Soil Biol. Biochetn.Vol. 26, No. 8, p. 1099, 1994 Copyright 0 1994 ElsevierScience Ltd Printed in Great Britain. All rights reserved
LETTER TO THE EDITOR METAL CONTAMINATION
ON SOIL MICROORGANISMS
I. L. PEPPER, J. W. BRENDECKE and R. D. AXEISON Department of Soil and Water Science, The University of Arizona, 429 Shantz Building No. 38, Tucson, AZ 85721, U.S.A. (Accepted 20 January 1994)
We read with interest the letter of E. Witter, K. E. Giller and S. P. McGrath (Soil Biol. Biochem. Vol. 26, No. 3, pp. 421-422, 1994) commenting on the longterm effects of metal contamination on soil microorganisms. The jist of their comments is that longterm applications of sewage sludge containing heavy metals may adversely affect microbial processes and soil fertility, if longterm loadings of sludge are continuously applied. Their point is well taken; however, their comparison of the situation in Europe to that in the Sonoran Desert is not. Soils of the Sonoran Desert are typically of high pH (> 8.0) and low in organic matter (>d.j%). This contrasts with most soils of Europe which are of lower pH, and higher in organic matter. Because of the influence of these par-
ameters on metal availability (particularly high pH), total metal concentrations are not nearly as relevant as available metal concentrations. Thus total metal concentrations in a Sonoran Desert soil will be less toxic than the same concentrations in a European soil. The sludge used in the article by Brendecke et al. (Soil Biol. Biochem. Vol. 25, No. 6, pp. 751-758, 1993) was very low in metals, and in Sonoran Desert soils is unlikely to affect microbial processes even after longterm applications. Further, municipal waste must be put into either soil, water or the atmosphere-there is no other choice. Given the situation in Tucson, we feel very comfortable recommending the practice of sewage sludge injections into farm land. We would be very interested to hear of a better alternative.