Red mud can be applied as soil ameliorant to acidic, sandy and micronutrient deficient soils. There are still knowledge gaps regarding the effects of red mud on the soil microbial community. The Biolog EcoPlate technique is a promising tool for community level physiological profiling. This study presents a detailed evaluation of Biolog EcoPlate data from two case studies. In experiment "A" red mud from Ajka (Hungary) was mixed into acidic sandy soil in soil microcosms at 5-50 w/w%. In experiement "B" red mud soil mixture was mixed into low quality subsoil in a field experiment at 5-50 w/w%. According to average well color development, substrate average well color development and substrate richness 5-20% red mud increased the microbial activity of the acidic sandy soil over the short term, but the effect did not last for 10 months. Shannon diversity index showed that red mud at up to 20% did not change microbial diversity over the short term, but the diversity decreased by the 10 th month. 30-50% red mud had deteriorating effect on the soil microflora. 5-20% red mud soil mixture in the low quality subsoil had a long lasting enhancing effect on the microbial community based on all Biolog EcoPlate parameters. However, 50% red mud soil mixture caused a decrease in diversity and substrate richness. With the Biolog EcoPlate we were able to monitor the changes of the microbial community in red mud affected soils and to assess the amount of red mud and red mud soil mixture applicable for soil treatment in these cases.

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BACKGROUND: Red mud is increasingly considered a valuable by-product requiring development of new re-use technologies instead of deposition as waste. This paper reports the results of a microcosm study aiming to reveal the beneficial effects of the Ajka red mud, as soil ameliorant, on a specific acidic sandy soil in eastern Hungary. RESULTS:The red mud amount mixed into the sandy soil ranged between 0 and 50%. The experiment was monitored by an integrated methodology combining physical and chemical methods with biological and ecotoxicological measurements. The adverse effects of red mud were examined by ecotoxicity tests using test organisms from three trophic levels: Aliivibrio fischeri (luminescent bacterium), Sinapis alba (plant), Triticum aestivum (plant) and Tetrahymena pyriformis (unicellular animal). The results showed that red mud applied at 5% had positive effects on the degraded acidic sandy soil, significantly increasing soil pH, improving soil texture and water holding capacity. In addition, according to the ecotoxicological test results 5% w/w red mud had no significant adverse effects on the tested organisms as members of the soil ecosystem. CONCLUSION:The results demonstrated the beneficial effects of red mud as an ameliorant of acidic sandy soils under continental climatic conditions.

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