Winter Cover Crops Increase Mycorrhization and Yield of Subsequent Cash Crops: A Meta-Analysis

Cover crops are critical tools for soil, weed and nutrient management in organic crop production systems. Similarly, arbuscular mycorrhiza (AM), symbiotic fungi that live in association with the roots of many crops, play an important role in crop phosphorus uptake, water relations and stress tolerance. We conducted a random effects model meta-analysis to determine the influence of cover crops on AM colonization and yield of subsequent cash crops. Eighty-seven studies were extracted from 21 peer-reviewed publications dating to 31 December, 2015. Overall, cover crops increased AM colonization of cash crops by 17% and AM spore density by 110%. When cover crops were known to be hosts of AM fungi, cash crop root colonization was increased by 29%. Conversely, monocultures of non-AM fungi host cover crops (all Brassicaceae) reduced the root colonization of subsequent cash crops by 17%. AM spore density in soils increased by over 400% when cover crops were allelopathic. Interestingly, spore density was also increased when non-allelopathic cover crops were grown, by a factor of 61% when compared to fallow field controls. When cover crops were monocots and dicot-legumes, subsequent cash crops had a 30% and 24% higher root colonization over controls, respectively. Higher yields were associated with cash crops that had higher % root colonization. Mycorrhizal cover crops appear to have a positive impact on the colonization of cash crops that follow. The few studies reporting cash crop colonization as influenced by non-AM cover crop hosts came from only two publications, however. Further investigation into non-AM cover crops is needed to confirm whether their negative influence on cash crop colonization is real and to better characterize consistency of their effect.