A great diversity of plants and fungi engage in mycorrhizal associations. In natural habitats, and in an ecologically meaningful time span, these associations have evolved to improve the fitness of both plant and fungal symbionts. In systems managed by humans, mycorrhizal associations often improve plant productivity, but this is not always the case. Mycorrhizal fungi might be considered to be parasitic on plants when net cost of the symbiosis exceeds net benefits. Parasitism can be developmentally induced, environmentally induced, or possibly genotypically induced. Morphological, phenological, and physiological characteristics of the symbionts influence the functioning of mycorrhizas at an individual scale. Biotic and abiotic factors at the rhizosphere, community, and ecosystem scales further mediate mycorrhizal functioning. Despite the complexity of mycorrhizal associations, it might be possible to construct predictive models of mycorrhizal functioning. These models will need to incorporate variables and parameters that account for differences in plant responses to, and control of, mycorrhizal fungi, and differences in fungal effects on, and responses to, the plant. Developing and testing quantitative models of mycorrhizal functioning in the real world requires creative experimental manipulations and measurements. This work will be facilitated by recent advances in molecular and biochemical techniques. A greater understanding of how mycorrhizas function in complex natural systems is a prerequisite to managing them in agriculture, forestry, and restoration.

http://max2.ese.u-psud.fr/epc/conservation/PDFs/HIPE/Johnson1997.pdf

Functioning of mycorrhizal associations along the mutualism–parasitism continuum*

This paper reviews the current knowledge of microbial processes affecting C sequestration in agroecosystems. The microbial contribution to soil C storage is directly related to microbial community dynamics and the balance between formation and degradation of microbial byproducts. Soil microbes also indirectly influence C cycling by improving soil aggregation, which physically protects soil organic matter (SOM). Consequently, the microbial contribution to C sequestration is governed by the interactions between the amount of microbial biomass, microbial community structure, microbial byproducts, and soil properties such as texture, clay mineralogy, pore-size distribution, and aggregate dynamics. The capacity of a soil to protect microbial biomass and microbially derived organic matter (MOM) is directly and/or indirectly (i.e., through physical protection by aggregates) related to the reactive properties of clays. However, the stabilization of MOM in the soil is also related to the efficiency with which microorganisms utilize substrate C and the chemical nature of the byproducts they produce. Crop rotations, reduced or no-tillage practices, organic farming, and cover crops increase total microbial biomass and shift the community structure toward a more fungal-dominated community, thereby enhancing the accumulation of MOM. A quantitative and qualitative improvement of SOM is generally observed in agroecosystems favoring a fungal-dominated community, but the mechanisms leading to this improvement are not completely understood. Gaps within our knowledge on MOM-C dynamics and how they are related to soil properties and agricultural practices are identified.

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Bacterial and Fungal Contributions to Carbon Sequestration in Agroecosystems

The paper focuses on conservation agriculture (CA), defined as minimal soil disturbance (no-till, NT) and permanent soil cover (mulch) combined with rotations, as a more sustainable cultivation system for the future. Cultivation and tillage play an important role in agriculture. The benefits of tillage in agriculture are explored before introducing conservation tillage (CT), a practice that was borne out of the American dust bowl of the 1930s. The paper then describes the benefits of CA, a suggested improvement on CT, where NT, mulch and rotations significantly improve soil properties and other biotic factors. The paper concludes that CA is a more sustainable and environmentally friendly management system for cultivating crops. Case studies from the rice-wheat areas of the Indo-Gangetic Plains of South Asia and the irrigated maize-wheat systems of Northwest Mexico are used to describe how CA practices have been used in these two environments to raise production sustainably and profitably. Benefits in terms of greenhouse gas emissions and their effect on global warming are also discussed. The paper concludes that agriculture in the next decade will have to sustainably produce more food from less land through more efficient use of natural resources and with minimal impact on the environment in order to meet growing population demands. Promoting and adopting CA management systems can help meet this goal.

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The role of conservation agriculture in sustainable agriculture.

Summary • Numerous field studies have measured mycorrhizal dynamics under additions of nitrogen (N), phosphorus (P), or atmospheric CO 2 to test the hypothesis that plants should invest in mycorrhizal fungi when soil nutrients are limiting. • Here meta-analyses were used to integrate nutrient responses across independent field-based studies. Responses were compared between ecto- and arbuscular mycorrhizal fungi, and among fertilizer types, methods of measurement, biomes, and lead investigators. Relationships between degree of response and study length, fertilization rates, total amounts of nutrients applied, and numbers of replicates were also tested. • Across studies, mycorrhizal abundance decreased 15% under N fertilization and 32% under P fertilization. Elevated CO 2 elicited a 47% increase. Nitrogen effects varied significantly among studies, and P effects varied significantly among lead investigators. Most other factors did not affect mycorrhizal responses. • These results support the plant investment hypothesis, and suggest that global standing stocks of mycorrhizal fungi may increase substantially under elevated CO 2 but decline moderately under P additions. Effects of N deposition may be difficult to predict for individual ecosystems, with a slightly negative influence overall.

/pdf/a-meta-analysis-of-mycorrhizal-responses-to-nitrogen-510ilwlfir.pdf

A meta‐analysis of mycorrhizal responses to nitrogen, phosphorus, and atmospheric CO2 in field studies

Because of their ability to transform atmospheric N2 into ammonia that can be used by the plant, researchers were originally very optimistic about the potential of associative diazotrophic bacteria to promote the growth of many cereals and grasses. However, multiple inoculation experiments during recent decades failed to show a substantial contribution of Biological Nitrogen Fixation (BNF) to plant growth in most cases. It is now clear that associative diazotrophs exert their positive effects on plant growth directly or indirectly through (a combination of) different mechanisms. Apart from fixing N2, diazotrophs can affect plant growth directly by the synthesis of phytohormones and vitamins, inhibition of plant ethylene synthesis, improved nutrient uptake, enhanced stress resistance, solubilization of inorganic phosphate and mineralization of organic phosphate. Indirectly, diazotrophs are able to decrease or prevent the deleterious effects of pathogenic microorganisms, mostly through the synthesis of anti...

