Greenhouse Gases Emissions and Potential Nitrogen Mineralization in Short Grass Steppe Soils Treated with Sewage Sludge.

The application of sewage sludge to grasslands is a solution to the problem of disposing sludge from wastewater treatment, and it also increases the fertility and structure of grassland soils. However, unintended consequences of this kind of management include the addition of toxic metals to the soils, and the potential for the increase in greenhouse gas emissions. Our objective was to evaluate the potential greenhouse gas emissions, and nitrogen mineralization in soils formerly dominated by native short grass steppe, where sewage sludge was applied during 20 years until 10 years ago. We also aimed to detect differences in microbial functional groups, as producers of greenhouse gases and mineral nitrogen. We collected soils from the Aurora reservoir close to Denver, Colorado, from sites treated with sewage sludge that differed in bulk density; and from sites dominated by native short grass steppe vegetation. The soils were incubated for 12 hours in the laboratory at 60 % WFPS, with the addition of glucose, and inhibitors that target fungi and bacteria. The air in the headspace of the incubation bottles was collected and analyzed for N₂O and CO₂ concentrations with a gas chromatograph. We extracted soil inorganic nitrogen before and after the incubations to determine potential nitrogen (N) mineralization. We found significant differences among sites but no differences among microbial functional types in the emissions of greenhouse gases and potential N mineralization. Sewage sludge treatment in high bulk density soils resulted in significantly higher N₂O emissions, whereas the short grass steppe and sewage sludge soils with low bulk density showed higher levels of CO₂ emissions and N mineralization. These results suggest that sewage sludge applications followed by trampling might increase N₂O emissions, whereas sewage sludge applications alone might increase CO₂ emissions, and N mineralization, with consequent leaching of excess inorganic nitrogen.