SITE-SPECIFIC UNDERESTIMATION OF WETFALL NH4+ USING NADP DATA

Estimates of NH4+ in weekly composited wetfall samples were measured by two laboratories, one at Kansas State University and that operated by the National Atmospheric Deposition Program (NADP) in Illinois. NH4+ samples were lower in the NADP estimates and exhibited a strong seasonal difference in concentrations. Losses were likely not due to volatilization; microbial immobilization of NH4+ likely occurred during transport of samples. Differences in estimates of annual ammonium ion deposition were larger than those observed for average weekly concentrations because of seasonal rainfall patterns. NADP values will require site-specific corrections for models describing or predicting regional or national patterns of nitrogen inputs in wetfall.