Dry deposition of nitrate to a deciduous forest

Because dry-deposition inputs are difficult to measure, they are often ignored in biogeochemical studies. In this study three separate methods were used to estimate dry deposition of nitrate to a deciduous forest (Walker Branch Watershed) in eastern Tennessee. The range of estimates of dry-deposition flux was from 1.8 to 9.1 kg NO₃ ^--N ha^-1 yr^-1 Using a hybrid approach that combines some aspects of all three methods, a best estimate of 4.8 kg NO₃ ^--N ha^-1 yr^-1 was derived. About 75% of this flux is attributable to deposition of HNO₃, vapor with large particles contributing most of the remainder; the contribution from small particles is negligible. The range of estimates obtained from the three techniques suggest that dry-deposition measurements should be interpreted with caution.Our best estimate indicates that dry deposition of NO₃ ^- is the largest single form of inorganic nitrogen (N) deposition to this forest, contributing almost half of the 10.1 kg N ha^-1 total annual input. All of the enhancement of NO₃ ^- deposition in stemflow and throughfall relative to incident precipitation can be explained by washoff of dry-deposited NO₃ ^-, and some canopy uptake of dry-deposited NO₃ ^- is suggested. This uptake occurs primarily during the growing season and contributes from 0.2 to 7.5 kg N ha^-1 yr^-1 to the N requirements of the ecosystem, with a best estimate of 3.2. Despite the uncertainties, the magnitude of the potential input fluxes to forested ecosystems necessitates consideration of nitrate dry deposition in ecosystem nitrogen cycling studies.