Aggregate-mediated redistribution and bioavailability of cadmium in paddy soils under alternating redox conditions

Unstable pe+pH (a soil redox parameter representing the total variation of pH and Eh, pe = -log₁₀(e^-) = Eh (mV)/59.2) environment caused by alternation flooding and drainage cycles can alter aggregate structure. However, the mechanism of aggregate restructuring and its impact on Cd distribution remains poorly understood. Contaminated soils were collected from three different rice-growing regions in China and then were subjected to a 40-day anaerobic incubation followed by a 20-day oxidation period. The contribution of known binders, including iron (Fe) oxides and soil organic carbon (SOC), to aggregate structural stability and its impact on DTPA-Cd distribution were investigated. The results show that flooding decreased laterite soil pe+pH from 9.56 to 1.73, resulting in the disintegration of macroaggregates and formation of microaggregates. In addition, free Fe oxides (Fe_DCB) were mainly distributed in macroaggregates, while amorphous Fe oxides (Fe_OA) and SOC tended to accumulate in microaggregates. The greatest decrease of 65.2 %-73.2 % was observed in aggregate mean weight diameter (MWD) after dithionite-citrate-bicarbonate (DCB) extraction, which indicates that Fe_DCB contributed more to aggregate stability than Fe_OA and SOC. Furthermore, flooding stage decreased the content of DTPA-Cd in bulk soil, and that tended to distribute in microaggregates (0.30 and 0.52 mg/kg for the > 2 and < 0.053 mm fraction, respectively). Taken together, unstable pe+pH changed the distribution of Fe oxides and SOC in different aggregate size, resulting in aggregate restructuring, which further impacted Cd distribution in paddy soil.