Impact of polyacrylic acid as soil amendment on soil microbial activity under different moisture regimes

Polyacrylic acid (PAA), a synthetic superabsorbent polymer (SAP), enhances the maximum water holding capacity (WHC_max), soil structural stability, and aeration of soil but may simultaneously affect soil microbiome by altering soil properties. However, its effects on microbial activity under different moisture regimes remain insufficiently understood. We examined the impact of PAA on soil microbial activity in a sand and loam treated with PAA at three concentrations (25, 250, 2500 mg kg^{- 1} dry soil) either incubated under constant moisture or subjected to ten drying-rewetting cycles. During incubation, soil WHC_max, pH, and soil microbial activity via headspace CO₂ and MicroResp assay were measured. PAA increased WHC_max in both soils, yet its effectiveness decreased in loam under static conditions. Initially, PAA acidified both soils, with pH shifts persisting in sand but dissipating in loam after one week. Drying-rewetting cycles counteracted acidification and partially maintained PAA swelling. In sand, high PAA concentrations consistently suppressed microbial respiration across substrate groups, whereas in loam, microbial responses were modulated by moisture dynamics: drying-rewetting enhanced, while static moisture reduced SIR under PAA treatment. Overall, PAA altered soil microbial activity in a concentration-, soil type-, and moisture-dependent manner, emphasizing the dual role of SAPs in improving soil water retention while potentially impairing microbial-mediated soil functions over time.