Soil organic carbon (SOC) in arable topsoil is known to have beneficial effects
on soil physical properties that are important for soil fertility. The effects of
SOC content on soil aggregate stability have been well documented; however,
few studies have investigated its relationship with the soil pore structure,
which has a strong influence on water dynamics and biogeochemical cycling.
In the present study, we examined the relationships between SOC and clay
contents and pore size distributions (PSDs) across an arable field with large
spatial variations in topsoil SOC and clay contents by combining X-ray tomography
and measurements of soil water retention. Additionally, we investigated
the relationships between fractionated SOC, reactive Fe and Al oxide contents
and soil pore structure. We found that porosities in the 0.2–720 μm diameter
class were positively correlated with SOC content. A unit increase of SOC content
was associated with a relatively large increase in porosity in the 0.2–5 and
480–720 μm diameter classes, which indicates that enhanced SOC content
would increase plant available water content and unsaturated hydraulic conductivity.
On the other hand, macroporosities (1200–3120 μm diameter classes)
and bioporosity were positively correlated with clay content but not with SOC
content. Due to strong correlations between soil texture, carbon-to-nitrogen
ratios and reactive iron contents, we could not separate the relative importance
of these soil properties for PSDs. Reactive aluminium and particulate organic
carbon contents were poorer predictors for PSDs compared with clay and SOC
contents. This study provides new insights on the relations between SOC and
soil pore structure in an arable soil and may lead to improved estimations of
the effects of enhanced SOC sequestration on soil water dynamics and soil
water supply to crops.