IMPACTS OF LAND USE ON MINERALS' INTERACTION WITH ORGANIC CARBON: FROM SOILS TO WATERSHED SCALES

Do sediment generation and transport affect the land-atmosphere carbon (C) exchange in regional to global scales? This question has encouraged many scientists to investigate the fate of organic C in sediments. However, relatively little attention has been given to minerals in sediments even though biogeochemistry literature is rich in documenting mineralogical control of soil C cycles. Focusing on minerals’ specific surface area (SSA) and its coverage by C in soils and sediments, we study a hilly watershed within the Christina River Basin Critical Zone Observatory in Piedmont Pennsylvania and an adjacent largely flat Coastal Plain in Delaware. At both places, we compared agricultural and adjacent forest soils. The reduction in plant C input by crop harvest results in reduced soil C contents, while tillage incorporates clay minerals and iron oxides with large SSA from the underlying B horizons into A horizons. Therefore, C adsorption on mineral surface shifts from SSA-limited in the forest soils to C-limited in the agricultural soils, which indicates that eroded minerals from agricultural upland will further adsorb organic matter onto their surface – with potential implications for the land-atmosphere carbon exchange - as they transit through fluvial systems. A simple modeling exercise suggests that such impact of agriculture on the mineral-C interaction at watershed scales would vary with the rates of erosion and plant C input as well as the initial weathering profile in the uplands. Physical mixing and erosion of minerals further alter the surface characteristics of minerals as the minerals transit across diverse geochemical environments. Notably, the contents of iron oxides, which are found to largely control mineral SSA in soils, decrease when sediments are temporarily stored in reducing settings. Lastly, addition of limes and fertilizer has significantly enriched soils with Ca and P despite their losses through chemical weathering, which also suggests that stream C cycle and the capacity of floodplain soils to store C will be distinct depending on whether the sediments originate from forests or agricultural field. Our study provides evidence that erosion and chemical weathering should significantly alter the mineralogical control of C cycle and that this control is sensitive to land use practices.