Field aging slows down biochar-mediated soil carbon dioxide emissions.

Biochar is widely used due to its potential in direct or indirect soil carbon sequestration. However, there is a lack of comprehensive studies on the changes in the physicochemical properties of biochar after long-term application in different types of soils and the effects on CO emissions. In this study, paddy soil and fluvisol were selected as typical acidic and alkaline soils. Rice biochar (RB) and maize biochar (MB) were incorporated into paddy soil and fluvisol for one year, and characterizations (e.g., SEM-EDS, FTIR, 3D-EEM, and TG-DTG) of pristine and aged biochars were analyzed. Incubation experiments were conducted to assess the impact of aged biochar on CO emissions from paddy soil and fluvisol. Results indicated consistent trends in the physicochemical properties of biochar after one year of aging in both acidic and alkaline soils. Aged biochars exhibited significant structural degradation, increased specific surface area, and increased oxygen-containing functional groups. The DOM fluorescence intensity of biochar decreased and the thermal stability increased after aging. Compared to pristine biochar, aged biochar promoted soil carbon sequestration, resulting in varied reductions in cumulative CO emissions from paddy soil and fluvisol in the short term. Spearman's correlation coefficient analysis and PCA loading plot revealed that field-aged biochar primarily influenced CO emissions from soil and carbon sequestration by reducing biochar DOC release and bioavailability of DOM, while enhancing the humification of biochar DOM. These findings suggest that aged biochar favors soil carbon sequestration in the short term, both in acidic and alkaline soils.