Cultivation and Soil Organic Matter Management in Low Input Cereal Production following the Ploughing out of Grass Leys

Nitrogen fertility of low input cereal systems following ploughing out of grass leys is investigated using the soil nitrogen dynamics model SOILN with its linked cereal growth model, supported by a set of experimental data for spring and winter barley cropping. Soil organic matter (OM) plays an important part in transfer of fertility from the fertility-building ley phase to the fertility exploitation arable phase of ley-arable rotations. One aspect investigated is a possible role of ‘protected organic matter’, a component of soil OM which generally mineralises at a slow rate, but which becomes ‘unprotected’ so it mineralises at a somewhat faster rate for a limited period following disturbance of the soil by ploughing. Measurements of CO2 emissions in the laboratory and field support the concept of protected OM. In simulations with the SOILN model, this is represented by a temporary increase in the value of the humus decomposition rate constant for a few weeks following ploughing. The assumption of raised humus decomposition rates after ploughing makes some improvements to fits of simulations with the linked models to measured grain and straw yields and N offtakes, as well as giving a more realistic decline in soil OM over successive years of cereal cropping. Predictive simulations demonstrate how, in the absence of mineral N fertiliser, cereal crop yields decline over the years following ploughing out of leys, but yields can be maintained by making applications of organic manure or slurry.