Short-term immobilisation and crop uptake of fertiliser nitrogen applied to winter wheat: effect of date of application in spring

Previous studies on the fate of fertiliser nitrogen applied to winter wheat in temperate climates have shown that nitrogen (N) applied early, at tille ring for wheat, was less efficiently taken up than N applied later in the g rowth cycle. We examined the extent to which the soil microbial N immobilis ation varied during the wheat spring growth cycle and how microbial immobil isation and plant uptake competed for nitrogen. We set up a pulse-N-15 labe lled field experiment in which N was applied at eight development stages fr om tillering (beginning of March) to anthesis (mid-June). Each application was 50 kg N ha(-1) as N-15 labelled urea except for the first application w hich was 25 kg N ha(-1). The distribution of fertiliser N-15 in shoots, roo ts, mineral and organic soil N was examined by destructive sampling 7 and 1 4 days after each N-15 pulse. The inorganic N-15 pool was almost depleted b y day 14. The N uptake efficiency increased with later applications from 45 % at tillering to 65% at flowering. N immobilisation was rather constant at 13-16% of N applied, whatever the date of application. The increase in pla nt N-15 uptake resulted in an increase in the total N-15 recovery in the pl ant-soil system (N-15 in soil + N-15 in plant), suggesting that gaseous los ses were lower at the later application dates.