Effects of Biochar Properties on Mineralization Rates, Microbial Communities, SOM Priming and N2O Emissions.

Biochar is a promising technology for climate mitigation, presenting benefits in terms of C sequestration and potentially reducing N₂O emissions. The C sequestration goal also requires that priming of the soil organic matter mineralization is negligible. Biochar mineralization rates, priming effect and N₂O emission are controlled by microbial processes in soil. The objective of the present study was to determine the effect of biochar production temperature and associated properties on mineralization rates in soil, microbial communities, priming and N₂O kinetics. We produced series of biochars from corncob and miscanthus grass at pyrolysis temperatures ranging from 250 to 800°C. Biochars were extensively characterized: proximate and elemental analyses, surface properties (BET, CEC) and organic structures (NMR, MIR, BPCAs). Long-term incubation for mineralization and priming effects and short-term incubations for denitrification kinetics were conducted. Bacterial and fungal community compositions after one-year incubation were studied using T-RFLP and ARISA.

As reported in other studies, largest changes in elemental and molecular compositions occurred at fairly low temperatures, i.e. below 370°C in our case. However, condensation degree of the pyrogenic structure, as measured by BPCA biomarkers, increased substantially until 700°C. Biochar stability increased greatly until 370°C, while no further stability gain was observed beyond this threshold. All biochars and un-pyrolysed feedstock generated a positive priming, which had mostly subsided by the end of the one-year incubation period. Fungal and bacterial community composition of the soil-biochar mixtures were significantly related to biochar properties (mainly pH and O/C) and correlated to pyrolysis temperature. The N₂O response to biochar displayed a sharply defined temperature threshold between 372 and 416°C, separating stimulatory from suppressing effects.  In summary, we observed a marked threshold in pyrolysis temperature for increased stability and reduced N₂O production, while microbial community responses appeared more progressive and the priming effect appeared unaffected.