Effects of single sewage sludge application on soil phytoremediation

Abstract

In European Union countries, disposal sewage sludge on agricultural lands is restricted by directives. Large-area degradation of surface soils in Europe is a major challenge to maintain and remediate due to their size and geomorphology, different historical uses and current biogeochemical conditions. These sites are often degraded with a mixture of contaminants mostly lacking a biologically active topsoil. The use of sewage sludge to restore such sites with soil organic matter and stabile plant cover have for long been explored, but the application is restricted by the chemical quality of the sewage sludge. Hence we designed a study to investigate the effect of a one-time amendment of a low-metal contaminated sewage sludge on a barren and contaminated soil to immobilize harmful trace metals and obtain sustainable plant cover. The present study was aimed to create plant cover on a barren soil by a single application of sewage sludge through the process of phytostabilization. The field experiment was conducted in the contaminated area near a zinc smelter with high metal concentration (cadmium, zinc, lead) and poor soil fertility. A single application of sewage sludge from food industry was made and forest species of Scots pine (Pinus silvestris L.), Norway spruce (Picea abies L.) and oak (Quercus robur L.) were used for phytoremediation. It was found that single application of sewage sludge was beneficial for development of plant cover and after a five years a significant increase in the biomass of trees (mainly spruce and pine) was achieved. Moreover, in the amended plots, the growth of various species of grasses, dicotyledonous plants, and spontaneous growth of birch was also observed. On control plots with no sewage sludge application, the planted trees gradually withered away during the same period. Sewage sludge from food industry provided plant nutrients effectively utilized by the plants. Its application also increased the water holding capacity in the test plots due to it contribution to soil organic matter. Compared to the control plots, the content of macroelements and carbon in soil was still higher at the end of the 5 year study. The high content of bioavailable macroelements in sewage sludge, poses a potential threat to their leaching and run-off into groundwater, contributing to the emergence of eutrophication of water bodies. However, the results of the experiments showed that the gradual release of macronutrients from sewage sludge were fully used by plants to stimulate their growth and development. The content of cadmium, zinc and lead in plant biomass was much lower compared to untreated plants. Also for treated plants the heavy metals were mainly accumulated in roots. The data from this field trial provided results supporting the use of high quality sewage sludge amendment for remediation of contaminated and barren soils, and large scale trials should be followed up on various degraded soil types.