High pyrolysis temperature biochars reduce nitrogen availability and nitrous oxide emissions from an acid soil

Biochar–bioenergy coproduction from biomass pyrolysis has the potential to contribute to climate change mitigation. Biochar produced at various pyrolysis temperatures (<600°C) has been widely studied. However, the effect of biochars, produced at high pyrolysis temperature (≥600°C), on soil nitrogen (N) dynamics and nitrous oxide (N₂O) emission is largely unknown. A pot trial was performed to examine the effect of high pyrolysis temperature (600, 700, 850 and 950°C) woody biochars on soil N dynamics, microbial gene abundance and N₂O emissions with (+N) and without N (−N) fertilization from an acid soil. Results showed that all biochar treatments significantly lowered the N₂O emissions in both fertilized and unfertilized regimes. However, the suppressive effect on N₂O emission among different high pyrolysis temperatures was not statistically different. Biochar amendment significantly decreased the concentration of soil NH₄ ⁺, and lower levels of soil NO₃ ⁻ were observed at the later stage of experiment. Under −N, plant biomass and N uptake were significantly lowered in all biochar treatments. Under +N, biochar addition significantly increased plant biomass, while only the 700°C biochar significantly increased N uptake. This suggests that single application of biochar could limit soil mineral N bioavailability and further decrease plant growth and N uptake in the plant–soil system. Biochar amendments tended to increase nitrous oxide reductase (nosZ) gene abundance, but this effect was only significant for biochar produced at 950°C under +N. In conclusion, high pyrolysis temperature biochars can be effectively used to reduce N₂O emission, while increases in nosZ gene abundance and decreases in NH₄ ⁺ and NO₃ ⁻ concentrations in the acid soil are likely to be responsible for the reduction in N₂O emission. Thus, woody biochars as a by-product produced at high pyrolysis temperature have the potential to mitigate soil N₂O emission via modifying N transformation and further affect climate change.