Abstract

Dissolved organic carbon (DOC) export from watersheds and soil organic carbon (SOC) storage are intimately linked in the terrestrial carbon cycle. However, predictions of hot spots and hot moments of DOC and SOC in watersheds remain uncertain because of high spatiotemporal variability and changing controls. In this study, we investigated the linkage between SOC storage and landform units across the 7.9-ha Shale Hills Critical Zone Observatory (CZO) and its implications for potential hot spots of DOC. We also examined the trends of DOC in soil pore water along two hillslopes of contrasting soils and topography and the impacts of rainfall, stream discharge, and stream temperature on DOC export to identify possible hot moments. Based on the SOC distribution throughout the entire catchment, swales (particularly south-facing swales) were identified as hot spots because they exhibited significantly higher SOC storage and more active hydrology as compared to the rest of the catchment. Along the two hillslopes reported here, average soil pore water DOC concentrations were noticeably higher (35 ± 12%) along the swale as compared to the planar hillslope. Soil pore water DOC concentrations were elevated at the soil–bedrock interface at the ridgetop and at the Bw–Bt horizon interface in the valley floor, suggesting transport-driven hot spots along restrictive layer interfaces. Stream water DOC concentration at the catchment outlet averaged 6.2 ± 5.3 mg L−1 from May 2008 to October 2010, which was significantly correlated with stream discharge and stream water temperature. Transport-driven hot moments of stream water DOC were observed during the periods of snowmelt and late summer to early fall wet-up, which together contributed ∼55% of total stream water DOC exported in 2009. This reflected the control of DOC export by flushing (linked to discharge) and biological activity (related to temperature) and its variation during different seasons of a year. This study showcased the impacts of complex soil and topography interactions—coupled with changing weather and seasonal biological activity—on the spatiotemporal dynamics of DOC export in a temperate forested catchment and its link to SOC distribution.

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