COMPARISON OF FOUR METHODS OF MEASURING VOLATILIZATION LOSSES OF NITROGEN FOLLOWING UREA FERTILIZATION OF FOREST SOILS

Abstract

Four methods were compared for measuring ammonia volatilization losses following urea application (220 kg N/ha) to a forest soil from Vancouver Island: (i) closed-static, (ii) semi-open, (iii) 15N-balance, and (iv) closed-dynamic. The first three methods were used in the field; the fourth in the laboratory. In addition, the effects of two levels of simulated rainfall were assessed with methods ii, iii and iv. Significantly greater [Formula: see text] amounts of volatile ammonia were measured by each of the following three methods in the order: closed-static (13%) < semi-open (17%) < closed-dynamic (22–26%). The 15N-balance method measured 35–42%, but these values could not be compared directly with the other three methods, because it measures losses from gases other than ammonia alone. The simulated 12-mm rainfall significantly [Formula: see text] decreased ammonia losses, but did not change the relationship among methods: semi-open < closed-dynamic < 15N-balance for 9, 12 and 22%, respectively. Since estimated ammonia losses (i.e. total minus other gases) in open microplots with 15N-urea approximated that obtained by the closed-dynamic method, the latter gives a more representative estimate of ammonia losses than the semi-open or closed-static systems. Losses from nitrogen oxides (NO and NO2), measured by the closed-dynamic method, were less than 1% of the applied fertilizer, while unaccountable losses by the 15N-balance method approached 23%. This suggests that losses as di-nitrogen (N2) and nitrous oxide (N2O), following urea application to forests, might be more important than is usually recognized. Significant differences in ammonia recovery by the closed-dynamic method from non-tracer (standard) granules and 15N-enriched granules (26 vs. 22%) was unexpected. Further research is warranted to determine the effects of re-processing techniques used to enrich the 15N content of urea.