Organic carbon dominates soil porosity across environmental gradients, with organic carbon–clay metrics providing secondary insight

Understanding how soil porosity varies across environmental gradients is critical for interpreting land–surface processes, including water storage, flow pathways, gas exchange, and carbon cycling. While organic carbon (OC) and soil texture are important influences on soil structure, their combined influence on porosity at landscape to continental scales remains incompletely resolved. Here, we compile topsoil datasets from Great Britain, Wales, the European LUCAS survey, and the USDA-NRCS NCSS database (n = 6335) to quantify relationships between porosity, OC, and clay content across diverse environmental conditions. Focusing on minimally disturbed soils, we evaluate individual and combined controls using generalized additive models.
We develop and test simple empirical descriptors of OC:clay ratio, including a signed metric (NonCOCC) representing deviation from a reference ratio of 1 g OC to 10 g clay, positioning soils along a continuum from clay excess to OC excess. Across all regions, OC has the strongest relationship to porosity, explaining substantially more variation than OC:clay ratio metrics. NonCOCC provided a modest improvement in model performance when combined with OC and clay.
Across regional datasets, the steepest portions of the fitted NonCOCC partial responses generally occurred at low positive values, suggesting a region of elevated porosity sensitivity. In a subset with aggregate-size data, this range of NonCOCC values also corresponds to a shift towards larger aggregates. This may suggest a structural pathway linking organic enrichment, aggregation, and porosity, but requires testing using data with measured OC partitions.
The findings confirm importance of OC for predicting porosity and identify added value of simple metrics (NonCOCC) from widely available soil observations.