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

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Abstract
Four methods were compared for measuring ammonia volatilization losses following urea application (220 kg N/ha) to a forest soil from Vancouver Island [Canada]: (i) closed-static, (ii) semi-open, (iii) 15N-balance and (iv) closed-dynamic. The first 3 methods were used in the field; the 4th in the laboratory. In addition, the effects of 2 levels of simulated rainfall were assessed with methods ii, iii and iv. Significantly greater (P .ltoreq. 0.05) amounts of volatile ammonia were measured by each of the following 3 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 3 methods, because it measures losses from gases other than ammonia alone. The simulated 12 mm rainfall significantly (P .ltoreq. 0.05) 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 < 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 reprocessing techniques used to enrich the 15N content of urea.

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