The uptake of 15 N‐labelled fertiliser by winter wheat and its immobilisation in a clay soil after direct drilling or ploughing

The uptake of fertiliser nitrogen (N) by winter wheat was measured using 15 N‐labelled calcium nitrate applied at 80 kg N ha‐ 1 rate in May to the soil in microplots contained by sections of rigid PVC tube (30 cm diameter, 20 cm long), inserted into a calcareous clay soil (Evesham series). The microplots and surrounding plots had been subjected to either 5 or 6 years of continuous ploughing or direct drilling or 1 and 2 years of direct drilling after 4 years of deep‐tined cultivation. The recovery of fertiliser N labelled with 15 N in the crop plants at harvest was in the range 60–67% of that applied in the first year and 0.5–0.7% in the second year; there were no significant treatment differences. The ‘apparent’ recovery of fertiliser based on a comparison of plots with and without fertiliser N was greater and ranged from 77–111 % of that applied. Estimates of the contribution of non‐fertiliser N to the crop by means of 15 N analysis suggest that if this N had been utilised with the same efficiency as that of fertiliser N about 120 kg N ha‐ 1 was available to the plants. This represents about 1% of the total N in the soil to 80 cm depth. Soil analysis (0–15 cm) showed that about 8% of the applied 15 N was retained in the upper layers of the soil; there were no treatment differences. Any changes due to mineralisation of the soil organic matter during the 2 years of the experiment were within the errors of the experiment.