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Sewage Sludge Builds Organic Matter in Depleted Soils

A sustainable destination for sewage sludge is an important challenge worldwide. Soil application is one option; however the impact of toxic metallic ions, pathogenic organisms and other organic contaminants must be carefully monitored, requiring long-term field experiments to study fate of contaminants.

Sewage sludge can be a source of plant nutrients, such as nitrogen, phosphorus, and soil organic matter. This aspect is more relevant in tropical regions, where organic matter decomposition is accelerated due to higher microbial activity. Soil organic matter plays a particularly crucial role in Brazil, with its predominance of soils with high clay to mineral ratios, which are especially poor with organic matter.

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 evaluate 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 journal is published by the Soil Science Society of America.

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 highs 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 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.

The full article is available for no charge for 30 days following the date of this summary. View the abstract at

Soil Science Society of America Journal,, is a peer-reviewed international journal published six times a year by the Soil Science Society of America. Its contents focus on research relating to physics; chemistry; biology and biochemistry; fertility and plant nutrition; genesis, morphology, and classification; water management and conservation; forest, range, and wildland soils; nutrient management and soil and plant analysis; mineralogy; and wetland soils.

The Soil Science Society of America (SSSA) is a progressive, international scientific society that fosters the transfer of knowledge and practices to sustain global soils. Based in Madison, WI, and founded in 1936, SSSA is the professional home for 6,000+ members dedicated to advancing the field of soil science. It provides information about soils in relation to crop production, environmental quality, ecosystem sustainability, bioremediation, waste management, recycling, and wise land use.

SSSA supports its members by providing quality research-based publications, educational programs, certifications, and science policy initiatives via a Washington, DC, office. For more information, visit

SSSA is the founding sponsor of an approximately 5,000-square foot exhibition, Dig It! The Secrets of Soil, which opened July 19, 2008 at the Smithsonian's National Museum of Natural History in Washington, DC.

Sara Uttech | Newswise Science News
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