15N-labelled nitrogen fertilizer (containing equal quantities of ammonium-N and nitrate-N) was applied in 4 consecutive years (1980–3) to different microplots located within the Broadbalk Wheat Experiment at Rothamsted, an experiment which has carried winter wheat continuously since 1843. Plots receiving 48, 96, 144 and 192 kg N/ha every year were given labelled fertilizer in mid-April at (nominally) these rates.
Grain yields ranged from 1–2 t/ha on plots given no N fertilizer since 1843 to a maximum of 7·3 t/ha with 196 kg N/ha. On plots given adequate P and K fertilizer, between 51 and 68% of the labelled N was recovered in the above-ground crop; only about 40% was recovered where P deficiency limited crop growth. In 1981 fertilizerderived N retained in soil (0–70 cm) at harvest increased from 16 kg/ha, where 48 kg/ha was applied, to 38 kg/ha, where 192 kg/ha was applied. More than 80% of this retained N was in the plough layer (0–23 cm).
Overall recovery of fertilizer N in crop plus soil ranged from 70 % to more than 90 % over the 4 years of the experiments. Losses of N were larger in years when spring rainfall was above average and when soil moisture deficits shortly after application were small.
Crop uptake of unlabelled N derived from soil increased from 28 kg N/ha on the plot given no fertilizer N to 67 kg N/ha on the plot given 144 kg N/ha. The extra uptake of unlabelled N was mainly, if not entirely, due to greater mineralization of soil N in the plots that had been given N fertilizer for many years. Presumably fertilizer N increased the annual return of crop residues, which in turn led to an accumulation of mineralizable organic N, although there was only a small increase in total soil N content.
Wheat given NH4-N grew less well and took up less N than wheat given N08-N in the relatively dry spring of 1980; there was little difference between the two forms of N in the wetter spring of 1981. In both years more fertilizer N was retained in the soil at harvest when fertilizer was applied as NH4-N than as N03-N.
The N content of the soil in several plots of the experiment has been constant for many years, so that the annual removal of N is balanced by the annual input. A nitrogen balance for the plot given 144 kg fertilizer N/ha showed an average annual input of non-fertilizer N of at least 48 kg/ha, of which N in rain and seed accounts for about 14 kg/ha. The remainder may come from biological fixation of atmospheric N2 by blue-green algae, or from dry deposition of oxides of nitrogen and/or NH3 onto crop and soil. The overall annual loss of N from the crop–soil system on this particular plot was 54 kg N/ha per year, 28% of the total annual input from fertilizer and nonfertilizer N.