Effects of combined biochar and maize straw applications on soil greenhouse gas emissions

Straw application is an effective and common measure to improve soil quality but generally increases greenhouse gas (GHG) emissions. The combined application of biochar and straw has been proposed to mitigate these emissions. However, the effects of the combined application of biochar and maize straw, particularly at different proportions, on GHG emissions remain inadequately understood. In this study, an incubation experiment was conducted with five treatments: soil only (CK), 1 % maize straw application (TS), 0.7 % maize straw +0.3 % biochar application (S7B3), 0.5 % maize straw +0.5 % biochar application (S5B5) and 0.3 % maize straw +0.7 % biochar application (S3B7). The study also considered the effect of 1 % biochar application (TB) from our previous study with the same soil type and incubation condition. Carbon isotope technology was utilized to trace CO₂ sources and assess the priming effects on soil organic carbon (SOC) mineralization. The results showed that TB reduced CO₂ emission, while TS increased CO₂ emission due to maize straw decomposition and its positive priming effect on native SOC mineralization. S7B3, S5B5 and S3B7 also increased CO₂ emissions, but with significantly lower emissions than TS, in the order of TS > S7B3 > S5B5 ≥ S3B7. The positive priming effects of S7B3, S5B5 and S3B7 on native SOC mineralization weakened over time and eventually turned negative. TB, TS and combined applications reduced N₂O emission due to decreased substrates (NH₄⁺ and NO₃⁻) for nitrification and denitrification, induced by promoting microbial fixation of inorganic nitrogen. The reduction effects on N₂O emission of combined applications were superior to those of TS and TB, with S5B5 demonstrating the best efficacy. TS increased CH₄ emission, while S7B3, S5B5 and S3B7 reduced CH₄ emission. The reduction effects of CH₄ emission with combined applications were superior to that of TB, and S5B5 exhibited the lowest CH₄ emission. Unlike soils with higher nitrogen content, where N₂O emission dominated the global warming potential (GWP), CO₂ emission dominated the GWP in this study, resulting in increased GWP in TS, S7B3, S5B5 and S3B7, but with significantly lower values for S7B3, S5B5 and S3B7 compared to TS. In conclusion, our study suggests that the combined application of biochar and maize straw, especially S5B5, could effectively mitigate GHG emissions promoted by maize straw application, especially in soils with higher nitrogen content.