Roots of Cynodon dactylon increase gas permeability and gas diffusion coefficient of highly compacted soils

Aims: This study experimentally investigated the short-term (about 6 months) effects of roots of Cynodon dactylon on the gas permeability and gas diffusion coefficient of soils with different degrees of compaction. Methods: Compacted soil planted with Cynodon dactylon were left outdoors for about 6 months for plant growth. The measurements of rooted and bare soils included gas permeability, gas diffusion coefficient, root characteristics and soil microstructure. The relative effects of different root characteristic parameters on gas permeability and gas diffusion coefficient were compared through grey relational analysis. Results: The root volume ratio had a greater effect on gas permeability and gas diffusion coefficient, compared with root area index, root area ratio, root length density, and root biomass ratio. When the degree of compaction ≥ 85% (porosity ≤ 0.41, bulk density ≥ 1.56 g cm⁻³), the macro-pores at the root–soil interface increased gas permeability and gas diffusion coefficient, while negligible effects of roots on gas movement existed under degree of compaction of 80%. The increase in gas permeability by roots was more significant than that in gas diffusion coefficient. However, roots’ increase of gas movement generally decreased at higher root volume ratio due to roots-occupied soil pores. Finally, gas permeability and gas diffusion coefficient of rooted soil were well predicted by newly-developed empirical models considering the effect of root volume ratio. Conclusions: Macro-pores at the root–soil interface tended to increase gas permeability and gas diffusion coefficient of soil with a degree of compaction ≥ 85%, while it is the opposite for root volume ratio.