Microorganisms in sewage sludge added to an extreme alkaline saline soil affect carbon and nitrogen dynamics

There are plans to vegetate soil of the former lake Texcoco and use wastewater sludge to provide nutrients. However, the Texcoco soil is N depleted, so the amount of N available to the vegetation might be limited and the dynamics of C and N affected. We investigated how emissions of CO₂, N₂O and N₂, and dynamics of mineral N were affected when different types of N fertilizer, i.e. NH₄⁺, NO₃^-, or unsterilized or sterilized wastewater sludge, were added to the Texcoco soil. An agricultural soil served as control. Sewage sludge added to an alkaline saline soil (Texcoco soil) induced a decrease in concentrations of NH₄⁺ and NO₃^-. Addition of sewage sludge increased the CO₂ emission rate>two times compared to soil amended with sterilized sludge. The NH₄⁺ concentration was lower when sludge was added to an agricultural soil for the first 28 days and in the Texcoco soil for 56 days compared to soil amended with sterilized sludge. Production of N₂O in the agricultural soil was mainly due to nitrification, even when sludge was added, but denitrification was the main source of N₂O in the Texcoco soil. Microorganisms in the sludge reduced N₂O to N₂, but not the soil microorganisms. It was found that microorganisms added with the sludge accelerated organic material decomposition, increased NH₄⁺ immobilization, and induced immobilization of NO₃^- (in Texcoco soil). These results suggest that wastewater sludge improves soil fertility at Otumba (an agricultural soil) and would favour the vegetation of the Texcoco soil (alkaline saline).