Does Biochar Stabilize the Bioreactive Carbon Fractions of Swine Manure?

The labile organic or bioreactive fraction in manure is the food and energy source that microbes continuously utilize. Biochar (BC) addition to manure following its application to soil improves the total C retention in soil. However, the relation between the BC application rate and the bioreactive C stability in manure is poorly understood. This study performed a short aging of swine manure with different loading rates of BC (0–25 wt %) and evaluated bioreactive C stability of these manure–BC mixtures using thermal, chemical, and spectral analyses. Findings suggest that increasing BC addition to manure causes (a) 13–42% less thermal decomposition of total labile C, (b) 65–96% reduction of labile C release in hot water, and (c) 22–69% decrease of respirational C loss as compared to manure; thus overall, it alters the manure–BC mixture's C quality. Interestingly, 1% BC addition decreased the manure–BC mixture's C stability indices (H/Corg and VM/FC molar ratios) compared to higher BC addition. The C-mineralization study revealed that the addition of BC to manure altered the measured inorganic C fraction of the mixture, which was further supported by Fourier-transform infrared and X-ray diffraction analyses. The 13C solid-state nuclear magnetic resonance suggests an increase in the aromatic index and a decrease in the alkyl to O-alkyl ratio as the BC to manure ratio increased. Inclusively, our results indicate that the manure–BC mixtures were substantially (P < 0.05) more stable than manure, and application of these mixtures can have a larger impact on the soil-C turnover when compared to manure application.