11 July 2019 | | South Downs groundwater
Results from Portsmouth Water’s cover crop trial for the 2018/19 period show a reduction of up to 60 % in nitrate leaching from plots sown with cover crops. This indicates a reduction in leaching to the South Downs groundwater and a reduction in fertilizer requirement for farmers in the using cover crops. If implemented across the catchment through a payments for ecosystem services scheme, this could also improve water quality and reduce treatment costs at their abstraction points.
The trial consists of three cover crop treatments drilled in August 2018 along a 100m length of a single ‘tramline’ width (36m) of a shallow silty clay loam soil over chalk. The cover crop species were selected to give a ‘simple’ low cost option (oats) compared to a mix that would qualify for an Ecological Focus Area green cover (EFAGC) payment (oats & phacelia), with both options compared to an untreated stubble (which became a weedy stubble during the course of the winter). Samples of soil water were collected and analysed for nitrate every two weeks over the 2018/19 winter and soils were collected early 2019 to estimate soil nitrogen supply (SNS) to the following spring barley crop.
The phacelia cover crop was very effective at reducing nitrate leaching losses, with just 10 kg/ha N lost by leaching and an average (flow-weighted) nitrate-nitrogen (NO3-N) concentration in the drainage water of 3 mg/l (Figure 1). The weedy stubble lost c.30 kg/ha with an average concentration of 11 mg/l (this concentration is the EU nitrate-N limit for drinking water), and the poorly-established, low cover oat cover crop c.60 kg NO3-N /ha, with an average concentration of 21 mg/l NO3-N in the drainage waters (i.e. almost double the EU limit).
Figure 1. Nitrate leaching losses October 2018-March 2019 (520 mm rainfall; 270-300mm drainage).
The lower leaching losses following the phacelia cover crop meant that more nitrogen was potentially available for the following spring barley crop; i.e. SNS was greatest following the phacelia cover crop (Figure 2). As a result, the amount of nitrogen fertiliser applied to the spring barley could be reduced by 30 kg/ha. The SNS of the stubble and oat treatments was not sufficient to warrant a change in fertiliser policy for these two treatments.
Figure 2. Nitrogen present in the soil, crop and lost by spring 2019 (c. 80-110 kg/ha was present in the soil in autumn 2018); SNS = soil nitrogen supply (potentially available for use by the following spring barley crop).
These results show the potential for cover crops to reduce leaching to the South Downs groundwater and to also benefit farmers who could end up spending less money on N fertilizers. If yields of the following spring barley crop are increased in the cover crop plots then this would a further benefit to the farmer. They also show the potential for cover crops as a practice that could be incentivised across Portsmouth water’s drinking water abstraction catchments to improve water quality and reduce treatment costs in the longer term.