Soil type and moisture dynamics govern the physicochemical aging of superabsorbent polyacrylic acid hydrogels in soil

Polyacrylic acid (PAA) is a synthetic superabsorbent polymer (SAP) used in agriculture to improve, inter alia, soil water retention and soil structural stability. However, the physicochemical transformation of PAA after incorporation into soil and the relative contributions of soil properties, moisture dynamics, and soil microbial activity remain insufficiently understood. We therefore investigated pre-swollen PAA incubated in sand and loam under initially sterilized and non-sterilized conditions for up to 10 weeks under static moisture conditions or 10 drying-rewetting cycles. Changes in polymer-water interactions, mechanical stability, surface chemistry, and morphology were assessed using ¹ H-NMR relaxometry, rheometry, ATR-FTIR, and ESEM. Soil incubation strongly restricted water mobility within the PAA hydrogel and increased its mechanical stability, accompanied by progressive condensation and association with soil mineral particles. These transformations occurred rapidly in loam, whereas PAA in sand retained a more open structure initially and showed more gradual structural reorganization. Under drying–rewetting, hydrogel condensation and particle-associated restructuring were particularly pronounced, although the magnitude and temporal development of these effects were soil-specific. Sterilization affected several individual endpoints but did not alter the general physicochemical aging pathway, indicating that microbial activity acted mainly as a secondary, context-dependent modifier relative to soil-matrix and incubation effects. Overall, the results demonstrate that loss of PAA swelling functionality is accompanied by pronounced physicochemical transformation and formation of dense polymer-soil residues and should therefore not be interpreted as evidence of polymer mineralization or disappearance.