Organic and inorganic nitrogen deposition in an urban evergreen broad-leaved forest in central Japan

Abstract

Chronic atmospheric nitrogen (N) deposition could influence the functioning of ecosystems as well as their biodiversity. However, N deposition in urban forest ecosystems, especially natural evergreen broad-leaved forests, is not well known. In this study, the concentrations and fluxes of dissolved inorganic N (DIN) and dissolved organic N (DON) in bulk deposition, throughfall, and stemflow were assessed in an urban evergreen broad-leaved forest site over three years, in order to clarify the characteristics of N deposition. At the study site, bulk DIN deposition was 3.7 kg N ha−1 year−1 (1.5 kg N ha−1 year−1 for NH4–N and 2.2 kg N ha−1 year−1 for NO3 + NO2–N), which is the same level as that found in rural areas. In contrast, 6.5 kg N ha−1 year−1 for bulk DON deposition contributed to 66% of the bulk N deposition, which suggests the importance of bulk DON deposition in Japanese forest ecosystems. Passing through the tree canopy, DIN was enriched by 8.8 kg N ha−1 year−1 (3.7 kg N ha−1 year−1 for NH4–N and 5.1 kg N ha−1 year−1 for NO3 + NO2–N) and DON was enriched by 1.5 kg N ha−1 year−1 as net throughfall in the evergreen broad-leaved forest. This reveals that dry deposition of DIN dominates the total DIN deposition onto the urban forest floor, compared to that found in the rural areas, due to the non-negligible N emissions from outside and possibly because of the evergreen broad-leaved forest's greater ability to capture N.