Effect of nitrogen fertilizer applied to winter oilseed rape (Brassica napus) on soil mineral nitrogen after harvest and on the response of a succeeding crop of winter wheat to nitrogen fertilizer

Soil mineral nitrogen (N-min) was measured to 90 cm at a total of 12 sites in the UK in the autumn after an oilseed rape experiment, which measured responses to fertilizer N. On average, N-min increased by 15 kg/ha per 100 kg/ha fertilizer nitrogen (N) applied to the rape, up to the economic optimum amount of N (N-opt). There were larger increases in N-min where fertilizer applications exceeded N-opt, thus super-optimal fertilizer applications disproportionately increased the amount of nitrate likely to leach over-winter. The small effects of sub-optimal N on N-min were associated with large increases in N offtake by the oilseed rape, whereas the larger effects of super-optimal N on N-min were associated with only small increases in N offtake. Over 70% of the variation in autumn N-min was explained by the previous rape's N fertilizer rate and the topsoil organic matter content. Nitrogen applied to the rape increased grain yields of the succeeding wheat crops when no further fertilizer N was applied to the wheat. It was concluded that N applied to oilseed rape significantly affected N-min after harvest, and these effects were not completely nullified by leaching over-winter, so soil N supply to the succeeding wheat crop was significantly increased. Responses in grain yield indicated that each 100 kg/ha N applied to the rape provided N equivalent to c. 30 kg/ha for the following cereal. Each 1% of soil organic matter further contributed N to the wheat, equivalent to 25 kg/ha. It is important to ensure that oilseed rape receives no more than the optimum amount of fertilizer N if subsequent leaching is to be minimized. Reductions below optimum amounts will have only a small effect on leaching. Substantial changes in the economic optimum N for rape production should be accompanied by adjustment in fertilizer N application to following wheat crops. Fertilizer recommendation systems for wheat should take account of the fertilizer N applied to the preceding oilseed rape and the topsoil organic matter content.