The two predominant soil bacterial phyla had opposite responses to black medic cover cropping and fertilizer nitrogen

A black medic (Medicago lupulina L.) cover crop produces a persistent seed bank and self-reseeds each year in North America, but its effects on soil microbial communities are not clear. A field trial was established in 2003 with the following experimental treatments: (a) black medic or no medic, (b) a 3-year crop rotation of flax (Linum usitatissimumL.)-oats (Avena sativa L.)-winter wheat (Sativum aestavum L.), and (c) fertilizer N applied to the rotation crops at three rates: 20, 60 and 100% of the recommended N based on soil testing. In 2011, soil bacterial communities were characterized by pyrosequencing. Black medic increased Shannon and Simpson indices of diversity, and both indicesincreased linearly with increasing N rate. Sixteen phyla were observed, and the most abundant (in bulk soil and rhizosphere, respectively) were: Actinobacteria (39.6 and 37.0%), Proteobacteria (34.0 and 32.9%), Acidobacteria (10.8 and 13.8%) and Bacteroidetes (6.2 and 6.7%). Black medic increased the abundance of Proteobacteria, butdecreased the abundances of Actinobacteria and Firmicutes in bulk soil, and Acidobacteria in the rhizosphere. Bacteroidetes increased, but Actinobacteria decreased, with increasing fertilizer N rate in bulk soil. The soil bacterial community structures reflected their relative abundances. The bacterial phyla in bulk soil that were most associatedwith medic treatments included Proteobacteria and Bacteoidetes, and those associated with no-medic treatments included Actinobacteria, Chloroflexi and Planctomycetes. The rotation crop effects were inconsistent. Therefore, the two predominant soil bacterial phyla, Actinobacteria and Proteobacteria, had opposite responses to black medic andfertilizer N.