Decomposition and nitrogen transforma- tions in digested sewage sludge applied to mine tailings-effects of temperature, soil moisture, pH and plants

Wennman, P. 2004. Decomposition and nitrogen transformations in digested sewage sludge applied to mine tailings -effects of temperature, soil moisture, pH and plants. Licentiate thesis. ISSN 1652-4748, ISBN 91-576-6628-8 Applying sewage sludge to mine tailings to encourage growth of vegetation in order to prevent environmental problems such as erosion and leaching of metals began around 1970. Use of sewage sludge for mine land reclamation is today an acceptable area of application and is in many cases preferable to spreading sludge as an organic fertilizer on agricultural land, since many sludges contain metals and pathogens. The sludge has been shown to be favourable compared to other additives that do not contain organic matter with respect to improved soil properties and nutrient cycling. The present work was conducted to elucidate the influence of temperature and soil moisture in an incubation experiment on the dynamics of decomposition and nitrogen (N) mineralization in digested sewage sludge mixed with pyrite mine tailings from the Aitik copper mine, Gällivare, Sweden. The data were fitted to a first order two-compartment model. The importance of pH and plants, barley (Hordeum vulgare) and red fescue (Festuca rubra) was studied in a greenhouse experiment. The experimental conditions (temperature, moisture and pH), were chosen to represent situations occurring under field conditions. Temperature and soil moisture had clear effects on microbial activity in the tailingssludge mixture and explained 93% of the variance in respiration rate constants. The proportions of carbon and nitrogen mineralized from the substrate during approximately 100 days, measured in the incubation and the pot experiment, were between 20 and 30% and 15 and 35% respectively. The C:N ratio was roughly constant (8-10) throughout the experimental periods. From these data, good estimates of the amounts mineralized during the first season under different climatic conditions can be obtained. Interestingly, the amounts mineralized N (net) in the pots were much lower than the determined changes in organic N, suggesting that large amounts were lost as gas, most likely via denitrification.