Dark formation of hydroxyl radicals (•OH) from soil/sediment oxygenation has been increasingly reported, and solid Fe­(II) is considered as the main electron donor for O2 activation. However, the role of solid organic matter (SOM) in •OH production is not clear, although it represents an important electron pool in the subsurface. In this study, •OH production from oxygenation of reduced solid humic acid (HAred) was investigated at pH 7.0. •OH production is linearly correlated with the electrons released from HAred suspension. Solid HAred transferred electrons rapidly to O2 via the surface-reduced moieties (hydroquinone groups), which was fueled by the slow electron transfer from the reduced moieties inside solid HA. Cycling of dissolved HA between oxidized and reduced states could mediate the electron transfer from solid HAred to O2 for •OH production enhancement. Modeling results predicted that reduced SOM played an important or even dominant role in •OH production for the soils and sediments possessing high molar ratios of SOC/Fe­(II) (e.g., >39). The significant contribution of SOM was further validated by the modeling results for oxygenation of 88 soils/sediments in the literature. Therefore, reduced SOM should be considered carefully to comprehensively understand •OH production in SOM-rich subsurface environments.

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Fourteen field cruises were carried out in a mariculture region of the northern Shandong Peninsula, North Yellow Sea, China from 2016 to 2017 for a better understanding of the biogeochemical behaviors, sources and export of dissolved inorganic nutrients. The spatial variations of nutrients were not obvious due to the influence of complex hydrological and biochemical conditions. Potential nutritional level was characterized in oligotrophy, and trophic status was rated at medium level. A preliminary estimation of nutrient budgets demonstrated that the dissolved inorganic nitrogen (DIN) load was mainly from atmospheric deposition and scallop excretion, accounting for 56.9% and 35.6% of its total influx. Scallop excretion and sediment release were the major source of phosphate (DIP), contributing to 25.2% and 44.3%, while dissolved silicon (DSi) was mainly from sediment release, accounting for 94.2%. In addition, about 136.7 × 10 3 , 7.3 × 10 3 and 485.5 × 10 3 mol km −2 yr −1 of DIN, DIP and DSi could be converted into other forms, e.g. organic and particulate matter and gas species.

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