Relationship Between Released Nickel and Zinc and Nitrification in Two Soils Amended with Biosolids #

Abstract

This study examined the effect of biosolids‐borne nickel (Ni) and zinc (Zn) on nitrification process in two soils (Ultisols of the Cecil and Tifton series). Biosolids from Athens, GA, were spiked singly with increasing amounts of Ni or Zn chloride and then mixed with the soils at a weight ratio of 98.5:1.5 (soil:biosolid). The concentration of the spiked metals in the soil‐biosolid mixtures ranged from 0 to 220 mg Ni and 0 to 1500 mg Zn kg− 1. The mixtures were repeatedly leached with water to observe changes in net nitrification and metal release into solution over time. Leached Zn concentrations that caused partial nitrification inhibition (reduction of net nitrification rates from 75% to 50% of control) were 5 to 19 mg kg− 1 and 6.6 to 10.5 mg kg− 1 in the Cecil and Tifton soil, respectively. Nitrification inhibition was manifested not only as net reduction in rates, but also as delay in the process. The prolonged recovery of inhibition caused lag phases of nitrification (10 and 20 days in the Cecil and the Tifton soil, respectively). In the Cecil soil the nitrification delays began when the leached Zn concentration reached 8.1 mg kg− 1 soil. The effect of Zn in the treatments with the least amount of Zn that caused full inhibition needed also time to be complete. It took 20 to 30 days (longer in the Cecil soil) until no NO3 − appeared in leachate, after the small, but still detectable amounts of NO3 −, which were present in the previous leachings. Inhibited nitrification was always associated with a reduced release of metals. This became evident after the easily soluble fraction of spiked metals was leached out. Therefore, it was a good bioindicator of Zn loading in biosolid treated soils. Excessive Zn was an inhibitor not only of nitrification, but also of its own release into the soil solution. Therefore, low concentrations of easily soluble Zn might well reflect impaired microbiological activity in the biosolid treated soils; they do not necessarily indicate lack of environmental hazard. Consequently, a low concentration of heavy metals in the solution of biosolid‐amended soils is not always ecologically safe enough and, in that case, it would be effective to take soil microbial activity into consideration. #Contribution from the Department of Crop and Soil Sciences, University of Georgia.