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.