Organic and inorganic nitrogen deposition on the red pine forests at the northern foot of Mt. Fuji, Japan

Abstract

Abstract In order to clarify the deposition amounts and processes of bioavailable nitrogen species including organic forms on forests, a simultaneous observation of rainfall, throughfall, stemflow, aerosols, and their precursor gases was conducted for a two-year period at a forested site on the northern foot of Mt. Fuji in Japan. About 80–90% of the rainfall waters reached the forest floor via the throughfall process and the remaining fraction can be considered to be intercepted by the forest canopy and not transported to the forest floor. Partitioning of the rainfall waters into stemflow waters was negligible. Deposition amounts of the total dissolved nitrogen by rainfall, throughfall, and stemflow were 2.204, 2.685, and 0.027 mgN m−2 day−1, respectively, on average. Contributions of the water-soluble organic nitrogen (WSON) to the dissolved nitrogen in the rainfall, throughfall, and stemflow deposition were about 51%, 58%, and 75%, respectively. The major fraction of the nitrogen species in the deposition was the organic forms. Among the inorganic nitrogen, the contribution of NH4+-N to the nitrogen deposition was significantly greater than that of NO3−-N in the rainfall deposition, whereas both were comparable in the throughfall deposition. The deposition amounts of NO3−-N and WSON by throughfall were greater than those by rainfall due to the influence of the dry deposition on the canopies and the leaching from trees, whereas the deposition amount of NH4+-N by throughfall was smaller than that by rainfall due to the uptake by trees. About 58% of the net nitrogen deposition on the forest floor, which is defined as the sum of the deposition amounts by the throughfall and stemflow, was explained by the WSON. In the remaining inorganic fraction, the contributions of NO3−-N and NH4+-N to the net nitrogen deposition were comparable, about 20% and 22%, respectively. The nitrogen deposition amount to the forest floor was estimated to be 2.711 mg m−2 day−1, which is equal to the value of the threshold for the condition of “nitrogen saturation” for the forest ecosystems. The modified canopy budget model can estimate that the contributions of the dry deposition to the sum of the dry and wet deposition were about 90%, 64%, 61%, and 22% for NO2−-N, NO3−-N, NH4+-N, and WSON, respectively. The canopy leaching was found only for the WSON, about 72%, 20%, and 8.5% of the net deposition of the WSON can be explained by the wet deposition, dry deposition, and canopy leaching, respectively. The remaining nitrogen compounds showed the canopy uptake. About 10%, 56% and 32% of NO2−-N, NO3−-N, and NH4+-N deposited on the canopy by the wet and dry processes would be transported to the forest floor and the remaining fractions (about 90%, 44%, and 68%, respectively) would be taken into the canopy. Comparisons of the dry deposition amount with the concentration in the surface atmosphere of the nitrogen compounds suggest that the modified canopy budget model can be considered to be better to estimate the amounts of the dry deposition and canopy exchange.