Microbial responses under sunlight-dark conditions accelerate sequestration and transformation of soil biogenic, redox and non-redox components, including As and Hg

Climatic warming and the resulting increase in soil respiration affect the sequestration and transformation of soil components, as well as their transport to the surrounding ecosystems. However, the integrated mechanistic details of these processes remain elusive. Here we apply an extraction protocol that utilizes two sequential extraction techniques to isolate and analyze both dissolved and solid-state soil components and to assess their dark and photoinduced fates under varying temperature conditions, intended to simulate global warming. We observe a net increase in total sulfur (1.2-41.0%), which is ascribed to S sequestration-redox reactions involving both sulfide oxidation to S^o ⇌ SO₄^2- and the reverse (S^2-/S₂^2- ⇌ SO₄^2-) via SO₄^2- plus soil organic sulfur plus sulfides/pyrites (SOS + S_n^2-) decrease and/or increase under sunlight, dark, and control conditions. Higher transformation and mineralization of various components occurs in dark/microbial conditions by the wide day-night experimental temperature variation (10-42 °C) in comparison with the control at constant temperature (25 °C). Remarkably, the photosynthetically-derived soil organic carbon (SOC)/humic substances (HS)-bound mineral neoformation through uptake and sequestration of various components, including As and Hg, is specifically detected under sunlight and control conditions. A major role is played by seven redox-active metals (Fe, Mn, Cu, Hg, Ni, As, and U), which are involved in both organo-mineral complexation and redox processes. Importantly, the dark/microbial dissolution of iron minerals is primarily responsible for the increased export of water-extractable or labile As (33.8-89.7%) over a period of 0-150 days, with no evidence of sequestration. In contrast, As sequestration and relatively low water-extractable As export occur under sunlight (9.0-25.5%) and control (17.4-38.4%) samples. A net decrease in Hg levels is observed over a period of 0 to 150 days, along with relatively low sequestration across three treatments, appearing the highest losses under sunlight conditions (9.8-17.4%) when compared to dark (5.2-11.4%) and control (3.6-11.6%) samples. This effect may be attributed to the reduction of Hg(I, II)‒DOM into gaseous Hg⁰. These findings could assist in managing soil components and predicting where and when the side effects of global warming-such as erosion-associated mobilization of soil components, including As and Hg into surrounding surface water, groundwater, and the atmosphere-are likely to manifest.