Root growth of sole and mixed cover crops in organic farming

Abstract

Cover crops are an important tool in arable farming systems. They provide many different agroecosystem services. For many of these functions, such as nutrient uptake, erosion prevention and soil structure formation, the roots of cover crops are of central importance. In recent years, cover crop mixtures have become increasingly popular in agricultural practice. Mixtures can have advantages over sole crops in terms of productivity and agroecosystem services. In agricultural practice and consulting, the advantages of mixtures are often mentioned with regard to the rooting of cover crop stands, and complementarity in the root zone as well as increased rooting across the entire soil profile are postulated. <br /> The aim of the present thesis is to test different cover crop species for their rooting intensity and, in a further step, to investigate the root traits in cover crop mixtures and possible below-ground mixture effects. In addition, the potential of cover crops to create large-sized biopores is assessed. Different methods for determining root length density are compared. <br /> In 2018/19 and 2019/20, sole and mixed cover crops were investigated for shoot and root traits in the field. The latter were determined using the monolith and profile wall method. Species-specific root masses in mixtures were analysed by Fourier transform infrared spectroscopy. <br /> The results showed that winter hardy cover crop species increased root length over winter. Oil radish, winter turnip rape and phacelia rooted intensively into large-sized biopores in the sub-soil. Positive mixture effects partly occurred in the shoot, whereby these were higher for nutrient content than for biomass. There were no significant mixing effects for root mass. Weak partners were suppressed in root mass. In mixtures with oil radish, there were positive mixture effects for root length and root mass density in the subsoil. For the specific root length, positive mixture effects were found in all mixtures. Neither a vertical niche differentiation for root mass density nor complementarity effects for root mass could be clearly found. The biopore genesis potential of cover crops was low, but highest for blue lupin. <br /> Compared to the monolith method, the root length density estimated with the profile wall method was underestimated, especially in highly rooted zones. High agreement between both methods was found for the cumulative root distribution. The influence of soil depth was very decisive for the comparison of both methods. <br /> Overall, it was shown that for the cover crop mixtures tested in this thesis mixture effects of root traits were rather small. Complementarity in the root zone and increased rooting over the entire soil profile could not be found. In addition, future perspectives for root research on cover crop mixtures as well as agronomic management options for cover cropping are discussed.