Organic matter characteristics and mineralization in ironstone gravel from two Plinthic soils in the Coastal Plain of Georgia

Agricultural soils can contain a significant proportion of rock fragments, but most routine procedures for the estimation of soil carbon (C) and nitrogen (N) stocks promote the exclusion of the >2 mm component. This fractioning eliminates ironstone gravel, which is a common but poorly understood rock fragment found in Plinthic soils. Thus, the purpose of this experiment was to study the role of ironstone gravel on soil C and N dynamics in two Plinthic soils of the southern Coastal Plain of Georgia. Specifically, the objective of this study was to conduct an incubation experiment using ground and unground ironstone gravel to (1) evaluate their C and N mineralization potential, (2) quantify and characterize the dissolved organic matter (DOM) generated from them, and (3) quantify their greenhouse gas evolution potential. Results from this incubation experiment revealed that ironstone gravel can contain 1%–5% of potentially mineralizable C and N, and that both ground and unground gravel can generate CO₂ and N₂O. The DOM released upon grinding the gravel is relatively fresh and of microbial origin. Thus, ironstone gravel is a chemically and biologically active component of Plinthic soils in the southern Coastal Plain. Disregard of the gravel fraction in soils where ironstone is present may result in an underestimation of the soil total C and N, which has implications for the accounting of C in agricultural soils and for the development of strategies to sequester soil C.