Effects of Cover Crops and Reduced Tillage on Arbuscular Mycorrhizal Abundance and Diversity: A Meta-Analysis.

Enhancing the relationship between crop roots and arbuscular mycorrhizal (AM) fungi could help reduce fertilizer inputs and nutrient losses, increase productivity in marginal soils, and improve crop resilience to drought stress. Practices such as winter cover cropping and reduced tillage may enhance these benefits by increasing AM colonization of crop roots and changing AM community structure and/or composition relative to typical practices like bare winter fallows and intensive tillage. Thus we carried out a meta-analysis and review of how such alternatives affect AM colonization of roots and AM communities in annual cropping systems. For cover crops, we compiled 93 comparisons of winter cover crops vs. fallow management from 17 field studies, encompassing a variety of cover crop species and main crops. Overall, cover crops increased mycorrhizal root colonization of subsequent main crops by 33%. Colonization increased by 35% vs. 20% for AM and non-AM cover crop hosts, respectively, relative to a winter fallow. Increases in colonization were also higher earlier in the growing season relative to later samplings. These results suggest that cover crops increase soil AM inoculum potential, particularly early in the growing season, and that even non-AM host cover crops may be beneficial, perhaps from AM host weeds present. Fewer studies examined changes in AM communities resulting from cover crops or reduced tillage, precluding a quantitative analysis, but a qualitative analysis showed that changes in community composition and increases in AM species richness can occur from both approaches.