Obtaining environmentally Sound Mixtures of Anthropogeomorphic Materials for Land Restoration.
Marta Camps Arbestain, NEIKER, Berreaga 1, Derio, Spain and Felipe Macias, Univ de Santiago de Compostela, Depto de Edafologia y Quimica Agricola, Facultad de Biologia, Santiago de Compostela, 15782, Spain.
Unconsolidated materials derived from anthropogenic activities can act as fresh parent materials which, under the influence of soil forming factors, may originate new soils. In the early stages of development these soils are Anthropic Regosols - qualified at the subgroup level as garbic, reductic, espolic, or urbic Regosols, depending on the origin of the anthropogeomorphic material - whereas at more advanced stages of development they can evolve to Anthrosols. The people of ancient cultures used to add organic amendments to the soils to maintain soil fertility and, in certain cases higher organic carbon contents and crop productivity was achieved than with nearby natural soils (e.g., Plaggen soils in Europe, “Terra Preta do Indio” soils in Brazil). However, environmental problems have recently arisen with the addition of wastes to soils because of (i) changes in the type residues added (e.g., industrial and urban residues), and (ii) the fact that the final goal of the addition of residues to soils is often the elimination of wastes rather than the enhancement of soil fertility. Nevertheless, the elaboration of soils from anthropogeomorphic materials can be an important and feasible method of re-using waste products and restored degraded areas, while at the same time recycling essential nutrients and stabilising the organic matter present in these materials, if they are of adequate quality and applied according to good practices. Environmental problems can be prevented if the characteristics of the materials used, as well as how the constituent mixtures evolve over time, are well known and adequate for such purposes, and the characteristics of the final products obtained are suited to the pedoclimatic conditions and to the soil uses of the area to be restored. Thus, residues such as fly ash, foundry sand, L-D slag, sewage sludge, poultry and lumber residues, straw, etc., can be effectively managed through the elaboration of more or less complex mixtures, in which the proportions of each ingredient should be adjusted to provide an adequate environment for the formation of a new soil. The new soil should represent an environmentally sound mixture (e.g., low pollutant availability, low ecotoxicity), and have (i) adequate mineralogical and biogeochemical conditions (e.g., reactive surfaces, acid-base and redox characteristics), (ii) a balanced essential nutrient content, (iii) adequate physical characteristics (e.g., porosity, particle size distribution, water retention capacity, aggregate stability), (iv) adequate soil biology, and (v) stable organic C forms. In other words, the soil newly formed from the mixture of wastes should fulfil main soil functions. The results of the present meso-scale experiments with mixtures of sewage sludge, fly ash, barley straw, L-D slag, and foundry sand suggest that the elaboration of soils from these materials, using the appropriate proportions, is feasible. The addition of organic residues with a high C/N ratio (barley straw) promotes the immobilisation of the N present in sewage sludge. Fly ash, on the other hand, promotes the stabilisation of organic carbon present in the sewage sludge through sorption on amorphous aluminosilicate surfaces, rendering a less oxidisable organic carbon, as indicated by the results of the permanganate reaction. The L-D slag has a liming effect that favours the immobilisation of heavy metals, and both L-D slag and foundry sand promote drainage. Foundry sand also increases the clay content, in addition to the sand fraction, and fly ash supplies most of the silt fraction. Essential nutrient elements in the different mixtures tested should be more balanced than with the separate use of each ingredient. We are currently carrying out greenhouse experiments to determine the extent to which, if any, deficiencies or toxicities affect plant growth. Results of these experiments, together with previous results, will provide data for predicting nutrient availability, heavy metal mobility, and organic matter stability, in order to carry out environmental risk assessment for the use of waste mixtures in soil environments. A formulation programme could also be generated from this information, which would allow attainment of the correct mixture, taking into account available ingredients, the final pedoclimatic environment, and the soil use of the area to be restored, and thus, the safe use of these mixtures. In any case, we must stress the need for controlled and monitored use of these materials in surface systems.