An experiment was conducted over seven sites in eastern England sown to win
ter cereals, whereby the soil mineral nitrogen (N) and N recovery in the cr
op were measured frequently during the active growth period. Plots received
100 kg/ha fertilizer N on one of three dates in spring (28 March, 11 April
and 25 April) on each site in each of the three years (1994 to 1996), and
these were compared with controls with no applied N. At one site, uncropped
plots were included, and measurements of gaseous N emission, soil respirat
ion, soil microbial biomass and root tissue N concentration were also made.
The fertilizer applications boosted yields by at least 3 t/ha at all sites
, but apparent recovery of fertilizer N varied throughout the previously kn
own range of 45-85 %. Soil type and timing of application had no effect on
N recovery, or on final yields. N was mineralized from soil sources through
out the growth period, but mineralization was outweighed by "immobilization
" of large amounts of N (around 30 kg/ha) in the soil, chiefly during May w
hen crop growth was most rapid. This occurred on all sites with a crop, but
not where the crop was absent. Measured losses of N-2 and N2O were very sm
all (< 70 g/ha/day N) on the site where they were measured, and no other ev
idence of loss or storage of N was found. The apparent recovery of fertiliz
er N at each site, was almost exactly explained by the amount immobilized d
uring May. We conclude that the poor recovery of spring-applied N was due,
not to losses as previously assumed but to temporary immobilization during
the period of most active crop uptake. Immobilization was caused primarily
by the presence of the crop.