Distribution of nitrogen-15 tracers applied to the canopy of a mature spruce-hemlock stand, Howland, Maine, USA.

In N-limited ecosystems, fertilization by N deposition may enhance plant growth and thus impact C sequestration. In many N deposition-C sequestration experiments, N is added directly to the soil, bypassing canopy processes and potentially favoring N immobilization by the soil. To understand the impact of enhanced N deposition on a low fertility unmanaged forest and better emulate natural N deposition processes, we added 18 kg N ha(-1) year(-1) as dissolved NH(4)NO(3) directly to the canopy of 21 ha of spruce-hemlock forest. In two 0.3-ha subplots, the added N was isotopically labeled as (15)NH(4) (+) or (15)NO(3) (-) (1% final enrichment). Among ecosystem pools, we recovered 38 and 67% of the (15)N added as (15)NH(4) (+) and (15)NO(3) (-), respectively. Of (15)N recoverable in plant biomass, only 3-6% was recovered in live foliage and bole wood. Tree twigs, branches, and bark constituted the most important plant sinks for both NO(3) (-) and NH(4) (+), together accounting for 25-50% of (15)N recovery for these ions, respectively. Forest floor and soil (15)N retention was small compared to previous studies; the litter layer and well-humified O horizon were important sinks for NH(4) (+) (9%) and NO(3) (-) (7%). Retention by canopy elements (surfaces of branches and boles) provided a substantial sink for N that may have been through physico-chemical processes rather than by N assimilation as indicated by poor recoveries in wood tissues. Canopy retention of precipitation-borne N added in this particular manner may thus not become plant-available N for several years. Despite a large canopy N retention potential in this forest, C sequestration into new wood growth as a result of the N addition was only ~16 g C m(-2) year(-1) or about 10% above the current net annual C sequestration for this site.