Soil pH and moisture affect cadmium fractionation in sewage sludge amended soils through humic and fulvic acid transformation

Despite some potential issues with long-term application, sewage sludge is considered beneficial as a valuable source of plant essential nutrients but there are some potential issues with long-term application. Detrimental effects can result due to changes in cadmium (Cd) bioavailability under varying soil pH, moisture and with ageing. We examined changes in soil Cd availability and fulvic and humic like acid fractions in soil together with Cd fractionation and availability with sewage sludge application during a 135 days incubation experiment. Sewage sludge was added @ 0, 15 and 30 g kg-1 to three contrasting soils (variable pH, 7.10, 8.48 and 9.52) and moisture levels, 50 and 70 % of the water holding capacity, (WHC). Fourier transform infrared (FTIR) analysis revealed that aromatic compounds were the main binding sites for Cd leading to its reduced availability. However, the humic like acid fractions exhibited humification with ageing due to the existence of recalcitrant mineral compounds. The high pH amended soils (8.48 and 9.52) at both moisture regimes did not significantly affect the spectral peaks. The percentage of labile Cd fractions was decreased in high pH soils, where soil exchangeable and reducible Cd fractions were decreased by 8 and 5 %, respectively. The stable Cd fractions were consequently increased, where soil oxidizable and residual Cd fractions increased by 8 and 5 %, respectively, especially at 50 % WHC. This indicated that the sewage sludge application at 50 % WHC to high pH soils (pH ??? 8.50) could become a significant sink for Cd, thus decreasing Cd availability steadily over time. Our results have implications for the effective use of sewage sludge for agricultural crop production in alkaline calcareous soils of Pakistan.