| Samples of two sewage sludges, one aerobic and other anaerobic, were taken for 15 months. The fluctuation of both carbon and nitrogen contents, as well as the changes in the ratio C/N, pH, electrical conductivities, the mineralization process of sewage sludges added to different soils, the variations of heavy metals (Fe, Cu, Mn, Zn, Ni, Cd, Pb) contents were studied. Both sludges have similar values of total nitrogen and they can be considerate as moderate fertilizers, although the mineral nitrogen, meanly ammonic nitrogen, is higher in the anaerobic sludge. The pH values of these materials (≈ 7) are similar to those of the agricultural soils of the S.E. of Spain. Their electrical conductivities are moderate ant their cation exchange capacities are 56.7 and 71.0 me/ 100 g in aerobic and anaerobic sludge. Both sludges have a high phosphorus, potassium and magnesium levels. Sodium and calcium content are also high. They have a large amount of ashes. From the values of the ratio C/N near 8, one may expect that these sludges will be able to improve the soil fertility. Furthermore, the results showed that the anaerobic sludge had higher contents of total and extractable heavy metals than the aerobic sludge, and that its variations were lower. The heavy metal levels in both sewage sludges never exceeded toxic limits.;The carbon mineralization of both sewage sludges, aerobic and anaerobic, added to five different agricultural typic soils of the S.E. of Spain also was studied. In the mineralization process it could be observed, in all cases, an initial stage in which the amounts of carbon given off were highest, corresponding to the onset of microbiological activity in the soil and to the degradation of the easily metabolizable compounds; and other stage, at the end, with daily almost constant losses of small amounts of carbon. The correlation found between microbiological activity of soils amended with sewage sludges and the C/N ratio of these soils might indicate that the mineralization process was influenced by the type of soil.;On the other hand, humic acids extracted from both sewage sludges were characterized by elemental and functional group analyses, infrared spectroscopy, gas chromatography-mass spectrometry, visible spectroscopy and coagulation threshold. A comparative study between these humic acids and other extracted from leonardite (an oxidized form of lignite coal) was made.;A calcareous soil was treated with an organic fertilizer mixture of grape debris and peat, or with pig manure, chicken manure, city refuse or with aerobic or anaerobic sewage sludges. Changes in different fractions of carbon and nitrogen were followed by analysis of the soil, immediately after these additions, after they had decomposed for six months in the soil, after a maize harvest, and after a subsequent barley harvest. The various forms of carbon had decreased after six months. The ratio of humic acids/fulvic acids had increased after six months of humification and, in general terms, the final values of the ratio oxidizable carbon/extractable carbon were lower than the initial ones, indicating a higher degree of humification in the organic matter by the end of the experiment. The total nitrogen level remained practically constant after the first six months of humification even after the second crop.;Sludge application enhanced the maize and barley yields and supplied a considerable amount of available N to the plants, whereas the supply of available K was low. In the barley plants it was observed that the higher the yield, the higher the N and K contents, while in the maize crop this trend was only observed for K. Plants grown on soil amended with sludges absorbed a larger amount of Fe, Cu, and Zn than those grown on the control soil. It was demonstrated that heavy metal absorption by the plants depended on the vegetal species.;Key words. sewage sludges, carbon, mineralization, humic acids, crop yield, heavy metal availability, macronutrients, harvests yield. |
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