Abstract: Crop-straw derived biochars inhibited the mobility and bioavailability of heavy metals in the upland cropping land due to complexation, precipitation, as well as increasing soil pH. However, information regarding the influence of biochar on heavy metal speciation in soils under submerged condition is limited. In the present study, the effect of the biochars on Cu speciation in an Oxisol was conducted under submerged condition. The biochars were derived from canola straw pyrolyzed at 400℃ and peanut straw at 300, 400 and 500℃, respectively. Soil was spiked with Cu(NO₃)₂ (5 mmol·kg^-1) and glucose (20 g·kg^-1), respectively, and the loading quantity of biochar is about 30 g·kg^-1. The homogenized soil samples were incubated under inundation condition for 49 days. BCR sequential extraction method was used to investigate the dynamic changes of Cu²⁺ species during the incubation. Results show that addition of activated carbon and crop straw derived biochars significantly increased soil pH, decreased acid-soluble fraction of Cu²⁺ evidently, and the reducible and oxidizable fraction of Cu²⁺ showed reverse trends in the early stage of flooding. Whilst, the acid-soluble fraction of Cu²⁺ increased with the incubation time, mainly due to the decreasing soil pH, which suggested the inhibition function of biochars on Cu²⁺ mobility faded progressively. Moreover, organic acids produced by decomposition of organic matter in the biochar, lead to a decreasing soil pH, and consequently increased the mobility of Cu²⁺.