A Nitrogen‐Saturated Plantation of Cryptomeria japonica and Chamaecyparis obtusa in Japan Is a Large Nonpoint Nitrogen Source

• Published in: Journal of environmental quality Vol. 44; no. 4; pp. 1225 - 1232
• Main Authors: Chiwa, Masaaki, Saito, Takami, Haga, Hirokazu, Kato, Hiroaki, Otsuki, Kyoichi, Onda, Yuichi
• Format: Journal Article
• Language: English
• Published: The American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc 01-07-2015
• ISSN: 0047-2425; 1537-2537
• DOI: 10.2134/jeq2014.09.0401

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 μmol 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.