Alteration of the organic wheat rhizobiome and enzyme activities by reduced tillage and diversified crop rotation

The acreage of organic agriculture continues to grow world-wide. Growing organic crops relies on soil tillage for weed control, and organic soil inputs for nutrient supply, because chemical inputs are not permitted. Organic inputs build soil organic C, a key determinant of soil quality, but tillage-induced C losses through erosion and decomposition reduce soil organic C. Crop rotations also supply nutrients and break pest cycles. In 2014 (normal rainfall) and 2015 (below-normal rainfall), we investigated whether reduced tillage (low tillage vs high tillage), and diversification of a 2-year forage pea green manure (GM)-wheat rotation to a 4-year rotation (simplified rotation vs diversified rotation) altered the soil microbiome and enzyme activities in the rhizosphere of organic wheat in a trial that had been established in 2010. Planctomycetes and Gemmatimonadetes were more abundant under high tillage than low tillage in both rotations. The Shannon index of α-diversity was greater in the diversified rotation than the simplified rotation under high tillage. Microbial biomass C (MBC) was greater in the diversified rotation than the simplified rotation in 2014. The relative abundances of Gemmatimonadetes, Proteobacteria and Tharmarchaeota were greater in the diversified rotation than the simplified rotation under high tillage. Low tillage increased the activity of acid phosphomonoesterase (P cycling), and in 2014, the activity of this enzyme was greater in the diversified rotation than the simplified rotation. The growing season precipitation shaped the soil microbiome (α- and β-diversity) the most: the drier-than-normal 2015 had less microbial biomass C (MBC), Actinobacteria, Bacteroidetes, Proteobacteria and enzyme activities than the normal 2014, but vice-versa for α-diversity, AD3, Armatimonadetes and Chloroflexi. Therefore, although the growing season affected these soil microbial properties more than tillage or crop rotation, both low tillage and crop diversification altered the soil microbial community composition and increased the soil P cycling potential