Origin of positive δ13C of emitted CO2 from soils after application of biogas residues

Bioenergy production from renewable organic material is known to be a clean energy source and therefore its use is currently much promoted in many countries. Biogas by-products also called biogas residues (BGR) are rich in partially stable organic carbon and can be used as an organic fertilizer for crop production. However so far, many environmental issues relevant when BGR are applied to agricultural land (soil C sequestration, increased denitrification and nutrient leaching) still have to be studied. Therefore a field experiment was set up to investigate the degradation of BGR and its impact on the decomposition of native soil organic matter based on a natural abundance stable isotope approach. Maize, a C4 plant has been used as bioenergy crop, therefore the δ13C of total C in BGR was −16.0‰PDB and soil organic matter was mostly derived from C3 plant based detritus, SOM thus showed a δ13C of −28.4‰PDB. Immediately after BGR application, soil-emitted CO2 showed unexpectedly high δ13C of up to +23.6‰PDB, which has never been reported earlier. A subsequent laboratory scale experiment confirmed the positive δ13C of soil-emitted CO2 after BGR addition and showed that obviously, the added BGR led to a consumption of dissolved inorganic C in soils. Additionally, it was observed that the δ13C of CO2 driven from inorganic C of BGR (BGR-IC) by acid treatment was +35.6‰PDB. Therefore, we suggest that also under field conditions the transformation of BGR-IC into CO2 contributed largely to CO2 emissions in addition to the decomposition of organic matter, which affected both the amount and the carbon isotope signature of emitted CO2 in the initial period after BGR application. Positive δ13C of inorganic C contained in BGR was attributed to processes with strong fractionation of C isotopes during anaerobic fermentation in the biogas formation process.