Objectives The application of biochar as a soil amendment method for improving carbon sequestration, improving soil structure, and mitigation of global climate change has received considerable attentions over the last decade. Soil pores play a key role in transportation of air, water, and heat. However, knowledge of the long-term effects of biochar on soil pore characteristics under field conditions is limited.

Methods We investigated the effect of successive addition of low-dose rice straw-derived biochar 1.5 t/(hm²∙a), C1.5, high-dose rice straw-derived biochar 3.0 t/(hm² ∙a), C3.0, and no biochar addition (control, C0) on soil pore structure in a paddy field over a 6-year (2013‒2018) period. Porosity, pore size distribution, connectivity, anisotropy, fractal dimension, porosity, and mean macropore diameter of the limiting layer were measured by X-ray computed tomography (CT) and image processing.

Results We found that biochar amendment significantly increased soil organic carbon content and total porosity by 15.5% and 10.5%, respectively, and decreased soil bulk density by 7.4%. There was no significant difference in soil total porosity under the two biochar amendment treatments (C1.5, C3.0), but the proportion of macropores was significantly different between them. Compared with C0, C1.5 treatment significantly increased porosity of 100‒500 μm and >500 μm macropores by 81.6% and 275.3%, respectively, while C3.0 treatment significantly decreased the porosity of 100‒500 μm macropores by 32.9%. C3.0 treatment had higher porosity of <25 μm micropores than C0 and C1.5. Meanwhile, connectivity, fractal dimension, porosity, and mean macropore diameter of the compacted layer were highest under C1.5, but anisotropy was similar for the three treatments.

Conclusions Our results indicate that the application of rice straw-derived biochar could increase the soil organic carbon content, total porosity, and decrease the soil bulk density. Proper application rate of biochar could increase the soil macroporosity and connectivity, but excessive application rate of biochar would reduce the soil macroporosity, soil hydraulic conductivity, and gas diffusivity. These results enhance our understanding of the relationship between soil pore structure and biochar application, and provide evidence for decision making process in choosing proper straw management.