Decoupling organic matter and macrostructure effects on water retention in No-Till Oxisols

Soil organic matter (SOM) is often cited as a major control of soil water retention, yet the dominant pathway in tropical no-till systems remains unclear: indirect effects mediated by soil structure, or direct effects on water retention. To isolate these pathways, we analyzed an 11-year experiment in the Brazilian Cerrado (Goiás) on a clayey Oxisol (47–61% clay; mean 54.7%), measured across treatments and depths, compiling 192 observations from eight long-term management systems across multiple depths. Texture was included as a covariate to account for this limited clay variability among treatments and depths. In multivariate models, bulk density dominated available water capacity (AWC), accounting for 53.4% of explained variation. After accounting for structural attributes (bulk density and macroporosity), SOM remained a significant predictor of AWC (25.6% relative importance, p < 0.001). Mediation analysis indicated that 91.4% of the SOM effect was direct, with only a minor share linked to macroporosity. The direct contribution was depth-specific: it was not significant at 0–10 cm, where macroporosity dominated, but it was significant at 10–30 cm (β = 0.019, p = 0.013). Clay content had negligible explanatory power (<1%), consistent with the narrow texture range. Overall, AWC variability reflected management-induced changes in compaction and pore-size distribution, with secondary SOM accrual. Because AWC was calculated on a volumetric basis, part of the bulk-density signal is consistent with the density dependence of volumetric water content. In this dataset, higher bulk density coincided with higher volumetric AWC despite reduced macroporosity, while SOM added a mostly direct subsurface benefit.