Low pe+pH induces inhibition of cadmium sulfide precipitation by methanogenesis in paddy soil

Soil flooding is associated with the formation of cadmium (Cd)-sulfide, which is known to decrease Cd solubility and extractability. However, the threshold for Cd-sulfide precipitation is largely unknown, particularly because carbon can capture electrons for methanogenesis during sulfate reduction in highly reducing soil conditions. Using soil microcosms with different water regimens applied, we analyzed the electrochemical and spectroscopic properties and revealed a key mechanism controlling Cd stabilization that is dependent on pe+pH (a comprehensive indicator of soil redox status). The extent of Cd-sulfide precipitation was limited, with the proportion decreasing from 58.5% to 49.6% under flooding conditions (corresponding to a decrease in pe+pH from 3.28 to 2.82). Our data suggest that the increase in Cd mobilization in highly reducing soil is due to methanogenesis outcompeting sulfate reduction for available electrons. Although glucose supply could reduce the competition between oxidized carbon and sulfur in the soil for seizing electrons, the role of glucose as an electron donor/shuttle became weaker when soil was more anaerobic. The optimal soil reductive environment for maximum Cd-sulfide precipitation was observed when pe+pH was between 4.45 and 6.58. Overall, this study provides a quantitative and mechanistic understanding of how redox status (pe+pH), sulfate reduction, and methanogenesis are coupled with Cd remobilization in over-reductive paddy soil.