Lowering N(2)O emissions from soils using eucalypt biochar: the importance of redox reactions

Agricultural soils are the primary anthropogenic source of atmospheric nitrous oxide (N(2)O), contributing to global warming and depletion of stratospheric ozone. Biochar addition has shown potential to lower soil N(2)O 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 N(2)O was injected at the base of the soil column, in the 0% biochar control 100% of expected injected N(2)O was released into headspace, declining to 67% in the 5% amendment. In a 100% biochar column at 6% WFPS, only 16% of the expected N(2)O was observed. X-ray photoelectron spectroscopy identified changes in surface functional groups suggesting interactions between N(2)O and the biochar surfaces. We have shown increases in -O-C = N /pyridine pyrrole/NH(3), suggesting reactions between N(2)O and the carbon (C) matrix upon exposure to N(2)O. 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 N(2)O to dinitrogen (N(2)), in addition to adsorption of N(2)O.