Oxygen is an important element for plant growth. Reducing its concentration in the soil affects plant physiological processes such as nutrient and water uptake as well as respiration, the redox potential of soil elements and the activity of microorganisms. The main mechanism of oxygen transport in the soil is by diffusion, a dynamic process greatly influenced by soil physical properties such as texture and structure, conditioning, pore size distribution, tortuosity and connectivity. Organic matter is a modifying agent of the soil's chemical and physical properties, affecting its structure and the porous matrix, which are determinants of oxygen transport. This study reviews the theory of soil gas diffusion and the effect of soil organic matter on the soil's physical properties and transport of gases. It also reviews gas diffusion models, particularly those including the effect of soil organic matter.

Arbuscular mycorrhizal fungi (AMF) are highly important for plant communities in dry or seasonally dry ecosystems, such as the South American Mediterranean-type ecosystem (MTE), considered a biodiversity hotspot. While AMF hold potential for sustainable MTE management and conservation, they have been under investigated on this ecosystem and little is known about AMF spore bank dynamics. In this study, we analyzed the effect of physico-chemical soil factors, phytobiont species, and seasonality on the AMF spore soil density in two sclerophyllous forests (Malloa and San Vicente). We sampled soil once per season during 1 year and beneath four representative tree species for each site. The results show a strong season effect at both sites, while physical-chemical parameters differed between sites. At Malloa, clay content and electrical conductivity were positively correlated with spore density, while available phosphorous showed a negative correlation. At San Vicente, clay content and total nitrogen were positively correlated with spore density, while soil organic matter showed a negative effect. Overall, spore number reached a minimum value in winter and higher values during the growing season at both sites. These results indicate a strong regulation of AMF spore density by seasonal climate, while physico-chemical soil properties exert a host-independent but site-specific effect in both forests.

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