Effects of Cadmium and Simulated Acid Rain on Ammonification and Nitrification in Soil

A perfusion technique was used to study the effects of l) simulated acid rain on the mineralization and nitrification of glycine (NH 2-N) or ammonium sulfate (NH4-N) by adjusting the pH of the perfusates to 3.0, 2.5, or 2.0 with H 2S0 4 ; and 2) 0, 50, I 00, 500, and I ,000 1-1-g Cd!g dry weight of soil on the mineralization and nitrification of NH 2-N, both in soil adjusted with H 2S04 to pH 3.0 and in soil not exposed to acidity. Ammonification was relatively insensitive to both Cd and acidity, occurring even in soil exposed to pH 2.0 or I ,000 ppm Cd. Nitrification was more sensitive, being retarded in NH 2-N-supplemented soils exposed to pH 2.5 and inhibited in soil exposed to pH 2.0. In NH 4-N-supplemented soils, nitrification was retarded at pH 3.0 and inhibited at pH 2.5. This retardation and inhibition occurred despite the increase in the pH of the perfusates to a noninhibitory level. Nitrification was retarded at 500 ppm Cd and to a greater extent at I ,000 ppm Cd, where nitrite accumulated. The combined effect of exposure to Cd and H 2S04 at pH 3.0 was additive rather than synergistic, and even with I ,000 ppm Cd at pH 3.0 some nitrification eventually occurred. Despite wide documentation concerning the importance of acid rain (Hutchinson and Havas I980) and cadmium (Cd) (Wong et a/. 1980) on ecosystems, there is little information regarding the effects of these pollutants, either alone or in combination, on the ecology, activity, and population dynamics of microorganisms in soils (Babich and Stotzky I978a, I978b, I978c. I980; Babich eta/. I980). The ' Present address: University of Calgary, Department of Biology, Calgary, Alberta T2N I N4. Canada disruption by these pollutants of biogeochemical cycling, particularly nitrogen transformations. in soils could have serious consequences for ecosystem stability and productivity. Application of sewage sludge from industrial areas to agricultural land may enrich the soil Cd content (Dudas and Pawluk I975; Soon eta/. I980). Cd is usually found in association with zinc (Zn) (Wong et al. I980). Nitrification was significantly reduced in two out of three soil types, and totally inhibited in all three, by applications of Zn as the sulfate salt, at concentrations of I 00 and I ,000 1-1-g Zn g1 soil respectively (Wilson I977); the pH of these soils ranged from 5.1 to 6.2. However, application of I ,000 ppm Zn to soil as the oxide had no significant effect on nitrogen mineralization or nitrification at pH 6.0, slightly reduced both processes at pH 7.0, and caused a greater reduction at pH 7. 7. (Bhuiya and Cornfield I974). Soil perfusion studies with "garden soil" demonstrated that the nitrification process was reduced by IO mM CdS0 4 and almost completely inhibited by 40 mM CdS04 (Lees and Quastel 1946). The addition of 500 or 1,000 1-1-g Cd/g soil temporarily depressed ammonification and nitrification rates, with complete suppression of these activities occurring only at IO,OOO ppm Cd (Morrissey et a/. I974). No investigations have been conducted, however, to determine whether the toxicity of Cd is influenced by the simultaneous presence of acid rain, a situation which may occur frequently under field conditions, as both pollutants are released by various smelting processes (Buchauer I973; Hutchinson and Havas I980; Bewley and Campbell I980). The heterotrophic process of ammonification appears to be less sensitive to pH than nitrification (Dancer et a/. I973; Alexander 1980a). There is controversy as to the pH at which autotrophic ni-