Tannery residues are potential pollution sources in several regions of Brazil, mainly in the State of Rio Grande do Sul. One of the alternatives for the final destination of the sludge resulting from the waste water treatment of tanning industries is the disposal and recycling in the soil. To evaluate the effects of two waste types originated from primary treatment of tannery residual water on soil microbial populations, an open air pot experiment was carried out in a Paleudult soil with ten percent clay from Estância Velha, Rio Grande do Sul State, Brazil, where several tannery industries are located. Mineral fertilization was compared to sludge application of 15, 30 and 60 t ha-1 of sludge originated either from tannin or Cr tanning agents. One treatment with cattle manure and a control were also included. Chromium soil, organic C concentrations, pH and bacteria, fungi and actinomyces populations were determined. Biological degradation was efficient for sludge with Cr up to 60 t ha-1 level and for tannin sludge up to 30 t ha-1 dose. Except for the sludge with Cr on the fungus population, the bacteria, fungi and actinomyces populations were stimulated by tannery sludge application. Due to the organic matter concentration in the soil, bacteria microbial populations were stimulated more than fungi and actinomyces. Chromium, pH and organic C increased as a result of the soil disposal of tannery sludges. The application of both tannery residues did not decrease the biological degradation process when compared to normal soil levels.
soil quality indicator; biological attribute; chromium; organic carbon; soil pH
