Quantification of geogenic carbon in anthropogenic alluvial coal soils of the Susquehanna River

Alluvial riparian soils act as a filtration system, improving the environmental quality of downstream soils and waters. In areas affected by coal mining, alluvial soils also serve as a modern “sink” of fossil carbon (C). To date, little research has been done on ecosystem services provided by alluvial landscapes (i.e., river islands and tributary deltas) in the retention of coal in coal‐mining regions. The objective of this study was to distinguish between and quantify geogenic and neogenetic C in alluvial soils of the North Branch of the Susquehanna River (NBSR). To investigate this, we compared five thermal analysis methods to quantify geogenic (coal) C in soils. Our results indicate that multivariate curve resolution of ramped thermal combustion data provided the most accurate estimate of coal content in soils. Our analysis found that NBSR alluvial soils have accumulated ∼375 Gg of anthropogenic, geogenic C (upper 1 m). In these soils, an average of ∼11% of soil mass is attributable to coal, yet ∼73% of the total soil C is attributable to geogenic C. These soil organic C stocks are substantially greater than locally mapped riparian soils unaffected by coal mining and are greater than regional organic soils (Histosols). Quantification of microbial decomposition of coal in alluvial soils and vulnerability to extreme flood events (potential remobilization) requires further investigation and will be important in determining the fate of this C sink. Core Ideas Alluvial soils downstream of mining areas are sinks for waste coal. Coal contributed 11% of soil mass and 73% of total C in sampled soils. North Branch of Susquehanna River alluvial soils contain 815 Mg geogenic C ha−1. These soils have also accumulated 266 Mg neogenetic soil C ha−1. Distinguishing and quantifying these two pools is essential to predicting fate of C sink.