Effects Of Biochar Addition On Earthworm-induced Soil N₂O Emissions

Soil is one of the main sources of nitrous oxide（N₂O）emissions,and the application of biochar in farmland can reduce soil N₂O emissions.However,most studies on the reduction mechanism of biochar focus on changes in soil physicochemical properties and soil microorganisms under biochar application.Earthworms are a representative group of soil animals,and their activities can significantly promote soil N₂O emissions.Therefore,investigating the impact of biochar addition on earthworm-induced N₂O emissions can help further enrich and improve the biochar emission reduction mechanism from the perspective of soil animals.In this study,indoor cultivation experiments were conducted to compare the effects of adding eight different types of biochar（rice straw char RS,nut shell char NS,corn straw char CS,apricot shell char AS,peach shell char PS,rice husk char RH,wheat straw char WS,and woody char WD）on earthworm-induced soil N₂O emissions,and to clarify their mechanisms by combining the characteristics of biochar,changes in soil properties in earthworm burrows,and changes in earthworm behavior.The main research results are as follows:（1）Compared with treatments without biochar and without earthworms,adding earthworms increased soil N₂O emissions by 263.3%.Compared with treatments with only earthworms,adding biochar reduced earthworm-induced N₂O emissions by 45.06%-81.28%,but the emission reduction effect of different types of biochar,with the following order:rice straw char（RS）>nut shell char（NS）>corn straw char（CS）>apricot shell char（AS）>peach shell char（PS）>rice husk char（RH）>wheat straw char（WS）>woody char（WD）.（2）Soil disturbed by earthworms（drilosphere）and soil undisturbed by earthworms（non-drilosphere）were carefully separated and tested.Regardless of whether biochar was added,the NH₄⁺-N,NO₃^--N,and MBC contents in drilosphere were higher than those in non-drilosphere in treatments with added earthworms.Biochar addition changed the physicochemical properties and nitrogen conversion-related functional gene abundance of drilosphere and non-drilosphere.After cultivation,compared with treatments with only earthworms,adding all eight types of biochar significantly increased the p H,C/N ratio,and nos Z gene abundance in non-drilosphere and the C/N ratio and nos Z gene abundance in drilosphere,while significantly reducing the MBC content in both drilosphere and non-drilosphere（p<0.05）.（3）Biochar addition limited earthworm burrowing and excretion behavior.Compared with treatments with only earthworms,biochar addition significantly reduced the cumulative burrow length of earthworms and the proportion of earthworm burrows in the soil,with reduction rates of 22.27%-68.12%and 16.23%-77.62%,respectively,and rice straw char（RS）had the most significant reduction（p<0.05）.Changes in earthworm burrowing behavior were closely related to the properties of biochar（p H,EC and PZC）,and the cumulative burrow length of earthworms was negatively correlated with the electrical conductivity（EC）of biochar（p<0.01）and positively correlated with the zero point charge（PZC）of biochar（p<0.05）.（4）Compared with the treatment of simply inoculating earthworms,the addition of biochar increased the p H of the earthworm gut environment and increased the concentration of O₂ in the gut.Redundancy analysis showed that biochar p H,C/N,TN,TC,electrical conductivity（EC）,total pore volume（TBV）,and mean pore size（PD）were the main factors affecting changes in the earthworm gut environment.The p H of the earthworm gut was positively correlated with the p H of the biochar,and negatively correlated with the electrical conductivity（EC）of the biochar.The concentration of O₂ in the gut was positively correlated with the total pore volume（TBV）of the biochar,and negatively correlated with the pore diameter（PD）of the biochar.（5）The properties of biochar influenced the soil N₂O emissions induced by earthworms.Path analysis showed that electrical conductivity（EC）,specific surface area（BET）,and pore diameter（PD）of the biochar were the main influencing factors,with total effect coefficients of0.651,0.534,0.236,and 0.137,respectively.In conclusion,the addition of biochar inhibited soil N₂O emissions induced by earthworms by affecting nitrogen transformation processes.The main mechanisms include inhibiting earthworm activity to reduce the range of earthworm burrow effects,altering the physicochemical properties and nitrogen cycling functional microbial abundance of drilosphere,and changing the anaerobic environment in the earthworm gut.