Ammonia and Nitrate Nitrogen in the Soil Profile and Its Relation to Various Nitrogen Treatments on Dry-Land Winter Wheat

In a dry-land winter wheat field, patterns of mineral nitrogen distributions were investigated before and after fertilizer additions. Large differences in the added mineral nitrogen recoverable three weeks after treatment were found to be specific to nitrogen source and treatment within source. Initial losses averaged 50 percent for urea treatments, 40 percent for calcium nitrate treatments, and varied from a loss of 18 percent to a gain of 22 percent for ammonium nitrate treatments. Ammonium sulfate proved the most variable with a 36 percent average loss for the before-planting treatment and a 61 percent gain for the after-planting treatment. This initial gain and loss behavior correlated at the 2 percent level of significance with the subsequent grain yield (r=0.774, 8 df), and was still discernable in soil test results of late April, where total mineral nitrogen depletion since before treatment correlated positively at the 10 percent level of significance with the nitrogen loss found 3 weeks after treatment. For a small sample of nine plots, a late July sampling revealed that depletions of mineral nitrogen since April were much more predictive of grain yields than were the actual April-N levels (r2=0.787 versus r2=0.460). This result confirms the large role played by differential moisture stress regimes in the field, since depletions during the drying season of late spring and early summer depend on the availability of moisture. Initial fertilizer behavior, determining fertilizer losses before the onset of crop usage, and a favorable later moisture regime were seen as the two largest determinants of yield under the conditions of this experiment. Since the latter factor is largely beyond further control, the former is the only factor open to manipulation. Generally, after planting treatments were lower in initial losses of mineral nitrogen, and also generally provided somewhat higher surface mineral nitrogen levels in early spring, which was found to be weakly correlated with yield. Surface accumulations in early spring can only be beneficial if sufficient spring moisture is available for downward transport into the root zone, however, and a drier spring than prevailed during this experiment could forseeably reverse this relationship by keeping such surface nitrogen accumulations from becoming available to the plant in spring