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 ), stability, and aeration of soil but may directly or indirectly impact the soil microbiome by altering soil properties. However, respective studies on its effects on microbial activity in terms of respiration and functional diversity remain scarce. In this study, 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 ) and either incubated under constant moisture or ten drying-rewetting cycles. During incubation, soil WHC _max , pH, and microbial activity were measured via headspace CO ₂ and MicroResp assay. PAA increased WHC _max in both soils and remained stable, except in loam under static moisture. Initially, PAA lowered pH in both soils, which persisted only in sand and disappeared in loam after one week. Further, drying-rewetting cycles raised pH in both soils compared to static conditions. PAA suppressed substrate-induced respiration (SIR) for carbohydrates, amines, and carboxylic acids, particularly in the sand, where high concentrations led to up to 100% suppression. Responses in the loam varied: drying-rewetting cycles increased, while static conditions reduced microbial respiration at higher PAA concentrations, respectively. Overall, PAA reduced microbial activity in sand, whereas moisture regimes and soil texture were dominant factors in loam. This highlights the dual impact of SAPs: improving water retention in a certain period, while potentially reducing soil microbial activity and nutrient cycling, depending on soil type, application rates, and environmental conditions. In the long term.