Is soil acidification the cause of biochemical responses when soils are amended with heavy metal salts?

Three New Zealand soils of contrasting texture, organic matter content and cation exchange capacity (CEC) were amended with solutions of the nitrate salts of Cd[II], Cr[III], Cu[II], Ni[II], Pb[II] and Zn[II], spanning the concentration range, 0–100 mmol heavy metal kg −1 soil. Additional treatment sets comprising: (1), the same range of Ca(NO 3) 2 concentrations to account for osmotic effects, and (2), the same range of NO 3 − concentrations, comprising NaNO 3 acidified with HNO 3, to account for the acidifying effects of metal salt amendment of the soils, were also included. Samples were assayed for phosphatase and sulphatase enzyme activities, and for basal respiration and substrate-induced respiration (SIR), approximately 1 week after amendment. Metal amendment resulted in considerable acidification of all three soils, with the metals which hydrolyse most (Cr, Cu and Pb) having the greatest effect, and the coarsest textured soil being the most affected. Phosphatase activity declined by up to 70% as a result of metal amendment in the finer-textured soils, but acid amendment had little or no effect. In the coarse-textured soil, neither acid nor most metals inhibited phosphatase activity until pH fell to below 4. In contrast, sulphatase activity was strongly inhibited by acid and by all metal amendments, including Ca, in all three soils, indicating that acidification was the dominant effect. Soil respiration declined markedly, and approximately equally, at low metal and acid amendment concentrations in the lighter-textured soils. Activity then stabilised with little further decline occurring until high metal amendment reduced soil pH to 4–4.5. In the coarse-textured soil, metal amendment resulted in a similar sharp initial decline of respiration, but acid had a proportionately lesser effect. In all three soils, metal amendment (except Ca) generally resulted in greater inhibition of SIR response than did acid amendment, although the acid effect was substantial. Cadmium and, especially, Ni were the most inhibitory metals. These results showed that, for all biochemical activities assayed except phosphatase, a considerable proportion of the inhibition occurring when soils are amended with heavy metal salts can be attributed to acidification and not a direct metal effect. In the extreme case, sulphatase activity, acidification may account for most or all of the inhibitory effect.