Ammonium Nitrogen Deposition as a Dominant Source of Nitrogen in a Forested Watershed Experiencing Acid Rain in Central Japan

Abstract

To clarify nitrogen (N) sources, the overall N budget in a forested watershed in Kanagawa Prefecture, Central Japan was estimated by measuring dissolved inorganic N (DIN; NH4 (+) + NO3 (-) + NO2 (-)) from Nov 2004 through Oct 2005. The estimated N budget (-1.43 kg N ha(-1) year(-1)) showed that the N output rate (stream water N) was higher than the N input rate (bulk deposition N) in the watershed. The annual NO2 (-) and NO3 (-) input rates were 0.02 and 1.99 kg N ha(-1) year(-1), respectively. NH4 (+) was the predominant source in this forested watershed, accounting for 71% (4.99 kg N ha(-1) year(-1)) of DIN input rate. In addition, this study estimated rainfall pH, air temperature, and wind direction, which were considered as controlling factors related to the atmospheric deposition rate of NH4 (+). This study showed that the rainfall NH4 (+) was inversely proportional to the initial pH of the rainfall, which was calculated by adding the amount of H+ consumed by the dissociation process of NH3(aq) to the measured rainfall pH. This result implies that acid rain can elevate the solubility of NH3(g) and the dissociation capacity of NH4 (+) throughout the process of precipitation. Also, this study provides strong evidence that the high NH4 (+) deposition rate is mainly derived from NH3(g) emitted from livestock wastes under the NH3 transport condition of warm summer and favorable wind direction.