Effect of Digestate Biochar Application on Methane Oxidation in Landfill Cover Soil

Methane emissions from landfills are a significant contributor to global warming. Harnessing methane-oxidizing microorganisms in the topsoil layer represents a highly efficient strategy to reduce uncontrolled methane release from landfills. Biochar is a material rich in carbon, derived from the thermal decomposition of organic waste. It boasts a range of superior physical and chemical attributes, which render it a potent amendment for boosting the methane-consuming capabilities of landfill cover soils. Digestate biochar is produced by the pyrolysis of digestate from anaerobic digestion. A large amount of digestate is produced during the anaerobic digestion process. Pyrolysis treatment can rapidly reduce the volume of digestate while yielding high nutrent (K, Na, Fe, Ca) biochar products. These nutrients that typically promote the growth and metabolism of microorganisms and plants. Applying digestate biochar to modify landfill cover may be a “treat waste with waste” approach, but the effects of biochar application in landfill cover soil still need further investigation.

Control groups without biochar and with woody biochar were established to contrast with digestate biochar, in order to investigate the impact of digestate biochar on the growth and metabolism of methanotrophs communities under conditions of 15% methane and 15% oxygen. Long-term semi-open ecological experiments were also set up to investigate the dynamic trends of elements in different biochar-amended soils, focusing on their downward migration with precipitation and upward migration through plant uptake.The digestate biochar exhibited superior performance in enhancing methane oxidation over woody biochar. The incorporation of both woody biochar and digestate biochar facilitated methane oxidation, with digestate biochar showing almost double the cumulative mass of methane oxidation (7.14 mg methane per gram) relative to woody biochar. It was determined that superior ion-exchange capacity of digestate biochar better supported the proliferation of Type I methanotrophs, which possess more effective metabolic routes for methane oxidation. In subsequent experiments, the highest daily methane oxidation rate in digestate biochar-amended soil was about 7 times that of the original soil and woody biochar after 6 months of plant growth. Moreover, the digestate biochar-amended soil consistently had the largest cation exchange capacity. The soil amended with digestate biochar had lower dissolved organic carbon (DOC) compared to the control group without biochar addition, while the recalcitrant organic carbon was higher than the control group. In contrast, the DOC and recalcitrant organic carbon in the woody biochar amended soils were significantly higher than those in the control group.

In general, applying digestate biochar to landfill cover soil for methane reduction is highly significant in terms of "treat waste with waste" engineering. Our research confirmed that digestate biochar does not possess heavy metal leaching toxicity; instead, it provides nutrients for the growth and metabolism of plants and microorganisms in the soil. Our results may offer crucial insights for devising and refining soil restoration approaches using biochar to reduce greenhouse gas emissions.

Keywords: Digestate biochar; Methane oxidation; Landfill cover soil; Cation exchange capacity; Organic carbon.