Study: Sewage Sludge Builds Organic Matter in Depleted Soils
A team of researchers analyzed these types of tropical soils in Brazil: Typic Eutrorthox (clayey) soil (left) and a Typic Haplorthox (sandy) soil (right). Photo courtesy of Ladislau Martin-Neto.
Researchers lead by Ladislau Martin-Neto, from the Brazilian Agricultural Research Corporation- Embrapa, have analyzed the impacts of sewage sludge applications on soil organic matter in a long-term experiment, conducted by Professor Wanderley Melo, from the State University of Sao Paulo-UNESP, using chemical and spectroscopic approaches.
Specifically, they evaluated changes in total soil organic carbon and in the chemical characteristics of the soil organic matter and its main constituents known as humic substances (from humus origin). Results were published in the January-February issue of the Soil Science Society of America Journal.
The sewage sludge applications to two soils classes (clay and sandy soils, from tropical areas of Brazil) during seven consecutive years caused an increase in organic content in both soils, but with higher relative increase in sandy soils. This is an important result for tropical soils where it is hard to maintain and/or increase soil organic matter, due to very intense microbial activity, generally stimulated by combination of high temperature and humidity.
Spectroscopic analysis detected chemical modifications in soil organic matter and humic acids, likely due to incorporation of less transformed organic compounds from sewage sludge to the indigenous organic matter. Instead of becoming an organic material that could easily convert to carbon dioxide (CO2) and augment greenhouse gas emission, the sewage sludge incorporated as humic substances, a more recalcitrant class of soil chemical compound with a longer lifetime in soils.
These findings support the humic substance model of relatively small molecules held together by weak forces, such as hydrogen and hydrophobic bonds, with a pseudo high molecular weight, instead of the traditional macromolecule model. Additional field and laboratory experiments are fundamental to improve the understanding of soil organic matter dynamics and tentative to carbon management in soils, including support to desired soil carbon sequestration conditions.
Funding was provided by the Sao Paulo Research Foundation, the Brazilian National Council for Scientific and Technological Development, and the Optics and Photonics Research Center.