First evidence for a major cover crop effect on arbuscular mycorrhizal fungi and organic maize growth

Abstract

Arbuscular mycorrhizal fungi are increasingly used in organic cropping systems to increase yields. Although cover crops are largely used in organic farming, there is little knowledge on the impact of cover crops on native mycorrhizal fungi. Here, we studied the effect of cover crop diversity on mycorrhizal colonization in subsequent organic maize cultivars differing in the level of genetic diversity. Experiments were conducted from 2010 to 2012 in a Mediterranean environment. First Indian mustard (Brassica juncea L. Czern.), hairy vetch (Vicia villosa Roth), a mix of seven cover crop species (Mix), and natural vegetation (Control) were cultivated as winter cover crops. Then, an organically and a conventionally bred maize hybrid and three organically bred composite cross populations were cultivated. Mycorrhizal propagule dynamics were measured. Results at juvenile stage show a higher mycorrhizal colonization in maize plants grown after hairy vetch, of 35.0 %, and Mix cover crops, of 29.4 %, compared to Indian mustard, of 20.9 %, and Control, of 21.3 %. The potential of soil mycorrhization decreased of 56.5 % following Indian mustard, higher than that of other cover crops, of 34.1-47.3 %. This finding could be explained by the release of isothiocyanates in soils. Moreover, maize shoot biomass, nitrogen, and phosphorus content across all maize genotypes at juvenile stage increased with mycorrhizal colonization. These findings provide the first evidence of the greater role played by cover crop identity in the enhancement of early mycorrhizal colonization of the subsequent crop and of soil mycorrhizal activity.

Changes in the composition of native root arbuscular mycorrhizal fungal communities during a short-term cover crop-maize succession

“…A previous work, based on morphological identification of spores, showed increases of AMF species richness in plots with high plant diversity (Njeru et al 2015). Other studies reported that a mixture of cover crops had a positive effect on AMF propagules and biomass (Lehman et al 2012) and on mycorrhizal colonization (Njeru et al 2014), compared to monocultures. We found that nearby plant species did not influence root AMF community composition in V. villosa, that hosted nearly the same AMF community when cultivated alone (LD) and together with other cover crop species (HD), in contrast with other data (Mummey et al 2005;Hausmann and Hawkes 2009).…”

“…Recently, we provided the first evidence of the important role played by winter cover crop identity in promoting early mycorrhizal colonization and growth of the subsequent crop and soil mycorrhizal activity in organic agroecosystems (Njeru et al 2014). Such data showed that the right choice of cover crop species opens the possibility of raising inoculum potential of AMF native strains, a fundamental approach in lowinput and organic farming, which rely more on the efficient use of natural soil resources than on the use of external inputs.…”

“…Climatic conditions at the experimental station are typical for Mediterranean regions, with rainfall mostly concentrated in autumn (October to December) and spring (March to April). The experimental field was part of the trials carried out within the EU-RTD FP7-funded project Strategies for Organic and Low input Integrated Breeding and Management (SOLIBAM 2010(SOLIBAM -2014, which aimed at investigating the role of cover crop diversity in promoting both the mycorrhizal symbiosis and the growth and productivity of subsequent different maize genotypes (Njeru et al 2014). The field was under a 5-year stockless arable crop rotation, managed as an organic system: weeds were not controlled and unfertilized maize was sown after cover crops mowing and incorporation into the soil by disc harrowing at a depth of 15 cm.…”

Biological and other soil parameters response to winter cover crops in a mediterranean irrigated system

“…The (Boswell et al, 1998;Deguchi et al, 2007;Kabir andKoide, 2000, 2002;White and Weil, 2010), even under Mediterranean climate conditions (Njeru et al, 2014). Studies involving other main crops are scarce, but the effect of CC was similar, e.g., in leeks (Sorensen et al, 2005).…”

“…Studies have shown that the residues of these species that remain in the field might be able to reduce the AMF colonization in the subsequent crop Koide and Peoples, 2012). On the other hand, there are studies that did not find a decrease of mycorrhizal colonization after Brassicaceae CC compared with fallow, suggesting that they did not always suppress the AMF activity in the subsequent crop (Njeru et al, 2014;White and Weil, 2010). An alternative to counterbalance the negative effect of these crops could be to mix them with AMF host plants that promote the colonization.…”