The dynamics of nitrogen uptake and its remobilization during the growth of sugar beet

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Abstract
SUMMARY: The uptake and distribution of N were examined in a series of sugar-beet crops grown on different sites (Broom's Barn, Suffolk and Trefloyne, Dyfed) or with 0 (No) or 125 kg N/ha (N125) between 1978 and 1982. Depletion of soil N was followed in some years. Initial rates of N uptake in spring for the N125 crops at Broom's Barn ranged from 2·3 kg/ha per day in 1980 to 5·8 kg/ha per day in 1981 and 1982 and at Trefloyne from 4·7 kg/ha per day in 1980 to 5·4 kg/ha per day in 1979. The initial phase of N uptake in No crops was shorter and at Broom's Barn the rate ranged from 1·6 kg/ha per day in 1979 to 5·1 kg/ha per day in 1982. Crops with high initial uptake rates had somewhat greater shoot N concentrations. There was no relation between the initial uptake rates or the total N uptake and the amounts of mineral N in the soil at the start of rapid growth in June. Simulations of early crop growth coupled with analysis of changes in the total N in the crop-plus-soil system showed that the rate of N uptake by the N125 crops was regulated by crop demand for N as determined by growth rate in 4 of the years and by soil supply in the 5th. The analysis of the crop-plus-soil N also showed that substantial losses of N occurred when the crop was actively growing in June and July in 1979 and 1980 due to excessive rainfall following early irrigations. There were serious consequences for N uptake, N concentration in developing leaves and the overall growth of these crops.N uptake rates in autumn ranged from no net uptake in 1979 and 1980 to 0·6 kg/ha per day in the other 3 years at Broom's Barn and 1·0 kg/ha per day at Trefloyne. Large amounts of N were remobilized from the shoot to sustain the growth of the storage root in years when uptakes from the soil in autumn were small. Remobilized N represented 80, 50 and 30% of the net increase in storage-root N between the end of August and harvest in 1979, 1980 and 1981 respectively. The amounts remobilized from shoots ranged from 8 to 18 kg N/ha and may therefore also represent a source of amino-N impurities in harvested beet. An analysis of N in individual leaves showed that remobilized N probably originated from leaf protein and that remobilization started at full expansion rather than at the onset of leaf senescence, which was often many weeks later.