Changes in soil mineral nitrogen during and after 3-year and 5-year set-aside and nitrate leaching losses after ploughing out the 5-year plant coversin the UK

The management and effects of 3-year and 5-year set-aside covers on soil mi neral nitrogen (SMN, 0.0-0.9 m) were studied at six sites in England. Soil mineral N was measured annually in autumn and spring during the period of s et-aside cover, with more frequent SMN sampling over the first winter after ploughing out the covers. Spring SMN was measured in the second year after set-aside. Nitrate leaching losses were also measured at three sites in th e first winter after destruction of the 5-year set-aside covers. Winter cer eals were grown in both test years after each set-aside period. Amounts of both autumn and spring SMN in the perennial rye-grass (PRG), per ennial rye-grass/white clover (PRG/WC) and natural regeneration (NR) covers were generally less than, or similar to those in the continuous arable tre atment during each year of set-aside, indicating a slightly smaller nitrate leaching risk under set-aside management. Slight increases in autumn SMN, and hence leaching potential were, however, observed under PRG/WC in the fo urth and fifth years, compared with continuous arable cropping. Ploughing out of both 3-year and 5-year covers increased soil N supply and potential nitrate leaching losses over winter, compared with continuous ara ble cropping. By the following spring, mean increases across all sites in a mounts of SMN after 3-year covers of PRG, NR and PRG/WC were 14, 18 and 33 kg ha(-1) N, respectively, compared with the arable rotation. Equivalent in creases in spring SMN following destruction of the 5-year set-aside covers were almost identical, at 17, 19 and 33 kg ha(-1), respectively, although o nly the ploughed-out PRG/WC covers increased SMN at the clay sites. Measure d nitrate leaching losses in the first winter after 5-year set-aside were g reatest after PRG/WC at two sites on shallow chalk but greatest after NR, w hich had a naturally large clover content, at the third site which was on a sandy soil. However, the leaching losses after set-aside were relatively s mall, relative to typical losses after ploughing out intensively managed gr ass or grass/clover swards, and would have been compensated for by potentia lly less leaching during set-aside. Spring SMN measurements in the second year after ploughing out the set-asid e covers, showed negligible or, for PRG/WC, only slight increases (12 - 18 kg ha(-1)) in residual soil N supply after both 3-year and 5-year covers, c ompared to continuous arable cropping. The extra N mineralisation after cov er destruction justified small reductions in fertiliser N inputs for the fi rst, but not second crop following either 3- or 5-year set-aside, unless th e cover had contained a large clover content. Both 3-year and 5-year set-as ide covers had minimal or no effect on either organic matter content, apart from a slight increase in the PRG/WC treatments, or extractable phosphorus , potassium and magnesium status in the topsoil.