Assessing the contribution of legume inclusion in crop rotations to soil health and ecosystem services: A case study from UK field trials

Grain legumes are currently subject to increasing interest as a potentially viable dietary alternative to livestock-derived protein. This is in part due to the contribution of the livestock industry to greenhouse gas production. There is a large body of previous work concerned with the contribution of grain legumes to soil fertility through increased biological nitrogen fixation, and the utilisation of grain legumes as part of a crop rotation has been shown to increase the yields of subsequent crops in the rotation. This has been particularly well explored in organic farming systems, however the uptake of grain legume cultivation in conventional farms in the UK (and much of northern Europe) has been low. Utilising long term field trials which are representative of a broad range of
farming systems, we investigated how the inclusion of grain legumes in both
conventionally and organically managed farming systems impacted the ability of the underlying soils to contribute to ecosystem services. Four conventionally managed rotations, all of which included spring peas in their cropping sequence, were included in the analysis. Each conventionally managed rotation received different organic amendment additions. One was untreated while the other three received green waste compost, turkey manure and paper crumble respectively. Two organic rotations were also considered. One rotation included fava beans in the cropping sequence and a one year grass and red clover green manure, and the other rotation had no grain legumes included, but did have a longer 3 year grass and white clover ley grazed by sheep. A mixture of soil chemical, physical and biological assessments were made including measurements of pH, soil macronutrient content as well as soil organic matter, potentially mineralizable nitrogen, macroporosity, water retention and bulk density. Field measures included VESS scores, earthworm counts and penetration resistance.
This range of field and laboratory measurements were used to assess soil health at all
stages of the rotation preceding and following legume inclusion. We were therefore
able to study the interaction between differing management systems and legume
cultivation, and the bearing this had on the ability of the underlying soils to provide ecosystem services. Preliminary results were interesting considering the differences in legume cultivation, inputs and ley periods. Soil organic matter was significantly higher in both organic rotations, however potentially mineralizable nitrogen was higher in all conventionally managed rotations. The organic rotations had higher VESS scores, with no difference between conventional organic amendment additions. Initial chemical results showed no change in soil phosphorus content between any of the rotations, while soil potassium was higher in the conventionally managed rotations.