Effect of sewage sludge on soil properties, the growth of Lycopersicon esculentum L. and the properties of the rhizosphere in a model experiment

Abstract

If soils are to be successfully treated with sewage sludge it is necessary to study the biological properties of the soil, which are closely correlated with soil quality. In general the biocomponents of the soil respond more rapidly to changes in soil conditions than the physical and chemical properties, so the microbe populations in the soil and the level of enzyme activity can be used as bio-indicators of soil fertility. In the present experiment a comparison was made of the physical, chemical and biological properties of three types of soil: meadow chernozem, "kovárvány" brown forest soil and brown forest soil with clay illuviation, treated with sewage sludge from Nyíregyháza and Hódmezovásárhely at soil:sludge ratios of 100:0, 20:80, 40:60, 60:40 and 0:100%, and the fertility of treated and untreated (control) soils was compared by growing tomato plants for 63 days. The results indicated that sewage sludge treatment increased the pH(KCl) value, organic matter content, CO 2 production (respiration) and moisture retention time of the soil. In all cases, the yields obtained on treated soils surpassed those recorded on control soils. The highest yield (1.5-2 times) was found for "kovárvány" brown forest soil treated with 60% sewage sludge from Nyíregyháza. A significant increase was also noted for the number of heterotrophic bacteria, aerobic heterotrophic spore-forming, cellulose-decomposing and phosphate-solubilising microorganisms, Bacillus , fluorescent Pseudomonas , Actinomycetes, Hyphomycetes and yeasts in the treated soils. As a consequence, there was a rise in the FDA, dehydrogenase, catalase, urease, protease, phosphatase, β-glucosidase, invertase and aryl sulphatase activity. The activity of these enzymes exhibited a linearly increasing correlation with the organic matter of the soil, thanks to the sewage sludge added to the soils. The proportion of Gram-negative and Gram-positive bacteria on meadow chernozem, "kovárvány" brown forest soil and brown forest soil with clay illuviation treated with sludge from Hódmezovásárhely and Nyíregyháza amounted to 1.924, 2.155, 1.825, 2.953, 2.745 and 3.032, respectively. The highest microbe population density was found for meadow chernozem treated with Nyíregyháza sewage sludge and for "kovárvány" brown forest soil treated with Hódmezovásárhely sludge. The most frequent bacterium isolates in the rhizosphere of tomato belonged to the following genera: Acinetobacter, Alcaligenes, Azotobacter, Bacillus, Burkholderia, Cellulomonas, Chromobacterium, Enterobacter, Escherichia, Flavobacterium, Microbacterium, Micrococcus, Pseudomonas, Proteus, Stenotrophomonas, Streptomyces, Streptococcus and Serratia . The fungus strains isolated in the model experiment belonged to the genera: Alternaria , Aspergillus , Cladosporium , Fusarium , Geotrichum , Mucor , Penicillium , Rhizopus and Trichoderma . In addition many strains were detected from the Saccharomyces genus, but these were only isolated from soils treated with sewage sludge from Nyíregyháza. In summary, the treatment of soils with sewage sludge stimulates plant growth, improves the physical, biochemical and microbial properties of the rhizosphere, helps to preserve the soil moisture content, and raises the soil pH, which is also beneficial for plant growth.