Effects of Growing Season Soil Temperature, Moisture, and Nh₄-N on Soil Nitrogen

Wood Mountain loam was wetted with water or (NH 4 ) 2 SO 4 solution to provide a factorial combination among three moisture and three NH 4 -N levels. Samples in polyethylene bags were incubated at 2.5-cm depths in fallow, and in an incubator that simulated the diurnal patterns of temperature fluctuation recorded in the field. During the growing season, treatments were sampled regularly for moisture, NO 3− and exchangeable NH 4 -N. Similar determinations were made on in situ samples taken in fallow Wood Mountain loam. The incubator simulated the effects of growing season temperatures on soil N transformations satisfactorily. Pronounced increases or decreases in temperature led to flushes in N mineralization. However, in the 1972 growing season, temperature was suboptimal and temperature changes were generally small. Consequently, when a stepwise multiple regression technique was used to analyze the data, neither ammonification nor nitrification showed a quantitative relationship to temperature. Comparison of the nitrification occurring in laboratory-incubated soils with that occurring in situ led to the conclusion that 70 to 90% of the NO 3 -N produced in surface soil resulted from wetting and drying. Estimates of potentially ammonifiable soil N(N o ) and its rate of mineralization (k) were derived from cumulative ammonification by assuming that the laws of first-order kinetics were applicable. In the 10, 15, and 20% moisture treatments the average N o was 27, 41, and 82 ppm, respectively. Under the conditions of this study, the time required to mineralize half of N o was about 7 wk.