Abstract
The effect of the addition of sewage sludge on the content of water-stable aggregates (WSA) in a stuctureless loessial soil was studied in a 54-d incubation experiment. The changes in the WSA content after addition of 5% sludge were characterized by a phase of WSA formation (lasting about 10 d at 25°C) and a phase of decreasing WSA content leading to a constant WSA level. The first phase was associated with a sharp increase in bacterial and fungal populations, a high rate of CO2 evolution and an increase in the water-soluble carbohydrate (WSC) content. At 25°C, the WSA content in the sludge-amended soil remained higher than in the unamended soil until the end of incubation. Selective inhibition treatments enabled the assessment of the relative importance of the main microbial groups in the formation of WSA. Fungal activity was best correlated with structural stability in the soil-sludge mixtures. Relatively high correlations were found between the WSA level and between both the fungal counts and the water-soluble anthrone-reactive carbohydrate content. This suggests that cementing by fungal carbohydrates and physical entanglement by mycelium may act as binding mechanisms involved in the formation of WSA when sludge is applied to soil.
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