Effects of recycled wastewater on soil enzymatic microbial activities during a long-term irrigation trial

Wastewater reuse offers a potential solution to global drought and water scarcity but may have environmental impacts, particularly on soil microbial communities and their metabolic activities. This study investigated the effect of selected irrigation water types on soil bacterial enzymatic activities. Three differently treated urban wastewaters (secondary effluent, ultraviolet (UV)-treated effluent, and ultrafiltration (UF)/UV-treated effluent) were compared with river water over a two-year irrigation period. Soil samples were regularly analysed for potential nitrification activity (PNA) and dehydrogenase activity (DHA). The results indicate that treated wastewater significantly increased DHA in surface soil (at depths of 0–25 cm), with the greatest increase observed for the UF/UV-treated effluent (up to a 59% increase relative to the initial value). In contrast, PNA decreased by up to 82% during the first year across all treatments, suggesting a shift in microbial community structure away from nitrifiers. In the second year, microbial activity stabilised across all treatments. Statistically significant correlations were identified between soil temperature, humus content, and enzymatic activity ( p < 0.001). These findings imply that wastewater reuse enhances total microbial biomass and potentially alters nitrogen cycling dynamics, underscoring the need for targeted monitoring of soil microbiological health in long-term wastewater reuse scenarios.