Carbon mineralization from sewage sludge-amended mine dump by response surface methodology

Application of sewage sludge to degraded soil has received considerable attention because of its potential for soil fertilization and also as an effective, low-cost disposal method. An understanding of the decomposition of organic substrates in soil is essential to determine the relative worth of different materials for the improvement of soil agrochemical properties. Therefore, this study was aimed to evaluate the C mineralization of mining soil amended with dried sewage sludge and co-composted with olive pruning by response surface methodology. We carried out an aerobic incubation in the laboratory using varying doses of waste (0–100 g kg^-1) following a D-optimal design over 30 days at 40 % field capacity and 28° C. The CO₂ evolution pattern was monitored throughout the incubation time. The experimental design allowed evaluating how the characteristics of the sludge affected the C mineralization process as showed by surface and contour plots and estimated the kinetic parameter of CO₂–C production at several application doses. The dried sludge supplied an easily degradable C pool that produces 5 times higher CO₂–C than that found for the co-composted one. Mineralization process, performed differently for each sludge, can be discriminated by mineralization and metabolic quotients. Additionally, fluorescence spectrometry also discriminates the composition of dissolved organic matter from each substrate. In both cases, soil biological activity was enhanced. The overall results suggest that response surface methodology was effective in the assessment of C mineralization over a wide range of sludge doses and times, in the identification of interactions between incubation factors and in the prediction of the dose of sludge and time for obtaining target C mineralization (and vice versa).

Keywords
Organic matter; D-optimal design; Mine dump; Dried sludge; Co-compost; Soil respiration; Soil biochemical properties