Rotational grass-legume leys increase arable crop yields, particularly at low N fertiliser rates

Including perennial leys in crop rotations can increase arable crop yield and soil organic carbon content. However, ley effects are often confounded by differences in manure addition, and it is unclear how the effects change over time or are impacted by ley species. Using 48 years of data from long term experiments at three locations in Sweden, this study examined the effects of including a two-year sole-grass or mixed grass-legume ley compared with only arable crops in six-year rotations, on crop production, and soil carbon and nitrogen under different nitrogen (N) fertiliser rates but without manure additions. Rotational leys resulted in greater oilseed and cereal grain yields at zero and low N fertilisation, particularly when legumes were included in the ley. The effect was evident for grain yields throughout the rotation and similar over crop rotation cycles. However, there were no yield differences between rotations at the highest N rate. With a grass-legume ley in the rotation, less N fertiliser was required to obtain similar grain N concentrations in winter wheat as in the rotation without ley. Concentrations of topsoil C and total-N, across cycles and N rates, were higher in rotations with ley. Topsoil C was maintained between the 2nd and 8th cycle in all treatments except at the low N fertiliser rate in the rotation without ley. Including short-term grass-legume leys in crop rotations with only arable crops may be a way to reduce the dependence on N fertiliser and still maintain topsoil C.