Increased CO2 fluxes from a sandy Cambisol under agricultural use in the Wendland region,Northern Germany,three years after biochar substrates application

In recent years, biochar has been discussed as an opportunity for carbon sequestration in arable soils. Field experiments under realistic conditions investigating the CO₂ emission from soil after biochar combined with fertilizer additions are scarce. Therefore, we investigated the CO₂ emission and its ¹³C signature after addition of compost, biogas digestate (originating from C4 feedstock) and mineral fertilizer with and without biochar (0, 3, 10, 40 Mg biochar/ha) to a sandy Cambisol in Northern Germany. Biomass residues were pyrolized at ~650°C to obtain biochar with C3 signature. Gas samples were taken biweekly during the growing season using static chambers three years after biochar substrate addition. The CO₂ concentration and its δ¹³C isotope signature were measured using a gas chromatograph coupled to an isotope ratio mass spectrometer. Results showed increased CO₂ emission (30%–60%) when high biochar amount (40 Mg/ha) was applied three years ago together with mineral fertilizer and biogas digestate. On average, 59% of the emitted CO₂ had a C3 signature (thus, deriving from biochar and/or soil organic matter), independent of the amount of biochar added. In addition, our results clearly demonstrated that only a small amount of released CO₂ derived from biochar. The results of this field experiment suggest that biochar most likely stimulates microbial activity in soil leading to increased CO₂ emissions derived from soil organic matter and fertilizers mineralization rather than from biochar. Nevertheless, compared to the amount of carbon added by biochar, additional CO₂ emission is marginal corroborating the C sequestration potential of biochar.