Biochar strategy for long-term N2O emission reduction: Insights into soil physical structure and microbial interaction

Abstract

Applying biochar to agricultural soils is a promising strategy for mitigating nitrous oxide (N₂O) emissions. Nitrogen (N) fertilizers are essential for crop production but also represent a significant source of N₂O emissions. The effectiveness of biochar in reducing N₂O emissions depends on the amount of N fertilizer applied and the morphological structure of the biochar. However, few studies have examined the impact of field-aged biochar on N₂O emissions under different N application levels, especially concerning the mechanisms by which biochar's morphological properties and soil characteristics influence microbial-driven N₂O production. We conducted a long-term field experiment over three winter wheat seasons, applying two N fertilizer doses (113.25 and 226.5 kg N ha⁻¹) and four biochar doses (0, 5, 10, 20 t ha⁻¹). In-situ N₂O measurements, combined with amplicon sequencing (16S rRNA, ITS rRNA), metagenomic sequencing, scanning electron microscopy, and Brunauer-Emmett-Teller analysis, were performed to explore the effects of combined application of biochar with N fertilizer on soil N₂O emissions and potential soil physicochemical and microbial mechanisms. The study demonstrated that biochar aged for several years consistently reduced soil N₂O emissions, likely due to modifications in soil physical properties such as specific surface area, pore size, and pore volume. The dose of N fertilizer had a significant effect on how biochar regulated soil pore structure, consequently impacting the abundance of N cycle genes and microbes. The intermediate biochar dose of 10 t ha⁻¹ biochar significantly increased soil mesopore size and the abundance of N₂O-reducing genes such as nosZ, while simultaneously suppressing the N₂O production genes such as napA and norB through enhanced soil specific surface area and pore volume, but further increasing the dose did not result in sustained improvement. The functional diversity of N-cycling genes proved to be a more reliable predictor of N₂O emissions than the diversity of fungal and bacterial taxa. Our findings advance the understanding of how biochar influences physical-microbial interactions that determine N₂O production in agricultural soils. These mechanistic insights are crucial for developing integrated biochar and fertilization management strategies to mitigate climate change effectively.