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Subsurface soil samples were collected from 20 forest sites of differing soil and cover type, atmospheric deposition history and physiographic location. Samples were analyzed for S pools, sulfate adsorption capacity, adsorption reversibility, and associated soil properties in order to compare the relative S chemistry and SO4 retention capacities of the sites. The SO4 adsorption capacity was determined by sequential equilibration of air‐dried soil samples with a percolating solution of 0.25 mM CaSO4, and adsorption reversibility by leaching both SO4 saturated and untreated soil samples with deionzed water. Phosphate extractable SO4 was measured at the end of each series of equilibrations as a means of estimating irreversibly adsorbed SO4. A total of 32 out of 36 soil horizons studied showed net SO4 adsorption, ranging from a desorption of 0.4 to an adsorption of 7.2 mmol SO4 kg−1. Thus, most of these soils were not yet saturated with respect to a solution concentration of 0.25 mM SO4. Most soil horizons showed irreversible SO4 adsorption (an average of 36% of adsorbed SO4 was retained irreversibly) as evidenced both by changes in phosphate extractable SO4 and by a comparison of desorption of SO4 before and after saturation by the 0.25 mM SO4 solution. When adsorption and adsorption reversibility observations were compared with measured soil properties, the presence of hydrous oxides of Al and native SO4 were the best indicators of SO4 adsorption potential. Most adsorption/desorption/extraction sequences showed mass conservation of SO4, indicating inorganic SO4 adsorption appeared to be far more important in these procdures than organic S incorporation or mineralization. It is unclear how much air‐drying, sample treatment, and the procedures followed might have changed the inorganic and organic S pools of these soils.

Most of our terrestrial carbon (C) storage occurs in soils as organic C derived from living organisms. Therefore, the fate of soil organic C (SOC) in response to changes in climate, land use, and management is of great concern. Here we provide a unified conceptual model for SOC cycling by gathering the available information on SOC sources, dissolved organic C (DOC) dynamics, and soil biogeochemical processes. The evidence suggests that belowground C inputs (from roots and microorganisms) are the dominant source of both SOC and DOC in most ecosystems. Considering our emerging understanding of SOC protection mechanisms and long-term storage, we highlight the present need to sample (often ignored) deeper soil layers. Contrary to long-held biases, deep SOC—which contains most of the global amount and is often hundreds to thousands of years old—is susceptible to decomposition on decadal timescales when the environmental conditions under which it accumulated change. Finally, we discuss the vulnerability of SOC in different soil types and ecosystems globally, as well as identify the need for methodological standardization of SOC quality and quantity analyses. Further study of SOC protection mechanisms and the deep soil biogeochemical environment will provide valuable information about controls on SOC cycling, which in turn may help prioritize C sequestration initiatives and provide key insights into climate-carbon feedbacks.