Carbon and nitrogen exports from forested headwater catchment in southwestern Japan

Accurate estimation of carbon and nitrogen exports from forest ecosystems via streams is important because of the critical role these losses play in the carbon and nitrogen budgets in forest ecosystems. However, little is known about the export of carbon and nitrogen from small hilly catchment under an East Asian monsoon climate In this study, carbon and nitrogen exports in both particulate and dissolved forms were measured under baseflow and stormflow conditions of a forested catchment in southwestern Japan. Coarse particulate matter (CPM) was collected continuously by using mesh frames deployed across a channel. Exports of fine particulate matter (FPM) and dissolved organic and inorganic matter were calculated by using load–discharge relationships. Total estimated yields for carbon and nitrogen were 369.6 kg ha−1 year−1 and 18.3 kg ha−1 year−1, respectively. FPM accounted for more than 60% of the total stream yield of both carbon and nitrogen, and was on the upper end of the range found in worldwide reviews. The contribution of CPM to the total yield was limited to 2% and 1% for carbon and nitrogen, respectively. Most of the carbon (81%) and nitrogen (93%) was exported by stormflows which occurred only 8% of the time. Large rainstorms occurred during the bai-u season and typhoons, which is a typical rainfall pattern in the monsoon climate. Therefore, to estimate the total yield of carbon and nitrogen from hilly catchments experiencing large frequent storms, adequate sampling and computation particularly of FPM export by stormflows is critical.