Plant Growth-Promoting Effects of Diazotrophs in the Rhizosphere

We selected three crop production practices; crop rotation, tillage and phosphorus fertilization, all known to affect arbuscular mycorrhiza (AM) development, to study early AM intraradical colonization in maize. Half of the plots were planted during the first year with either a host (maize, Zea mays L.) or a non-host (canola, Brassica napus L.) crop, and all of them with maize for the second year. Tillage and P fertilization treatments were applied to the plots in the second year. Mycorrhiza development in maize was measured in pot culture bioassays conducted before planting and after harvest of the previous and the subsequent crops, and in the field during the second crop season. Previous cropping of a soil with canola (Brassica napus L.), a non-host plant species, delayed mycorrhiza development of maize in a bioassay conducted with that soil in comparison with a previous cropping cycle with maize (Zea mays L.) or with the original plant species in the field site, bromegrass (Bromus inermis Leys.) and alfalfa (Medicago sativa L.). The delay in mycorrhiza development after cropping with canola was also observed in samples taken from the field and in a bioassay, both conducted at the beginning of the subsequent cropping cycle. Tillage had, on average, little effect on intraradical colonization either in the field or in the bioassays. Phosphorus fertilization also had little effect on mycorrhiza development in the field. Crop rotation with a non-host had the strongest effect on intraradical mycorrhiza development of the three practices studied.

Changes in mycorrhiza development in maize induced by crop management practices

The importance of vesicular arbuscular mycorrhizas (VAM) in an agricultural crop production system depends largely on our ability, through soil management, to increase the effectiveness of the indigenous mycorrhizal fungal population. To do so requires a good understanding of the functional ecology of the symbiosis.In this article, we discuss primarily our programs at Guelph, which have focused on two aspects of the symbiosis: the influence of phosphate (P) fertilization on colonization and the influence of soil disturbance by tillage on colonization and P absorption.Although it is generally accepted that the level of colonization of roots by VAM fungi decreases with increased P availability, we have found that the decrease is not as marked as thought. A reasonable degree of colonization was observed at available P levels well above those required for maximum yield. We have also found that the reduction in colonization occurs to a greater extent in the roots growing in a fertilized zone than in th...

Functional Ecology of Vesicular Arbuscular Mycorrhizas as Influenced by Phosphate Fertilization and Tillage in an Agricultural Ecosystem

Tillage translocation and tillage erosion were measured throughout the topographically complex landscapes of two fields in the upland region of southwestern Ontario. Translocation of soil by tillage was measured by labelling plots of soil with chloride and measuring the tracer's forward displacement in response to single passes by four tillage implements (mouldboard plough, chisel plough, tandem disc and field cultivator). The change in translocation within the landscape was used to measure tillage erosion. All four implements were erosive. A relationship between tillage translocation and slope gradient was observed; however, the variability in translocation could not be explained by slope gradient alone. Slope curvature was responsible for some translocation through the planning action of tillage implements. Tillage depth and speed were subject to considerable discontinuous and inconsistent manipulation by the operator in response to changing topographic and soil conditions. Tillage speed decreased by as much as 60% during upslope tillage and increased by as much as 30% during downslope tillage, relative to that on level ground. Tillage depth decreased by as much as 20% and increased by as much as 30%, relative to that on level ground. This manipulation is typical for tillage in complex landscapes and was presumed largely responsible for the variability in the results. The manipulation of tillage depth and speed are affected by the tractor-implement match and the responsiveness of the tillage operator.

Tillage translocation and tillage erosion in the complex upland landscapes of southwestern Ontario, Canada

Mycorrhizal Development and Phosphorus Absorption in Maize under Conventional and Reduced Tillage

We conducted a field experiment to test the hypothesis that improved phosphorus nutrition occurs in maize plants with rapid arbuscular (AM) mycorrhizae development at early developmental stages and that this also is reflected in dry matter allocation and final yield. A split-split plot design was used with previous crop (Zea mays L.-maize and Brassica napus L.-canola), tillage practices (no-tillage or conventional tillage) and P fertilization (5 levels) as factors chosen to modify mycorrhizae development at early developmental stages of maize. Previous cropping with canola resulted in decreased shoot-P concentration and shoot growth of maize at early stages. No-tillage resulted in higher shoot-P concentration but lower shoot weight than conventional tillage. Greater shoot-P uptake was related to a rapid intraradical development of mycorrhizae (previous crop of maize) or rapid connection to a mycorrhizal mycelium network (no-tillage treatments). Maize yield and harvest index were lower after cropping with canola. The yield for conventional tillage was higher than that for no-tillage but the harvest index was lower. The hypothesis was supported at early stages of maize growth by the effect of previous crop but not by results of tillage, because an unknown factor reduced growth in the no-tillage system. The hypothesis was supported at maturity by increased biomass allocation to grain relative to total shoot weight in treatments with greater shoot-P concentration at early stages.

Murray H. Miller

Papers

Changes in mycorrhiza development in maize induced by crop management practices

Functional Ecology of Vesicular Arbuscular Mycorrhizas as Influenced by Phosphate Fertilization and Tillage in an Agricultural Ecosystem

Tillage translocation and tillage erosion in the complex upland landscapes of southwestern Ontario, Canada

Mycorrhizal Development and Phosphorus Absorption in Maize under Conventional and Reduced Tillage

Early phosphorus nutrition, mycorrhizae development, dry matter partitioning and yield of maize