

Cassava is generally grown by poor farmers living in marginal areas with adverse climatic and soil conditions. The crop is very suitable for these conditions because of its exceptional tolerance to drought and to acid, infertile soils. It is often grown on sloping land because of its minimal requirement for land preparation, and its ability to produce reasonably good yields on eroded and degraded soils, where other crops would fail. It has been shown (Quintiliano et al., 1961; Margolis and Campos Filho, 1981; Putthacharoen et al., 1998), however, that growing cassava on slopes can result in severe erosion, with high soil and nutrient losses. Thus cassava cultivation on slopes requires adequate cultural and soil conservation practices that minimize erosion (Howeler, 1994). Cassava is well adapted to poor or degraded soils because of its tolerance to low pH, high levels of exchangeable aluminium (Al) and low concentrations of phosphorus (P) in the soil solution. Studying the effect of pH on the growth of several crops grown in flowing nutrient solution, Islam et al. (1980) reported that cassava and ginger (Zingiber officinale) were more tolerant of low pH (< 4) than tomatoes (Lycopersicon esculentum), wheat (Triticum aestivum L.) or maize (Zea mays L.). Centro Internacional de Agricultura Tropical (CIAT; 1978) and Howeler (1991a) also reported that cassava and cowpeas (Vigna unguiculata) were more tolerant of acid soils with high levels of exchangeable Al, and were much less responsive to lime applications than common beans (Phaseolus vulgaris), rice (Oryza sativa), maize and sorghum (Sorghum vulgaris).

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Cassava mineral nutrition and fertilization

Greenhouse and field experiments were conducted on the effect of VA mycorrhiza (VAM) on the growth of cassava, various tropical grass and legume species, as well as beans, coffee and tea. A large number of VAM fungal species were evaluated for effectivity in increasing cassava growth and P uptake in acid low-P soils. The effectivity of VAM species and isolates was highly variable and dependent on soil pH and fertilizer applications, as well as on soil temperature and humidity. Two species, Glomus manihotis and Entrophospora colombiana were found to be most effective for a range of crops and pastures, at low pH and at a wide range of N, P, and K levels.

Reinhardt H. Howeler

Papers

Save and grow : cassava : a guide to sustainable production intensification

Cassava mineral nutrition and fertilization

Practical aspects of mycorrhizal technology in some tropical crops and pastures

Long-term effect of cassava cultivation on soil productivity

Accumulation and distribution of dry matter and nutrients during a 12-month growth cycle of cassava