R. Sylvester-bradley et al., Dynamics of nitrogen capture without fertilizer: the baseline for fertilizing winter wheat in the UK, J AGR SCI, 136, 2001, pp. 15-33
Experiments at three sites in 1993. six sites in 1993 and eight sites in 19
95, mostly after oilseed rape, tested effects of previous fertilizer N (dif
fering by 200 kg/ha for 1993 and 1994 and 300 kg/ha for 1995) and date of s
owing (differing by about 2 months) on soil mineral N and N uptake by winte
r wheat cv. Mercia which received no fertilizer N. Soil mineral N to 90 cm
plus crop N ('soil N supply'; SNS) in February was 103 and 76 kg/ha after l
arge and small amounts of previous fertilizer N respectively but was not af
fected by date of sowing. Previous fertilizer N seldom affected crop N in s
pring because sowing was too late for N capture during autumn, but it did a
ffect soil mineral N, particularly in the 60 90 cm soil horizon. presumably
due to over-winter leaching. Tillering generally occurred in spring, and w
as delayed but not diminished by later sowing. Previous fertilizer N increa
sed shoot survival more than it increased shoot production. Final shoot num
ber was affected by previous fertilizer N, but not by date of sowing. Overa
ll, there were 29 surviving tillers/g SNS.
N uptakes at Fortnightly intervals from spring to harvest at two core sites
were described well by linear rates. The difference between sowings in the
fitted date with 10 kg/ha crop N was 1 month. these dates were not signifi
cantly affected by previous fertilizer. N uptake rates were increased by bo
th previous fertilizer N and late sowing. Rates of N uptake related closely
to soil mineral N in February such that 'equivalent recovery' was achieved
in late May or early June. At one site there was evidence that most of the
residue from previous fertilizer N had moved below 90 cm by February, but
N uptake was nevertheless increased. Two further 'satellite' sites behaved
similarly. Thus at 14 out of 17 sites. N uptake until harvest related direc
tly and with approximate parity to soil mineral N in February (R-2 = 0.79).
a significant intercept bring in keeping with an atmospheric contribution
of 20-40 kg/ha N at all sites.
It is concluded that, on retentive soils in the UK, SNS in early spring was
a good indicator of N availability throughout growth of unfertilized wheat
. because the N residues arising from previous fertilizer mineralized befor
e analysis, yet remained largely within root range. The steady rates of soi
l mineral N recovery were taken as being dependent on progressively deeper
root development. Thus, even if soil mineral N equated with a crop's N requ
irement, fresh fertilizer applications might be needed before 'equivalent r
ecovery' of soil N, to encourage the earlier processes of tiller production
and canopy expansion. The later process of grain filling was sustained by
continued N uptake (mean 41 kg/ha) coming apparently from N leached to the
subsoil(relating to previous fertilizer use) as well as from sources not re
lated to previous fertilizer use. significant net mineralization was appare
nt in some subsoils.