FORMATION OF STABLE AGGREGATES IN DEGRADED SOIL BY AMENDMENT WITH URBAN REFUSE AND PEAT

Soil structure has been destroyed over large areas of arid and semi-arid regions by soil degradation processes. This study was conducted to investigate the effects of two organic amendments, urban refuse and peat, on the improvement of soil structure and to analyse correlations between organic carbon content, fungal and bacterial populations and aggregate stability. Two series of five plots were established in the southeast of Spain, in typical Mediterranean semi-arid to arid conditions. To one series different initial doses of urban refuse (0, 6.5, 13, 19.5 and 26 kg m-2) were added, whereas to the other series different doses of peat (0, 10, 20, 30 and 40 kg m-2) were added. The average percentage of stable aggregates showed a significant increase (31.6, 41.1, 53.7, 63.2%) with increased levels of urban refuse with respect to the control. On the other hand, peat was not effective in improving stable aggregates. The beneficial effect which appeared with urban refuse remained in the soil two years after application, probably due to the growth of natural vegetal cover in the treated plots. A marked increase in fungal and bacterial populations and a decrease in extractable organic carbon was observed in the plots into which urban refuse was incorporated. This, together with the high correlation coefficients between the percentage of stable aggregates and the microbial population, suggested that the combined action of polysaccharides from the urban refuse and the increase in microbiological activity was responsible for the initial formation of soil aggregates.