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.