A nitrogen-saturated plantation of Cryptomeria japonica and Chamaecyparis obtusa in Japan is a large nonpoint nitrogen source

Japanese cedar (Cryptomeria japonica) and Japanese cypress (Chamaecyparis obtusa) plantations account for approximately 30% of the total forested area in Japan. Both are arbuscular mycorrhizal trees that leach more NO₃ ^- in response to nitrogen (N) deposition than do forests of ectomycorrhizal trees. However, little information is available about the size of N exports from these plantations. The aim of this study was to evaluate nonpoint source N exports from a N-saturated plantation. We collected stream water samples in base-flow (25 samples) and storm-flow conditions (20 events) in a watershed (2.98 ha) where Japanese cypress and Japanese cedar were planted in 1969 (41 yr old). The annual NO₃ ^- export was calculated from load-discharge relationships. Atmospheric N deposition was also determined. The stream water contained high NO₃ ^- concentrations (160 and 165 mmol L^-1 during base flow and storm flow, respectively), indicating N saturation in the watershed. High bulk atmospheric N deposition (16.5 kg N ha^-1 yr^-1) could explain the N saturation. There were only small variations in NO₃ ^- concentrations in stream water in response to discharge volume, because of the N saturation of the forest ecosystem. Consequently, there were only small errors in estimating annual NO₃ ^- exports from the studied watershed. The annual NO₃ ^- export was high (36.1 kg N ha^-1 yr^-1), comparable to values reported for agricultural and urbanized areas. These results suggest that N-saturated forest plantations can become important nonpoint N sources. Our results also suggest that N exports from forest plantations across Japan should be quantified to evaluate nonpoint source N accurately.