Site dominance and the role of soil organic matter and clays in controlling plant-available water in Mediterranean perennial cropping systems

Soil and water conservation is a critical challenge for agricultural systems in Spain. Mediterranean environments are characterized by semiarid conditions, where water scarcity, high evapotranspiration, and irregular rainfall constrain crop productivity and ecosystem sustainability. Soils typically exhibit low organic matter content, structural vulnerability, and a high susceptibility to degradation processes, including erosion, compaction, and reductions in water-holding capacity. Perennial woody crops such as vineyards and olive groves dominate large areas of central Spain and are frequently managed under conditions that intensify soil exposure, particularly through tillage and limited vegetation cover. Under these circumstances, the capacity of soils to retain and supply plant-available water becomes a key determinant of agronomic resilience and long-term soil functioning. Improving soil available water through management therefore requires a mechanistic understanding of how intrinsic soil properties interact with modifiable factors across contrasting edaphic contexts.

The relative importance of soil organic matter (SOM) and clay content in controlling variability in soil available water (AW) is central to understanding soil water retention mechanisms. Both constituents contribute to water storage but operate within strong site-specific constraints and are differentially influenced by management. In this study, the variability of volumetric available water (% vol) was analyzed as the dependent variable in vineyard and olive grove soils under semiarid conditions in central Spain. A linear model was applied including SOM, total clay content, above-ground biomass, management system, site, and their interactions. Univariate tests of significance were used to quantify the independent contribution of each factor while accounting for covariance among variables.

The model explained a large proportion of AW variability (adjusted R² = 0.7; p < 0.05), indicating strong overall performance despite the inherent heterogeneity of Mediterranean soils. Site emerged as the dominant source of variability (p < 0.05), reflecting the influence of inherent soil properties and pedogenic controls. Within this context, SOM, explained a significant proportion of AW variability independent of clay content (p = 0.05), highlighting its role as the main modifiable edaphic factor affecting soil water availability. Clay content exerted a weaker but still significant effect, consistent with its structural control on water retention. Management effects were strongly site dependent, as indicated by a significant management × site interaction (p < 0.05), whereas management alone was not significant, underscoring the absence of uniform management responses across sites. Above-ground biomass did not explain additional variability once soil properties were accounted for, suggesting limited relevance for explaining AW at the 0–30 cm depth considered.

Overall, these results indicate that SOM plays a key role in enhancing soil water availability beyond textural effects, but always within a framework dominated by site-specific soil conditions. This provides a robust basis for further analyses addressing the role of clay mineralogy and soil–organic matter interactions in controlling water retention in Mediterranean perennial cropping systems.

Acknowledgements: This work was supported by the European Joint Programme for SOIL (EJP SOIL), funded by the EU Horizon 2020 Programme (Grant Agreement No. 862695), within the SANCHOTHIRST project. J. González Canales was supported by grant PRE2021-097966 (MCIU/AEI/10.13039/501100011033) and the European Social Fund (ESF). We thank the farmers for field access and management information.