AN INCUBATION EXPERIMENT TO DETERMINE FACTORS INVOLVING AGGREGATION CHANGES IN AN ARID SOIL RECEIVING URBAN REFUSE

The effect of the addition of urban refuse (UR) on the percentage of s table aggregates in a semi-arid structureless soil was studied in a 67 -day incubation experiment. Twelve systems were established combining amendments with two doses of UR (2.4 and 4.8%) and treatments with cyc loheximide, chloramphenicol or formaldehyde to evaluate the relative e fficiency of carbohydrates and microbial (fungi and bacteria) communit ies in promoting changes in aggregate stability. The addition of UR to the soil increased the abundance of stable aggregates and this increa se was proportional to the quantity of UR added. The degree of aggrega te stability was related to all the factors considered, although this relationship was more evident for the fungal communities (R = 0.81, P < 0.001) and, to a lesser extent, for the bacterial communities and ca rbohydrate content. Carbohydrates were significantly correlated with s table aggregates during the first stages of incubation (R = 0.83 P < 0 .001, at day 4), but this effect was short-lived and the maintenance a nd increase of the aggregate stability in the subsequent phases was at tributable to the increases in microbial populations, and particularly to the proliferation of fungal mycelium.