Evaluating the effects of elevated concentrations of urea, phosphogypsum and paper mill sludge on soil chemical, microbial and exoenzyme dynamics

Urea, phosphogypsum (PG) and paper mill sludge (PMS) are commonly used to address soil nutrient deficiency. In this study, these chemicals were used at recommended agricultural dose (RAD) along with low and high concentrations to assess their effects on soil chemical and biological quality. Urea was applied at 0.5, 1.0, 1.5 g/kg soil, PG at 50, 100, 150 g/kg soil and PMS at 5, 10, 15 g/kg soil. Significant variation in soil pH, electrical conductivity and organic carbon (OC) were noticed in response to chemical concentration and treatment types. The percent OC reduction was high in PG and PMS treated soils. Percent nitrogen and phosphorous in soil increased with the concentrations of urea and PG. Invariably, lower bacterial and fungal population were observed in treated soils relative to control. Activities of amylase, cellulase, invertase, dehydrogenase and protease were inhibited at high concentration of all test chemicals with minor deviations. The study indicated that bacterial population and exoenzymes could be useful markers to evaluate soil toxicity due to hyper concentrations of urea, PG and PMS in soil.