Organic matter in degraded agricultural soils amended with composted and thermally-dried sewage sludges

The cumulative and residual effects of composted and thermally-dried sewage sludge (CSS and TSS, respectively) on soil organic matter and its humified fraction were evaluated in a field experiment under Mediterranean conditions. The experimental design included soil plots either unamended (control) or amended with mineral fertilizer, CSS and TSS at rates of 20 and 80 t ha− 1. After the first year of sewage sludge application, each plot was divided into two subplots. In one subplot group, no additional application of SS was made in the following 3 years, and the residual effect of the first applications was evaluated. In the second subplot group, the cumulative effect of sludge amendments was evaluated by applying CSS and TSS also in the following three consecutive years. Nine months after the yearly sludge and mineral fertilizer applications, surface soil samples from control and amended soils were collected and analyzed for total organic C (TOC), total extractable C (TEC), and humified C fractions, both humic acid C (HAC) and fulvic acid C (FAC) fractions. Compared with the control and mineral treatments, which showed similar results, the repeated application to soil of TSS, and specially CSS, induced an increase on the content of the organic fractions examined, as well as HA percentage (%HA = HAC/TOC) and degree of polymerisation (DP = HAC/FAC). In the residual experiment, the TOC, TEC, HAC and FAC content of soils amended once with CSS and TSS decreased slightly when increasing the time from the amendment, whereas the %HA and DP tended to increase. Further, three years after the sludge applications, with respect to the control soil, the soils amended once with CSS exhibited similar TOC, TEC, and FAC content, and slightly larger HAC content, %HA and DP; whereas those amended once with TSS, featured still larger TOC, TEC, HAC content, similar FAC content, and slightly larger %HA and DP values. As a whole, the results obtained suggested that both kind of sludges contribute to improved soil organic matter levels and humified fractions, although the CSS contribution can be considered as a more efficient organic amendment than the TSS, which may present problems of maturity and degradability.