Lowering N2O emissions from soils using eucalypt biochar: the importance of redox reactions

Agricultural soils are the primary anthropogenic source of atmospheric nitrous oxide (N₂O), contributing to global warming and depletion of stratospheric ozone. Biochar addition has shown potential to lower soil N₂O emission, with the mechanisms remaining unclear. We incubated eucalypt biochar (550 °C) – 0, 1 and 5% (w/w) in Ferralsol at 3 water regimes (12, 39 and 54% WFPS) – in a soil column, following gamma irradiation. After N₂O was injected at the base of the soil column, in the 0% biochar control 100% of expected injected N₂O was released into headspace, declining to 67% in the 5% amendment. In a 100% biochar column at 6% WFPS, only 16% of the expected N₂O was observed. X-ray photoelectron spectroscopy identified changes in surface functional groups suggesting interactions between N₂O and the biochar surfaces. We have shown increases in -O-C = N /pyridine pyrrole/NH₃, suggesting reactions between N₂O and the carbon (C) matrix upon exposure to N₂O. With increasing rates of biochar application, higher pH adjusted redox potentials were observed at the lower water contents. Evidence suggests that biochar has taken part in redox reactions reducing N₂O to dinitrogen (N₂), in addition to adsorption of N₂O.