LOSSES OF NITRATE-NITROGEN IN WATER DRAINING FROM UNDER AUTUMN-SOWN CROPS ESTABLISHED BY DIRECT DRILLING OR MOLDBOARD PLOWING

The leaching of nitrate-N under autumn-sown arable crops was measured using hydrologically isolated plots, about 0.24 ha in area, from 1984 1988. Fluxes of water and nitrate moving over the soil surface (surfac e runoff), at the interface between topsoil and subsoil (interflow), a nd in the subsoil (drainflow) were monitored in plots with mole-and-pi pe drain systems (drained plots); surface runoff and interflow only we re monitored in 'undrained' plots. Half the drained and undrained plot s were direct-drilled, and on the other half seedbeds were prepared by tillage to 200 mm. Tillage increased the total leaching loss of nitra te by 21% compared with direct drilling in drained plots. About 95% of the nitrate moving from the soil was present in the water intercepted by the subsoil drains in these plots. In undrained plots less water a nd nitrate were collected in total; more of the nitrate was present in interflow on ploughed plots and in surface runoff in direct-drilled l and. Losses of nitrate for the whole experiment from 1978-1988 were an alysed. This showed that. between the harvest of one crop and the spri ng application of fertilizer to the next, loss of nitrate-N from ploug hed land (L(p)) was approximated by L(p) 22+ F kg N ha-1. where F was the autumn fertilizer-N applied. After fertilizer was applied in sprin g, loss of nitrate-N depended on rainfall such that for 100 mm rainfal l about 30% of the fertilizer-N was lost by leaching. About 18% more n itrate-N was lost form direct-drilled land than from ploughed land in spring, but the total loss was generally small compared to that over w inter. The apparent net mineralization of organic-N was measured in 19 88. In autumn and winter there was little effect of tillage treatment (26 and 31 kg N ha-1 on direct-drilled and tilled plots respectively). However, over the year 83 kg N ha-1 were mineralized in tilled plots, and 67 kg N ha-1 in direct-drilled plots. Five factors governing the leaching of nitrate are assessed and this identified that fertilizer n itrogen application to the seedbed of winter sown crops and the minera lization of nitrogen from the residues of the previous crop are the mo st significant factors for nitrogen leaching in the UK.