Assessment of tillage strategies to decrease nitrate leaching in the Brimstone Farm Experiment, Oxfordshire, UK

Tillage may in ̄uence nitrate losses from agricultural soils. Losses of nitrate were measured in drain ̄ow at 60 cm depth and in combined surface runoff and inter ̄ow in the A horizon (ˆsurface layer ̄ow) on hydrologically sealed plots with a two-year comparison (1988±1990) of shallow-tine cultivation vs. mouldboard ploughing. Ploughing increased concentrations and loadings of nitrate in drain ̄ow and surface layer ̄ow, especially in the ®rst year. After these two years the shallow-tined plots were ploughed to plant winter beans (Vicia faba L.), and nitrate in drain ̄ow then increased over the next three winters, slightly exceeding that from the plots which had been ploughed throughout for winter cereals. The composition of the surface layer ̄ow did not show this effect, however. Calculations of net winter mineralisation of soil organic nitrogen showed that shallowtine cultivation may have decreased mineralisation slightly compared with ploughing in the ®rst two years. These calculations did not indicate any increase in mineralisation for two winters after the minimally cultivated plots were ploughed in autumn 1990, probably because the soil was then very dry. This increase was apparently delayed until the ®fth winter (1992/1993), which was much wetter than any since autumn 1990. In the previous eight years (1980±1988) half of the plots had been ploughed and half had been direct drilled. Averaged over the ®ve winters 1988/1989±1992/1993, the ®ve measures of nitrate loss in drain ̄ow from plots previously direct drilled were 6±57% more than from plots previously ploughed, and winter mineralisation was 20% more, with no evidence of any decline in either with time. The nitrate produced by mineralisation of organic matter conserved by the eight years of direct drilling was mainly lost by leaching or denitri®cation; it was of little or no bene®t to the crops. The results suggest that in the long term more nitrate is leached from land subject to periods of minimal or zero tillage and ploughing than from land ploughed every year. # 2000 Elsevier Science B.V. All rights reserved.