In 1977, a four-course rotation was set up at Ropsley (UK) to study crop response to eight rates of nitrogen application (35–265 kg ha−1). This rotation continued until 1990 when continuous winter wheat was introduced. Results from 1978 to 1990 provide an opportunity to study the initial phase of cumulative effects from different rates of N fertilizer application on the recovery of N by cereals and the retention of N in the soil.
From 1978 to 1990, considerable variation in the recovery of nitrogen by winter wheat was observed. Neither rainfall nor drainage, as indicators of possible denitrification or leaching losses, provided a useful explanation for this, possibly because of the relatively dry conditions prevailing after spring fertilizer application. There was no evidence of increased soil N fertility, beyond single year residues, as a result of large N applications over the 13-year period.
In order to achieve the economic optimum grain yield, it was necessary to use N applications which produced inefficient recovery of N. Thus, greater return of N in crop residues and immobilization at relatively large N applications (>150 kg ha−1) contributed to an observed build-up in soil organic N over the period of study. Plots receiving, on average, 265 kg ha−1 appeared to gain c. 250 kg ha−1 N over control plots (35 kg ha−1) after 13 years of N application. Reducing the N application rate from the economic optimum to a more biologically efficient N rate (156 kg ha−1) was calculated to result in an average yield loss of 0·305 t ha−1 and cause an estimated £17 ha−1 loss in profit.