Estimation of the biogeochemical reactivities of dissolved organic matter from modified biochars using color.

Modified biochar is widely used as a soil amendment in agricultural systems to improve crop yields and remove environmental pollutants. The water-soluble fraction of biochar, called biochar-derived dissolved organic matter (DOM), is the most active biochar component. However, the correlation between the optical properties of DOM and its biogeochemical activity remain unclear. In this study, one biochar and six modified derivatives were used to extract DOM and characterize its optical properties. The biogeochemical reactivities of DOM were determined using biodegradation, photodegradation, and electron-donating capacity assays. The results show that modification changes the biochar characteristics, leading to a variety of DOM properties. The DOM from modified biochars degrades more rapidly than the original biochar. On the other hand, modification reduces the redox functional groups in DOM, resulting in a lower electron-donating capacity of DOM samples. However, the modifications did not seem to affect photodegradation. Not all spectral parameters provide information about the correlations between the DOM properties and biogeochemical reactivity. However, two fundamental properties, that is, the specific UV absorbance at 254 nm (SUVA, showing aromaticity) and spectral slopes over the ranges of 275-295 nm of the UV absorbance (S, showing molecular weight), are the dominant factors affecting the biodegradation and electron-donating capacities of DOM. In this study, a rapid and straightforward method is presented, which can be used to characterize DOM and predict the reactivity of biochar that is used as an environmental amendment to minimize toxic organic compounds.