Nitrogen Deposition to a Southeastern U.S. Forest

During the Duke Forest CELTIC study, we used the eddy covariance flux method to measure NOy deposition along with gradients of HNO3, NH3, NH4+ and NO3- in order to elucidate components of dry deposition of nitrogen. These measurements have been viewed in light of continuous CASTNET-based concentrations to estimate annual nitrogen deposition for the Duke Forest area. Significant findings include (1) HNO3 vapor concentrations may be overestimated in the CASTNET data set, probably due to re-volatilization of aerosol-NH4NO3 and (2) total oxidized nitrogen (NOy) deposited is significantly greater than HNO3 deposition alone (~ 30-40%). The effect of (1) is an overestimate of N-deposition when using the CASTNET data; however, this is more than offset by including the total NOy flux in (2). Scaling up to an annual budget yields a best estimate of 13.4 kg N m-2 deposited in the Duke Forest area in 2003. Approximately 48% of this was due to dry deposition. Figure 1 shows that dry deposition of NOy (which includes gas phase HNO3) is a major component of N-deposition during fall and winter periods when wet deposition is at a minimum.