Chemical characterization of sewage sludges in chile and their potential utilization as amendment to reclaim soils for forestation purposes

Soils open for forestation in Chile are characterized by their very low organic carbon content; therefore, new forest plantations, needed to restore soil ecological equilibria, are scarcely developed. Stabilized sewage sludge contain organic compounds which have been demonstrated can serve as good soil amendments. To evaluate their actual uses in Chile it is necessary to characterize the carbon (C)-distribution pattern of such sludges in order to foresee their potential contribution for soil reclamation and plant growth, acting both as a source of stable C-reservoir and as readily available C-source. The molecular weight (m.w.) distribution and carbon balance in Chilean sewage sludges are quite similar to soil humus and some woody by-products, as sawdust and bark dust. Total C varies from 30 to 35%, N from 5 to 9%, and P from 10,000 to 12,000 ppm. No significant content was found for heavy metals. The C-balance indicates that around 70% of total-C is under stabilized forms as humine and humic macromolecule structures. The sewage sludge can be considered as good amendments for forest soils, especially in areas heavily eroded and depleted of natural humus. Their soluble-C fractions (13% of total-C) will serve as good energy starter source to strengthen indigenous soil microbial ecology, while their high m.w. fractions (humic acid-like and humine-like macromolecules) will serve both as good humus reservoir and as microelement frame in soils treated with such sewage sludges.