Fate of ¹⁵N-labelled fertilizer applied on snow at two forest sites in British Columbia

Abstract

Fertilizer was applied on snow in January 1981 at 100 kg N•ha −1 as [ 15 N]urea, 15 NH 4 NO 3 , and NH 4 15 NO 3 to 11-year-old lodgepole pine (Pinuscontorta Dougl. var. latifolia Engelm.) at Spillimacheen in the British Columbia interior and as [ 15 N]urea (200 kg N•ha −1 ) to 13-year-old Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) at Green Mountain, a coastal site in British Columbia. Recovery of labelled N after one growing season was determined in soil and biomass at both sites, and it was also monitored during the growing season in snow and soil at Spillimacheen. At Green Mountain, 5.5% of urea N was recovered in tree biomass, 10.8% in understory, and 33.4% in soil organic N (total recovery 49.7%). Leaching may have contributed to N losses at Green Mountain, but was probably not a direct consequence of the application on snow. At Spillimacheen, total recovery of [ 15 N]urea was 93.3%, with 10.1% in tree biomass, 2.4% in understory, and 80.8% in soil. For 15 NH 4 NO 3 , recoveries were 5.3% in tree biomass, 2.9% in understory, and 87.0% in soil, for a total of 95.2%. For NH 4 15 NO 3 , recoveries were 1.9% in tree biomass, 3.4% in understory, and 39.1% in soil, for a total of 44.4%. At Spillimacheen, the performance of 15 NH 4 NO 3 was comparable to that of urea in tree uptake and soil retention. There were large losses with the NH 4 15 NO 3 source, however, most likely due to leaching and denitrification during snowmelt. For this reason, fertilization with nitrate on snow is not recommended because of nitrate's susceptibility to leaching, but urea and ammonium sources may be applied under these conditions.