Distribution of 15N tracers applied to the canopy of a mature spruce-hemlock stand, Howland, Maine, USA
In nitrogen (N) limited ecosystems fertilization by increases in N deposition may enhance plant growth and thus impact carbon (C) sequestration. However in many experimental N fertilization experiments that test this interaction, soils and not plants are the main fate of additional N. A key methodological feature in many N deposition-C sequestration experiments is the addition of N directly to the soil which bypasses canopy processes thought to be important to plant nutrient needs and thus may favor N immobilization by the soil. In 2001 we began a wet foliar addition of 18 kg N/ha to a 21 ha plot in a Maine spruce-hemlock forest. The addition was as dissolved NH4NO3 and was added 5-6 times during the growing season for three years. In 2–0.3 ha subplots, we made additional foliar sprays of 1% (final enrichment) 15NH4+ or 15NO3-. In 2003 we sampled forest floor, soil, green foliage, woody biomass, litter, canopy gaseous N losses, and dissolved N fluxes in the 15N subplots to determine ecosystem N retention and estimate the fate of three years of N addition to a site previously determined to be N limited. We recovered 38 and 70% of the 15N added among ecosystem pools for 15NH4+ and 15NO3-, respectively. An additional 5-10% was lost from the canopy by gaseous processes and 2-5% lost from the rooting zone by leaching. Of 15N recoverable in plant biomass, only 3-6% was recovered in green foliage and wood. Tree twigs, branches, and bark constituted the most important plant sinks for both NO3- and NH4+, together accounting for 25 to 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%), however r Retention by canopy elements (surfaces of branches and boles) may have been through physico-chemical processes rather than by N assimilation as indicated by poor recoveries in wood tissues. Physical interception and retention of N in the intermediate term (years) may not have a strong influence upon tree growth and thus C sequestration in this forest. Consistent with the localization of 15N, we did not see significant increased biomass in the treated area as assessed through forest inventory and analysis and wood increment cores. The potential for this closed canopy, late successional forest to retain experimental N inputs was large and may have been optimized by the manner in which the N was delivered; as a dissolved N fertilizer in a volume of spray that did not exceed canopy water storage capacity and timed to occur only during the growing season. Canopy foliage, branches, wood, bark and roots retained a total of 29 and 61% of 15NH4+ and 15NO3-, respectively, but very little of this (<5%) was present in new wood as determined from increment cores. Our findings indicate that canopy retention of precipitation-borne N added in this particular manner may not become plant available N for several years but that canopy surfaces (e.g., bark and epiphytes) provide a substantial sink for N that is in contrast to earlier findings that soils are a more important short term N sink under enhanced N deposition. Despite a large canopy N retention potential in this forest, C sequestration into new wood growth was ~4-5 g C m-2 y-1 or about 2% above the current net annual C sequestration for this site.