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.

Biochar can have a positive effect on agricultural soils and plant yields. The underlying mechanisms that deliver beneficial outcomes are still poorly understood. Soils contain complex communities of hundreds or thousands of distinct microorganisms, and it has been shown that biochar can have an impact on their composition and function. Here we analyse the microbial communities in a controlled field trial that compared the effect of enhanced biochars (EBs) against a farmer practice (FP) of traditional fertilisation (urea, superphosphate and potash) on sweet corn yield. During sequential crop cycles (barley and sweet corn) two types of EBs were applied at low and high levels (total of 1.1 and 5.44 t ha−1, respectively). Samples were taken at the end of a second crop cycle and over 50,000 16S ribosomal RNA (16S rRNA) tag sequences were generated per sample to characterise microbial communities. Despite the lower amounts of nutrients provided by EBs, their amendment to soil produced similar crop yields to the FP. In addition, significant differences in microbial community composition were observed between the high EB and FP treatments. This was driven by differences in the relative abundances of only a few community members. Community level differences were also correlated with a higher soil pH associated with EB laden soil. Network analysis showed that the low EB application had more correlation patterns (co-occurrences and exclusions) between microbial taxa, and highlighted the importance of associations between members of the phyla Acidobacteria and Verrucomicrobia in the biochar environment. Overall, a large number of microorganisms appear to be influenced by EB amendment compared with fertiliser use leading to a complex re-wiring of community composition and associations.