EFFECTS OF FERTILIZER N AND SOIL MOISTURE ON MINERALIZATION, N RECOVERY AND A-VALUES, UNDER SPRING WHEAT GROWN IN SMALL LYSIMETERS

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Abstract
The influence of rate of fertilizer N and soil moisture on N uptake by spring wheat, N mineralization, A-values and N recovery was determined in small lysimeters on stubble land by means of 15N-labelled KNO3. Net mineralization was enhanced by frequent irrigations but depressed by cropping. In contrast to most growth chamber results, N uptake was not linearly but logarithmically related to rate of application; uptake from fertilizer was positively related to rate, but only up to a point beyond which it levelled off; uptake from native soil N was generally negatively related to rate; A-values were not constant but negatively related to rate except at the highest rates (123–164 kg N/ha) when they were positively related especially under dry conditions. These differences in results were credited to the fact that the pot system does not adequately simulate the field situation. On dryland an average of 68, 20 and 12% of the plant N was located in the grain, straw and roots, respectively; under irrigation the corresponding values were 75, 16 and 9%. Average recovery of fertilizer N on dryland was: soil 34.6%, grain 37.3%, straw 12.2%, roots 2.6%, error 6.0%, and unaccounted 7%; under irrigation it was 15.4, 58.3, 13.0, 3.5, 6.0 and 4.0%, respectively. On dryland about 28% of the fertilizer N was left in the soil at rates up to 82 kg N/ha, while 57% was left at 164 kg N/ha; under irrigation the corresponding values were 15 and 21%, respectively. On dryland > 70% of the residual N was located in the 0- to 30-cm soil segment at fertilizer rates < 82 kg N/ha; at higher rates > 50% was in the 30- to 60-cm segment. Only at 164 kg N/ha was there appreciable residual N in the 30- to 60-cm segment under irrigation. There was negligible fertilizer N below 60 cm in all treatments.