Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4529 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8502, Japan.
Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering and Engineering, Tokyo Institute of Technology, 4529 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8502, Japan.
Sci Total Environ. 2019 Nov 10;690:290-299. doi: 10.1016/j.scitotenv.2019.06.507. Epub 2019 Jul 2.
The Hokuriku district of central Japan receives high levels of precipitation during winter, largely in the form of snow. This study aimed to elucidate the internal nitrogen dynamics in this temperate forested region with heavy snowfall using the triple oxygen and nitrogen isotopic compositions of NO. The isotopic compositions of NO in atmospheric depositions (P and Tf), with terrestrial components of the soil layer (A0, S25, S55, and S90), ground water (G), and output (St) were measured from 2015 to 2016 in a forested catchment located in the southern area of the Ishikawa Prefecture, Japan. Seasonal distributions of ΔO(NO) showed a decreasing trend from the inputs to outputs of the ecosystem. We found relatively constant ΔO(NO) values in the output components (G and St), but found highly fluctuating ΔO(NO) values resulting from the seasonal variations in the nitrification activity within soil waters. Specifically, we observed a lower nitrifying activity in the top soil layer throughout cold periods, presumably due to the input of cold melted snow water. The general trend of increasing δN(NO) value from the input to output components, with the changes in denitrification hotspots from shallow to deeper soil layer, can be observed between warm and cold periods. Thus, the seasonal changes of hotspots related to microbial nitrification and denitrification could be noted due to the seasonal changes in the isotopic compositions of nitrate. The estimated ecosystem-scale gross nitrification and denitrification rates are low; however, the output components are relatively stable with low concentrations of nitrate, indicating that the plant uptake of nitrogen most probably occurs at greater rates and scales in this forested ecosystem. Future nitrogen deposition and the vulnerable dynamics of snow melting are likely to have impactful consequences on such localities.
日本中部的北陆地区在冬季会有大量的降水，其中大部分以雪的形式出现。本研究旨在利用氮氧化物的三重氧和氮同位素组成，阐明这个降雪量大的温带森林地区的内部氮动态。在日本石川县南部的一个森林流域，从 2015 年到 2016 年，测量了大气沉降物（P 和 Tf）、土壤层的陆地成分（A0、S25、S55 和 S90）、地下水（G）和输出物（St）中 NO 的同位素组成。ΔO(NO) 的季节分布表现出从生态系统输入到输出的递减趋势。我们发现输出成分（G 和 St）中的 ΔO(NO) 值相对稳定，但由于土壤水中硝化活性的季节性变化，发现了高度波动的 ΔO(NO) 值。具体来说，我们观察到在整个寒冷时期，表层土壤中的硝化活性较低，这可能是由于冷融雪水的输入。在温暖和寒冷时期之间，可以观察到 δN(NO) 值从输入到输出成分增加的总体趋势，以及反硝化热点从浅层到深层土壤层的变化。因此，由于硝酸盐同位素组成的季节性变化，可以注意到与微生物硝化和反硝化相关的热点的季节性变化。估计的生态系统尺度的总硝化和反硝化速率较低；然而，输出成分相对稳定，硝酸盐浓度较低，这表明在这个森林生态系统中，植物对氮的吸收可能以更大的速率和规模发生。未来的氮沉积和脆弱的融雪动态可能对这些地方产生重大影响。
