Unused fertiliser nitrogen in arable soils--its contribution to nitrate leaching

Nitrate present in arable soils in autumn is at risk to leaching during the following winter. To see whether unused nitrogen fertiliser was a major source of this nitrate, 15N-labelled fertiliser was applied to 11 winter wheat crops at rates of between 47 and 234 kg N ha-1 in spring. The experiments were on three contrasting soil types in south-east England. On average, 17% of the N from spring-applied labelled fertiliser remained in the 0-23 cm soil layer at harvest (range, 7-36%) but only a small proportion was in inorganic forms (ammonium + nitrate). This was never more than 5 kg N ha-1 and averaged only 1.3% of the fertiliser N applied (range, 0.4-3.6%). Between 79 and 98% of the inorganic N in soils at harvest was unlabelled, being derived from the mineralisation of organic N rather than from unused fertiliser. The amount of unlabelled N was much greater where wheat was grown after ploughing up grass or grass/clover leys than where it was grown in all-arable rotations. When wheat was grown without N fertiliser, soil inorganic N content at harvest was no lower than in plots given fertiliser at rates up to 234 kg N ha-1. This work indicates that, for soil growing winter wheat, almost all of the nitrate at risk to leaching over the winter period comes from mineralisation of organic N, not from unused fertiliser applied in spring. Consequently, even a drastic reduction in N fertiliser use would have little effect on nitrate leaching.