ATMOSPHERIC DEPOSITION AND CANOPY INTERACTIONS OF NITROGEN IN FORESTS

Wet deposition of nitrogen compounds was measured and dry and cloud wa ter deposition were estimated at 11 forested sites in North America an d one site in Europe. Dry deposition was a significant pathway of N in put to all the forests, averaging 46% of the total deposition. At most of these sites, NH4+ was the dominant form of fixed N in the air. but HNO3 vapor dominated the dry deposition of N. Coarse-particle deposit ion was often important, but fine-particle deposition usually contribu ted only a small amount of the dry-deposited N. The deposition rates o f inorganic N, which ranged between 4.8 and 27 kg N.ha-1.year-1, were generally much higher than has been reported by other studies measurin g only wet or bulk deposition. The highest deposition rates were at th e high-elevation sites in the southeastern and northeastern United Sta tes and much of the deposition at these sites was attributed to cloud water. Throughfall and stemflow (TF + SF) flux was also measured at al l sites, and the net canopy exchange (NCE = (TF + SF) - total depositi on) was found to be negative (indicating consumption of N in the canop y) for NH4+ and NO3-, and positive (indicating canopy release) for org anic N. Past reports of canopy release of NO3- can probably be attribu ted to washoff of dry-deposited NO3- species. Consumption of inorganic N in the canopy ranged from 1 to 12 kg N . ha-1.year-1, and was highe st in the spruce and spruce-fir stands. When organic N was included in the canopy N balance, the net canopy uptake of N was generally <15% o f the total system N requirement. Total N deposition was a linear func tion of wet deposition for low-elevation sites, and dry deposition was a linear function of the net throughfall flux for NO3-.