Chiwa Masaaki, Saito Takami, Haga Hirokazu, Kato Hiroaki, Otsuki Kyoichi, Onda Yuichi
J Environ Qual. 2015 Jul;44(4):1225-32. doi: 10.2134/jeq2014.09.0401.
Japanese cedar () and Japanese cypress () plantations account for approximately 30% of the total forested area in Japan. Both are arbuscular mycorrhizal trees that leach more NO 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 NO export was calculated from load-discharge relationships. Atmospheric N deposition was also determined. The stream water contained high NO concentrations (160 and 165 μmol L during base flow and storm flow, respectively), indicating N saturation in the watershed. High bulk atmospheric N deposition (16.5 kg N ha yr) could explain the N saturation. There were only small variations in NO 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 NO exports from the studied watershed. The annual NO export was high (36.1 kg N ha yr), 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.
日本柳杉（Cryptomeria japonica）和日本扁柏（Chamaecyparis obtusa）人工林占日本森林总面积的约30%。二者均为丛枝菌根树，相较于外生菌根树森林，它们在应对氮（N）沉降时会淋溶出更多的NO₃⁻。然而，关于这些人工林氮输出量的信息却很少。本研究的目的是评估氮饱和人工林的非点源氮输出。我们在一个集水区（2.98公顷）收集了基流（25个样本）和暴雨径流条件下（20次事件）的溪水样本，该集水区于1969年种植了日本扁柏和日本柳杉（树龄41年）。根据负荷 - 流量关系计算年NO₃⁻输出量。同时也测定了大气氮沉降。溪水中NO₃⁻浓度较高（基流和暴雨径流期间分别为160和165 μmol L⁻¹），表明该集水区存在氮饱和现象。高大气氮沉降量（16.5 kg N ha⁻¹ yr⁻¹）可以解释氮饱和现象。由于森林生态系统的氮饱和，溪水中NO₃⁻浓度随径流量的变化很小。因此，在所研究集水区估算年NO₃⁻输出量时误差很小。年NO₃⁻输出量很高（36.1 kg N ha⁻¹ yr⁻¹），与农业和城市化地区报道的值相当。这些结果表明，氮饱和的森林人工林可能成为重要的非点源氮源。我们的结果还表明，应该对日本各地森林人工林的氮输出进行量化，以便准确评估非点源氮。
