PRIMING EFFECTS OF N-15-LABELED AMMONIUM-NITRATE ON UPTAKE OF SOIL N BY WHEAT (TRITICUM-AESTIVUM L) UNDER FIELD CONDITIONS

The uptake of labelled and unlabelled N by wheat was measured in a field experiment using N-15-labelled ammonium nitrate fertilizer. The dry matter yield and N yields were significantly increased with fertilizer N application compared to those from unfertilized soil. The uptake of applied N by wheat ranged between 25 and 34%. Fertilizer N application increased the uptake of unlabelled soil N which was attributed to a positive ''priming effect'' or added N interaction. The added N interaction observed by applying 20, 60, and 120 kg fertilizer N was 11.4, 19. 1, and 27.9 kg, corresponding to 26, 44 and 64%, respectively of the N taken up from unfertilized soil. The A values did not alter with the increase in fertilizer N application. The observed added N interaction may have been the result of pool substitution whereby added labelled fertilizer N stood ''proxy'' for unlabelled soil N. A significant correlation coefficient (r = 0.996**) between the uptake of soil N and the dry matter yield showed that soil N was more important than fertilizer N in wheat production.