The effects of biochar on the amount and characteristics of dissolved organic matter (DOM) and the relationship between the DOM and the availability of trace elements in soils are poorly understood. An incubation experiment was used to investigate the effects of biochar on DOM properties and the mobility and availability of As, Cd, and Pb in two types of contaminated soils (vegetable plot vs. paddy). The availability of trace elements in the soil was assessed by diffusive gradients using the thin film (DGT) technique. The results showed that mixing biochar at a 10 wt% dose increased the amount of dissolved organic carbon (DOC) from 12.8 to 42.9 mg L^-1 in soils, and this corresponded to an increase in soil pH. Fluorescence excitation–emission matrices (EEMs) indicated that rice straw biochar (RSBC) amendment to soil changed the DOM composition towards higher aromaticity. The DGT-extractable Cd and Pb concentrations were significantly decreased with biochar amendments, while DGT-extractable As was significantly increased RSBC treatment decreased acid-soluble Cd and Pb fractions but increased the acid-soluble As fraction in soil solutions compared to control samples. RSBC amendment can effectively adjust soil metal availability, as well as DOM amount and composition, and that the amendment conditions must be tailored according to the specific trace elements and soil types.