NITROGEN TRANSFORMATION IN SEWAGE-SLUDGE AMENDED SOIL AS INFLUENCED BY ADDITION OF ZINC, CADMIUM AND NICKEL

An incubation experiment was conducted to study the effect of Zn, Cd a nd Ni alone and in combination on the N transformation in loamy-sand s oil amended with sewage-sludge. Nitrification decreased significantly at Zn, Cd and Ni concentration > 1.5, greater than or equal to 0.25 an d greater than or equal to 1.5 meq/kg, respectively at the end of 16 w eeks incubation. However, the decrease was more marked during later pe riod of incubation. Ammonification was affected at much higher rates o f metal addition. The threshold metal concentration for 5% decrease in the total mineralized N was 6 (196.1 ppm), 0.25 (14.1 ppm) and 6 (176 .1 ppm) meq/kg Zn, Cd and Ni, respectively. The effect of Zn and Ni on the N transformation was comparable, whereas Cd was 6-8 and 4-6 times more inhibiting than Zn and Ni. Addition of metals combinedly caused severe reduction in the nitrification and ammonification. The effect w as additive on the nitrification up to 6, 1 and 6 meq/kg Zn, Cd and Ni , respectively when used in combination. Further increase in the metal concentration resulted in mon than additive inhibitory effect except the 9, 1.5 and 9 meq/kg Zn, Cd and Ni, respectively showing less than additive effect when combinedly added. Addition of metals combinedly c aused 9.5-86.1 and 14-44.2% decrease in the nitrification and ammonifi cation, respectively after 16 weeks of incubation.