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 NH4NO3 directly to the canopy of 21 ha of spruce-hemlock forest. In two 0.3-ha subplots, the added N was isotopically labeled as 15NH4 + or 15NO3 − (1% final enrichment). Among ecosystem pools, we recovered 38 and 67% of the 15N added as 15NH4 + and 15NO3 −, respectively. Of 15N 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 NO3 − and NH4 +, together accounting for 25–50% of 15N recovery for these ions, respectively. Forest floor and soil 15N retention was small compared to previous studies; the litter layer and well-humified O horizon were important sinks for NH4 + (9%) and NO3 − (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.