Effects of biochar addition on earthworm enhanced N2O emission

The application of biochar has been shown to suppress soil nitrous oxide (N₂O) emissions. Earthworms, a key component of soil fauna, are known to increase N₂O production. While existing research has focused mainly on soil physicochemical management and microbial interactions, limited attention has been paid to how biochar interacts with soil fauna in relation to N₂O emissions. To investigate this, an incubation experiment was conducted to analyze how various biochars, including corn straw (CS), rice straw (RS), wheat straw (WS), nutshell (NS), wood chip (WC), rice husk (RH), apricot shell (AS), and peach shell (PS) biochar, affect earthworm (Amynthas cortices) enhanced N₂O emissions. Biochar addition reduced earthworm enhanced N₂O production and decreased the cumulative earthworm burrowing length compared to control. Rice straw biochar was the most effective, releasing the lowest earthworm enhanced N₂O emission at 73 μg kg⁻¹ soil and having the shortest cumulative burrowing length at 48.6 cm, whereas wood chip biochar had the least impact, with earthworm enhanced N₂O reaching 307 μg kg⁻¹ soil. The drilosphere influenced by earthworms' activity demonstrated increased pH, C/N ratio, mineral nitrogen (MN), dissolved organic carbon (DOC), and microbial biomass carbon (MBC) compared to the bulk soil, though the extent of these changes varied with the type of biochar applied. The biochar addition altered the micro-environment within the earthworm gut, including O₂ concentration and pH levels, thereby affecting the N₂O related microbial community in the drilosphere. This was evidenced by changes in the ratio of nirK + nirS to nosZ genes and the abundance of ammonia-oxidizing archaea and bacteria gene copies. Hierarchical partitioning analysis revealed that the biochar's properties primarily influenced earthworm burrowing activity, the dominant factor affecting earthworm enhanced N₂O emissions, followed by MN, DOC, and MBC content in the drilosphere. The impact of gut-derived microbes on N₂O emissions was comparatively insignificant. These findings highlight that biochar amendment can mitigate earthworm induced N₂O emissions, primarily by modifying earthworm activity, which is strongly influenced by the biochar's physicochemical characteristics.