Carbon-sensitive pedotransfer functions for plant available water

Currently accepted pedotransfer functions show negligible effect of management-induced changes to soil organic carbon (SOC) on plant available water holding capacity (θ_AWHC), while some studies show the ability to substantially increase θ_AWHC through management. The Soil Health Institute's North America Project to Evaluate Soil Health Measurements measured water content at field capacity using intact soil cores across 124 long-term research sites that contained increases in SOC as a result of management treatments such as reduced tillage and cover cropping. Pedotransfer functions were created for volumetric water content at field capacity (θ_FC) and permanent wilting point (θ_PWP). New pedotransfer functions had predictions of θ_AWHC that were similarly accurate compared with Saxton and Rawls when tested on samples from the National Soil Characterization database. Further, the new pedotransfer functions showed substantial effects of soil calcareousness and SOC on θ_AWHC. For an increase in SOC of 10 g kg^–1 (1%) in noncalcareous soils, an average increase in θ_AWHC of 3.0 mm 100 mm^–1 soil (0.03 m³ m^–3) on average across all soil texture classes was found. This SOC related increase in θ_AWHC is about double previous estimates. Calcareous soils had an increase in θ_AWHC of 1.2 mm 100 mm^–1 soil associated with a 10 g kg^–1 increase in SOC, across all soil texture classes. New equations can aid in quantifying benefits of soil management practices that increase SOC and can be used to model the effect of changes in management on drought resilience.