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Sewage Sludge Builds Organic Matter in Depleted Soils

A long-term study on the effect of sewage sludge on tropical soil

MADISON, WI, March 10, 2010-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.

Soil Science Society of America Journal is the flagship journal of the SSSA. It publishes basic and applied soil research in soil chemistry, soil physics, soil pedology, and hydrology in agricultural, forest, wetlands, and urban settings. SSSAJ supports a comprehensive venue for interdisciplinary soil scientists, biogeochemists, and agronomists.

The Soil Science Society of America is an international scientific and professional society with its headquarters in Madison, WI. Our members are dedicated to advancing the field of soil science in relation to food production, environmental quality, sustainability, waste management and wise land use. We work at universities, government research facilities and private businesses across the United States and the world.