Chemical Stability of Organic Matter in Biochar Amended Soils.

The potential of biochar (charcoal obtained by heating biomass in a low/no oxygen environment) to increase carbon sequestration in soil while improving soil quality has been widely discussed. Application of biochar to soils might increase the chemical and physical stability of organic matter. However, the interaction of biochar with soil particles and the distribution of such carbon input in soil may vary with biochar feedstock but also with soil characteristics.

The ability of three biochar types to increase the chemical stability of organic matter was evaluated for an agricultural soil. Undisturbed samples were collected from the topsoil (15 cm) and amended with different biochar types, prepared in a furnace oven (450°C) using forest waste (FW), peanut shell (P) or dry maize straw (M) as feedstock. Soil samples were then incubated in air-tight jars at 25°C and in the darkness. The CO₂ released was measured periodically during 30 days using a LI-820 CO2 Analyzer. After 30 days, samples were collected and analysed with a FTIR-microscope Agilent 600.

After 30 days of incubation, FTIR spectra revealed the presence of significant amounts of polysaccharides and traces of aromatic compounds (assigned to the signals at 1200 and 1600 cm^-1 respectively) in the microaggregates surface for soil amended with the M based biochar. No signal was detected for soil amended with FW-biochar while only traces of aromatic compounds were determined for the application of P-biochar. Carbon respiration rates were consistent with FTIR spectra results. Thus, soil amendment with FW and P based biochar resulted in a slight decrease of carbon respiration rates (mg C respired kg soil^-1 day^-1) compared to non-amended soil, while application of M-biochar, increased C respiration rate up to 2-fold at 30 days.

Results indicate that selection of biochar feedstock is critical to develop carbon sequestration strategies.