Fall cover cropping can increase arbuscular mycorrhizae in soils supporting intensive agricultural production

Abstract

Intensive agricultural practices, such as tillage, monocropping, seasonal fallow periods, and inorganic nutrient application have been shown to reduce arbuscular mycorrrhizal fungi (AMF) populations and thus may reduce benefits frequently provided to crops by AMF, such as nutrient acquisition, disease resistance and drought tolerance. We have evaluated the ability of different cover crops to elevate the native mycorrhizal inoculum potential of soils under soil–climatic conditions typical of the upper Midwest U.S. production agricultural region. We measured the number of soil AMF propagules at three sites in the late fall following cover crops that were seeded into summer-harvested small grains within a no-till rotation. At all three sites, soil AMF propagule numbers were generally low (≤1 propagule g⁻¹). Fall cover crops significantly increased the mycorrhizal inoculum potential of the soils. Forage oats (Avena sativa (L.) Hausskn.), by itself or in mixtures, was most effective at both sites where it was planted. At the third site, a cover crop mixture doubled the inoculum potential of these soils. The effect of cover crop treatments on AMF propagules was corroborated at one site over two seasons by measuring AMF biomass with the neutral lipid fatty acid mycorrhizal biomarker, C16:1cis11. Identification of AMF-promoting cover crops for inclusion in diversified, no till cropping rotations in the upper Midwest U.S. will provide opportunity for reduced inorganic nutrient application with economic and environmental benefit.

Introduction

The addition of cover crops into agricultural production systems reduces seasonal fallow and thus provides many benefits to the following cash crops and the health of the soil (Clark, 2007). The presence of a living plant provides a host for obligate mutualists like arbuscular mycorrhizal fungi (AMF) that can protect the host plant against pathogens, extend environmental tolerances, and provide substantial nutritional benefits, particularly improved phosphorus uptake (Rillig, 2004, Lekberg and Koide, 2005, Jansa et al., 2006). Studies evaluating the effect of cover crops on AMF in agricultural soils date back to at least 1995 (Galvez et al., 1995, Boswell et al., 1998, Kabir and Koide, 2000, Deguchi et al., 2007, White and Weil, 2010); however, much of this data is limited to soil-climatic regions or agricultural practices that are not representative of production agricultural systems for corn (Zea mays (L.)), soybean (Glycine max (L.) Merrill), and wheat (Triticum aestivum) that are dominant in the Midwest United States.

Because of dwindling phosphate reserves and soaring prices for phosphate fertilizer (Cordell et al., 2009, Van Vuuren et al., 2010), there is a renewed interest in the capability of AMF to efficiently supply plant-available phosphorous in production agricultural systems, especially for corn. AMF have been shown to support nutrient uptake in corn and may increase yield in some cases (Vivekanandan and Fixen, 1991, Murray, 2000, Kabir and Koide, 2002, Deguchi et al., 2007). Intensively farmed soils are often depleted of AMF (Douds et al., 1993, Mader et al., 2000, Oehl et al., 2004, Jansa et al., 2006) and a meta-analysis of published data has concluded that low AMF inoculum potential limits mycorrhizal colonization and plant performance (Lekberg and Koide, 2005). There are two possible strategies to increase AMF in agricultural soils: inoculation or selective management. Successful field-scale AMF inoculation for commodity crops is economically prohibitive using root cuttings, and only a single species, Glomus intradices, can be routinely produced in sufficient densities for spore applications (Atunes et al., 2009). Inoculation strategies are further constrained by the absence of information on which AMF strains are highly beneficial for a particular crop and which will be competitive in the ambient soil environment. With respect to management, many production agricultural systems rely on tillage, monocultures, seasonal or annual fallow, inorganic fertilizer and pesticide application, all of which can negatively impact AMF numbers and or diversity (Jansa et al., 2006).

We evaluated the ability of specific cover crops and cover crop mixtures to elevate the native AMF inoculum potential prior to corn planting within no-till, small grains–corn–soybean crop rotations under soil-climatic conditions representative of the upper Midwest U.S. The AMF inoculum potential of soils in agricultural fields with and without cover crops at three locations in South Dakota was determined by most-probable number enumeration of arbuscular mycorrhizal fungal propagules (spores, vegetative hyphae, infected root fragments) and by AMF biomass estimates using fatty acid analysis.