Biochar induced changes of soil dissolved organic matter: The release and adsorption of dissolved organic matter by biochar and soil

Highlights: • Biochar increased the humification of soil DOM. • Biochar pyrolyzed at 300 °C increased DOM content mainly by releasing intrinsic DOM. • Biochar pyrolyzed at 700 °C decreased DOM content mainly by adsorbing soil DOM. • The release and adsorption of DOM by biochar were the keys determining DOM content. Dissolved organic matter (DOM) is an important organic matter fraction that affects many biological and chemical processes in soil. Biochar can change soil DOM while the effects were paradoxical, and contributions of biochar to soil DOM was not clear yet. In this study, excitation-emission matrix (EEM) fluorescence spectroscopy was applied to determine the biochar-induced changes of DOM composition. Batch experiments were conducted to quantify the contributions of biochar to soil DOM. Biochars were prepared by pyrolyzing wheat straw (S300/700) and cow manure (M300/700) at 300 and 700 °C, respectively. Generally, biochar increased the humification of soil DOM possibly by the release of indigenous DOM and selective adsorption of the small molecule DOM. Besides, contributions of S300 and M300 to soil DOM (37–91%) were higher than that of S700 and M700 (2–19%) irrespective of application rates. The indigenous DOM released from S300 and M300 was 6.4–12.1 times more than the soil DOM adsorbed by S300 and M300, leading to the increase of DOM content. Contrarily, the DOM from S700 and M700 was only 11–17% of the soil DOM adsorbed by them, resulting in the decrease of DOM content. In addition, contributions of biochar to soil DOM increased as the application rate increased, especially for S300 and M300. This study indicated that the release and adsorption of DOM were the key processes determining the effects of biochar on soil DOM, which were closely related to the pyrolysis temperature and application rate of biochar.