PARENT SESSION
Contributed Oral Session 146: Biogeochemistry: Nutrient Dynamics; Carbon Sequestration
Thursday, August 11, 1:30 PM - 5:00 PM, Meeting Room 524 C, Level 5, Palais des congrès de Montréal

Uptake and recycling of nitrogen two years after fertilizer addition to a riparian ecosystem.

Chambers, Chris^*,1, Marshall, John ², Danehy, Robert ³, ¹ School of Biological Sciences, Pullman, WA, USA² Department of Forest Resources, Moscow, ID, 99164³ Weyerhaueser Company, Springfield, OR, 97478

ABSTRACT- A pulse of fertilizer N incorporating a ¹⁵N tracer was added to 2 streams in Central Idaho in the fall of 2001. The treatments included 448 kg N/ha, 224 kg N/ha, and 0 added fertilizer. Fertilizer uptake and turnover after 1 year has been reported previously. In 2003, two years after fertilizer application, we sampled the same sites to determine whether uptake and turnover patterns had changed from the previous year. Current-year foliage of deciduous shrubs decreased slightly in ¹⁵N between 2002 and 2003 (difference in means = 2.5‰ ± 1.0‰). Current-year needles of trees increased in ¹⁵N in the 448kg N/ha treatment between 2002 and 2003 (difference in means of 0.8‰ ± 0.27‰), but decreased in the 224 kg N/ha treatment between 2002 and 2003 (difference in means of 4.6‰ ± 0.53) Older needle age classes showed significant increases in ¹⁵N compared to the untreated reaches, suggesting fertilizer N storage (p < 0.0001). Although on average foliar ¹⁵N remained constant between 2002 and 2003, foliar percent N decreased (2002: 1.84% ± 0.04%, 2003: 1.51% ± 0.04%) indicating a significant growth response. Alnus incana, which had taken up miniscule amounts of fertilizer N in 2002, did not show a significant portion of fertilizer N in its foliage in 2003 (p = 0.5). An analysis of leaf litter for 2003 is pending, and should illuminate the differences in retention of a pulse of nitrogen between conifers and deciduous shrubs. The continued storage and uptake of fertilizer N in coniferous trees contrasts starkly with the downturn in co-occurring deciduous shrubs. If current trends continue, the fertilizer nitrogen will gradually move into and accumulate within the conifer trees.

Key words: nitrogen stable isotopes, retranslocation, foliar N