Assessment of the application of a cover crop and conservation tillage on soil and water properties dissolved nitrous oxide in an arable system

Agriculture is a major contributor to environmental pollution. About quarter of water bodies in England are classified as being good ecological and chemical status. To tackle agricultural pollution, a range of on-farm mitigation measures are recommended. The overall aim of this study was to assess the effectiveness of cover cropping and reduced cultivation methods as infield mitigation measures to reduce diffuse water pollution, improve soil quality and reduce nitrous oxide greenhouse gas emissions. These mitigation measures were applied to seven fields within the intensive arable River Wensum catchment, eastern England, with a further two fields kept under conventional cultivation as a control. Soil and water chemistry, principally water discharging from subsurface agricultural field drains, were regularly sampled and analysed from these fields over a two-year period. The results revealed the mitigation measures had no positive impact on soil quality. The soil chemical condition, including soil organic carbon, phosphorus, magnesium and sulphate concentrations were not improved by the use of a cover crop or reduced cultivation, whilst soil physical condition deteriorated through increased compaction, as highlighted by increased bulk density, penetration resistance and lower infiltration rates. Conversely, field drain water quality improved markedly. The presence of a winter cover crop significantly reduced mean dissolved nitrate concentrations from 13.9 mg N L-1 to 2.5 mg N L-1, an 82% reduction. Different inversion intensity of the soil tended to have no effect on nitrate concentrations. Regarding dissolved N2O, a slightly higher N2O concentration was recorded in field drains under a cover crop than without cover crop. This finding suggests that whilst the use of a winter cover crop is highly effective at reducing soil nitrate losses to rivers, it does not represent an effective strategy for reducing N2O emissions. Indirect nitrous oxide emission factors (EF5g and EF5r) were calculated using two approaches (IPCC 2006 and the N2O-N/NO3̄-N ratio) for both field drain and stream water samples. Values for these two EFs obtained were found to be below the IPCC default value of 0.0025. If the IPCC were to revise EF5 values in future then, regardless of soil type, crop type, and land use practices, a value of 0.0009 (about one third of the current value) for EF5g and a value of 0.0002 (one order of magnitude lower than the current value) for EF5r may be a more reasonable estimates. Such radical downward revision would at least halve the current estimates of N2O emissions associated with N leaching and runoff from agriculture for both the UK and globally