Little evidence that farmers should consider abundance or diversity of arbuscular mycorrhizal fungi when managing crops

Abstract

Contents Summary 1092 I. Introduction 1093 II. Investigating activity of AMF in agroecosystems 1093 III. Crop benefit from AMF: agronomic and mycorrhizal literature differ 1094 IV. Flawed methodology leads to benefits of mycorrhizas being overstated 1094 V. Rigorous methodology suggests low colonisation by AMF can sometimes reduce crop yield 1095 VI. Predicting when mycorrhizas matter for crop yield 1096 VII. Crop genotype 1099 VIII. Fungal genotype 1100 IX. Complex interactions between the mycorrhizal fungal and soil microbial communities 1102 X. Phosphorus-efficient agroecosystems 1102 XI. Conclusions 1103 Acknowledgements 1104 References 1104 SUMMARY: Arbuscular mycorrhizal fungi (AMF) are ubiquitous in agroecosystems and often stated to be critical for crop yield and agroecosystem sustainability. However, should farmers modify management to enhance the abundance and diversity of AMF? We address this question with a focus on field experiments that manipulated colonisation by indigenous AMF and report crop yield, or investigated community structure and diversity of AMF. We find that the literature presents an overly optimistic view of the importance of AMF in crop yield due, in part, to flawed methodology in field experiments. A small body of rigorous research only sometimes reports a positive impact of high colonisation on crop yield, even under phosphorus limitation. We suggest that studies vary due to the interaction of environment and genotype (crop and mycorrhizal fungal). We also find that the literature can be overly pessimistic about the impact of some common agricultural practices on mycorrhizal fungal communities and that interactions between AMF and soil microbes are complex and poorly understood. We provide a template for future field experiments and a list of research priorities, including phosphorus-efficient agroecosystems. However, we conclude that management of AMF by farmers will not be warranted until benefits are demonstrated at the field scale under prescribed agronomic management.

Research must use a systems agronomy approach if management of the arbuscular mycorrhizal symbiosis is to contribute to sustainable intensification

“…() note other potential benefits, including biofortification, improved soil aggregation and reduced nutrient leaching. We contend that the literature pertaining to these processes expresses similar undue optimism as described for yield by Ryan & Graham () because of a paucity of field‐relevant experimentation, experimental rigour (in some instances) and agricultural systems context. In this Letter, we concentrate on the last issue, as Ryan & Graham () provide specific examples of the first two.…”

“…We welcome the comments of Rillig et al . () on Ryan & Graham (). A robust debate concerning the need to manage arbuscular mycorrhizal fungi (AMF) in agricultural systems has been long overdue.…”

“…Thus, deliberate management can be warranted which favours AMF, but usually only under certain conditions and contexts. Although the role of AMF for crop yield was the main focus of Ryan & Graham (), Rillig et al . () note other potential benefits, including biofortification, improved soil aggregation and reduced nutrient leaching.…”

Cross‐scale integration of mycorrhizal function

“…The scientific rewards from comprehensive research programs on mycorrhizal symbioses include a greater fundamental understanding of the interactions between organisms at the community level and benefits for sustainable agriculture and forestry. A deeper understanding of the complex array of factors affecting host–fungus interactions and co‐evolution could indeed ensure efficient selection of fungal symbionts for large‐scale inoculation methods in forest and agricultural ecosystems, although all scientists are not so optimistic (Ryan & Graham, , pp. 1092–1107).…”

“…An area where consistent and easy‐to‐communicate naming is required is the application of AM fungi. Ryan & Graham (; in this issue of New Phytologist , pp. 1092–1107) provoke the research community to take a fresh look on what is known about functional roles of AM fungi in agroecosystems and what would the crop growers need from scientists to be able to harness the potential of soil biodiversity for more environment‐friendly and sustainable crop production, but also for minimizing destruction of natural habitats.…”