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 yr1), 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. A Nitrogen-Saturated Plantation of Cryptomeria japonica and Chamaecyparis obtusa in Japan Is a Large Nonpoint Nitrogen Source