Biological and physical resilience of soil amended with heavy metal-contaminated sewage sludge

Stability and resilience of a variety of soil properties and processes are emerging as key components of soil quality. We applied recently developed measures of biological and physical resilience to soils from an experimental site treated with metal-contaminated sewage sludge. Soils treated with cadmium-, copper- or zinc-contaminated, digested or undigested sewage sludge were studied. Biological stability and resilience indices were: (i) the time-dependent effects of either a transient stress (heating to 40degreesC for 18 hours) or a persistent stress (amendment with CuSO4) on decomposition, and (ii) the mineralization of dissolved organic carbon (DOC) released by drying-rewetting cycles. Physical stability and resilience measures were: (i) compression and expansion indices of the soils, and (ii) resistance to prolonged wetting and structural regeneration through drying-rewetting cycles. Soil total carbon and DOC levels were greater in the sludge-amended soils, but there were no differential effects due to metal contamination of the sewage sludge. Effects of metals on physical resilience were greater than effects on soil C, there being marked reductions in the expansion indices with Cd- and Cu-contaminated sludge, and pointed to changes in soil aggregation. The rate of mineralization of DOC released by drying and wetting was reduced by Zn contamination, while biological resilience was increased in the Zn-contaminated soil and reduced by Cd contamination. We argue that physical and biological resilience are potentially coupled through the microbial community. This needs to be tested in a wider range of soils, but demonstrates the benefits from a combined approach to the biological and physical resilience of soils.