Impacts of biochar on nitrous oxide emissions and ammonia volatilisation in wheat and maize cropping systems

Agriculture and other land use practices are major contributors to greenhouse gas emissions. To meet the needs of an increasing global food demand while mitigating climate change, sustainable agricultural practices are necessary. Biochar seems to be a promising tool to support this transition to sustainability in agriculture. The application of nitrogen fertilizers increases N₂O emissions and NH₃ volatilisation. Nitrous oxide (N₂O) is a highly potent greenhouse gas and ammonia (NH₃) can re-react with soil and forms N₂O or can lead to other environmental issues in the surrounding. Besides its carbon sequestration potential, it is known that biochar can positively influence soil properties like water holding capacity, nutrient leaching and mitigation of nitrous oxide emissions and ammonia volatilisation. However, these effects depend on pedoclimatic conditions, the properties of the applied biochar, and other agricultural practises. Therefore, it is necessary to expand the knowledge of these effects, especially under field conditions, to generate valid estimates on biochar’s mitigation potential for N₂O and NH₃ emissions. A good and extensive data basis is essential for recommendations and a large-scale application in agriculture. In a two-year field experiment in Grabenegg (Lower Austria) we cultivated silage maize (Zea mays) in 2022 and spring wheat (Triticum aestivum) in 2023 with different organic (external organic matter, EOM) and inorganic (NPK) fertilisers. For the biochar treatments we applied 7 t/ha hardwood biochar additionally. The original soil was loamy, low in organic carbon and slightly acidic. We found substantial reductions with 36% (NPK) and 53% (compost) for N₂O and 56% (NPK) and 40% (compost) for NH₃ emissions. There are several factors discussed in literature how biochar mitigates N₂O and NH₃ emissions. We suggest that the immobilisation effect of biochar on NH₄⁺ and NO₃^- (which was observed in the soil), and possibly an increased dinitrogen monoxide reductase activity are responsible for this reduction. Our data support that biochar can be a suitable amendment for highly productive agroecosystems where high amounts of fertiliser are needed and often applied at one timepoint. Still, further investigations on the long-term effect on emission mitigation of biochar and the mechanisms behind are necessary.