Cover Crop Diversity to Improve Soil Health in Dryland Wheat Systems of Montana.

Farmers in Montana are experimenting with cover crop mixtures (CCMs) as partial summer fallow replacement largely to increase soil health. Summer fallow has been crucial for soil nutrient storage and moisture recharge in this severely water-limited region, but also increases erosion and nitrogen leaching and reduces the quantity and quality of residues returned to soil. Legume green manure (LGM) and now CCM as fallow replacement introduce biomass that decomposes quickly, which may promote biological activity. We conducted a 3-yr on-farm study at four locations across Montana and compared soil biological activity and wheat yields following the first year of cover crop implementation in three systems: single species LGM, an 8-species CCM, and conventional summer fallow. The 8 species represent four functional groups: nitrogen fixers, graminoids, tap-rooted plants, and brassicaceae. Cover crops biomass ranged from 0.4 Mg ha^-1 in a record dry year to 3.7 Mg ha^-1 with no differences in total aboveground biomass between monoculture pea and an 8-species mix (P>0.10). Soil enzymatic activity of β-glucosidase, β-glucosaminidase, arylsulfatase, and acid and alkaline phosphatases showed no differences between these two cover crop treatments, but all enzymes showed significant positive correlation with biomass produced the previous year. Similarly, microbial biomass was similar between treatments but was positively correlated to cover crop biomass from the previous year. Mycorrhizal colonization showed differences between treatments only after high cover crop biomass; colonization of wheat roots was higher following cover crop treatment than summer fallow at one site and at another site mycorrhizal colonization levels were greater in the CCM than in the LGM or summer fallow. Our results suggest that in low biomass systems, the quantity of plant biomass is more important than plant biodiversity for soil processes, except when activity is highly dependent on species present, as is mycorrhizal infectivity.