How do earthworms affect the soil organic carbon fractions and CO2 emissions after incorporation of different maize straw-derived materials

Applying carbon-rich materials, such as those resulting from pyrolysis (biochar) or composting (compost), to the agro-ecosystems has been proven to improve soil organic carbon (SOC) storage and reduce CO2 emissions. However, it is not clear how earthworms drive changes in soil C sequestration and mineralization under biochar/compost incorporation. A 288-day microcosm experiment was conducted to investigate the effects of earthworms on the concentrations of SOC and its fractions, the structural characteristics of SOC and CO2 emissions following three straw incorporation treatments (maize straw, maize straw compost, and maize straw biochar), and no straw addition was used as the control. Each treatment had two variants: one with earthworms (Eisenia nordenskioldi) and one without earthworms. The elemental analysis, 13C NMR spectroscopy, and three-dimensional fluorescence technologies were used to characterize the features of SOC. Results indicated that the incorporation of straw, compost, and biochar significantly improved the SOC contents compared with the control. Earthworms did not affect the SOC fractions in the control soil but led to the increase in SOC and humic acid (HA) under compost incorporation. The molecular structures of HA showed features pointing to high degrees of humification and aromaticity, as presented by elemental composition, fluorescence intensities, and 13C NMR. The increase of SOC and a slight reduction of CO2 emission in compost-amended soil could be attributed to the accumulation of more stable HA caused by earthworms. In contrast, earthworm presence decreased the aromaticity of HA in biochar-amended soil, and made its structure more hydrophilic, which was not conducive to C sequestration. In addition, the mineralization of microbial C by earthworms also led to an increased CO2 emission from biochar-amended soil. Earthworms weaken the ability of biochar to reduce soil CO2 emissions but improve the C sequestration capacity of compost-amended soil. This study improves our understanding of the contributions of soil macrofauna on the SOM formation and stabilization after incorporation of different maize straw-derived materials.