Effects of Synthetic Root Exudate on Drying-Induced Hardening in Sandy Soils

In arid and semi-arid regions where soil sandification is widespread, soil drying simultaneously reduces water availability and increases mechanical impedance, yet how rhizosphere carbon inputs regulate this coupling remains unclear. We investigated whether a synthetic root exudate (SRE, glucose) alters the moisture range and time window in which penetrometer resistance ( PR ) increases during drying across soils with contrasting sand contents. Volumetric water content ( θ ) and PR were measured concurrently at fixed drying times, from which PR - θ sensitivity metrics and a reference threshold ( PR = 2 MPa) were derived. Relative to the control, SRE maintained a higher θ from day 3 onward but also increased PR , shifting the main PR sensitivity window toward wetter conditions and maximum sensitivity was amplified about 3.5-fold at intermediate sand contents. SRE also caused responsive soils to cross the 2 MPa threshold 1.0–1.5 days earlier. Overall, this model system highlights a rhizosphere-driven trade-off: low-molecular-weight carbon inputs can retain moisture while accelerating drying-induced hardening risk toward wetter conditions, with the strongest effects at intermediate sandification levels. These findings provide process-level insight that may inform sustainable soil and water management in arid and semi-arid sandy agroecosystems.