THE AVAILABILITY OF THE NITROGEN IN THE CROP RESIDUES OF WINTER-WHEAT TO SUBSEQUENT CROPS

Three field experiments in Eastern England, in which N-15-labelled fertilizer had been applied to winter wheat, were used to measure the persistence of the labelled N remaining in soil and stubble at harvest and the availability of this N to up to four subsequent wheat crops. A portion of the labelled fertilizer N quickly became stabilized in the soil, with only small and ever-decreasing amounts recovered by subsequent crops. Combining all sites, all years and all applications of fertilizer, 6.6 +/- 1.92 (S.D.) % of the labelled fertilizer remaining in soil (0-70 cm) plus stubble in the year of application was taken up by the next wheat crop, i.e. by the first 'residual year' crop. A further 3.5 +/- 0.39 % was taken up in the second residual year, 2.2 +/- 0.43 % in the third and 22 % in the fourth. Loss of residual labelled N was more rapid from a sandy soil than from two heavier-textured soils, particularly in the first residual year. After four residual crops on one of the heavier soils (at Rothamsted), 16 % of the labelled N remaining in soil (0-70 cm) and stubble in the year of application had been taken up by the crops, c. 29 % had been lost from the soil/crop system and 55 % remained in the soil.