Root traits of cover crops and carbon inputs in an organic grain rotation

Cover crops are widely used to increase the quantity of organic carbon (C) returned to the soil between cash crops. Roots play an important role in increasing soil organic carbon (SOC) levels, but the root traits that impact SOC likely vary widely among cover crop species and this variation has yet to be characterized. Recently, cover crop mixtures have expanded in popularity as a way to increase the diversity of cover crop benefits. We tested the quantity, quality and spatial distribution of roots in three monocultures and one mixture to increase our understanding of root trait variation among species, and how that variation impacts mixture design. Root cores were taken from in-row and between-row locations to a depth of 40 cm from cover crops planted after winter wheat during the 2016-2017 growing season. These samples were taken from a larger maize-soybean-winter wheat organic grain rotation experiment (2012-2018) located in central Pennsylvania, USA. Cover crop treatments included monocultures of triticale (X Triticosecale Wittmack cv. 'Trical 815'), canola (Brassica napus L. cv. 'Wichita'), crimson clover (Trifolium incarnatum L. cv. 'Dixie') and a five species mixture dominated by those three species. Additionally, cumulative carbon (C) inputs were assessed for the entire rotation to determine cover crop and cash crop root C contributions. Root biomass C vertical and horizontal distribution, root-to-shoot (R:S) ratio, and root carbon-to-nitrogen (C:N) ratio differed among cover crop treatments. Triticale produced more root biomass in the between-row space at all depth intervals compared to other cover crop treatments. The five species mixture had more total 0-5 cm and between-row 0-5 cm root biomass than crimson clover in spring 2017. Cover crop and cash crop roots increased cumulative C estimates by between 37% (crimson clover) and 46% (triticale) compared to shoot C alone. Cover crop root trait information can inform the belowground benefits from combining different species into cover crop mixtures. Crimson clover produced less root biomass, surface root biomass and between-row root biomass than other cover crop treatments. Therefore, combining crimson clover with grass and certain brassica species can improve total root biomass production, and root distribution compared to crimson clover monocultures, whereas reducing the C:N ratio of roots compared to grass species monocultures. The five species mixture led to greater cumulative carbon inputs compared to monoculture treatments, which was due to greater cover crop biomass C and its influence on the following corn crop's biomass C.