Carbon and nitrogen dynamics in alkaline saline soil of the former Lake Texcoco (Mexico) as affected by application of sewage sludge

Sewage sludge is being applied to alkaline saline soil of the former Lake Texcoco (Mexico) as part of a scheme to vegetate the land and prevent erosion by wind. However, its effect on the microbial functioning of the soil is unknown. We investigated under laboratory conditions the decomposition of sewage sludge and its effect on carbon and nitrogen mineralization and ammonia (NH3) volatilization. Soil was sampled from four locations, one undrained and three drained, for 1, 5 and 8 years. The samples were divided into two groups, one of which was amended with sewage sludge, and were aerobically incubated for 70 days at 22 +/- 1 degrees C. The microbial biomass activity in the unamended soil, as indicated by CO2 production as a percentage of total soil organic C, was inhibited neither by high pH nor by large salt concentrations. Addition of 1.5 g of dry sewage sludge kg(-1) dry soil, equivalent to 200 mg C kg(-1), increased CO2 production significantly in the soil with the least soil C content and lowest pH, which had been drained for 8 years. It did not influence production of CO2 in the soil from the other treatments even when 1000 mg sewage sludge C kg(-1) was added. Volatilization of NH3 increased with soil pH and application of sludge. Concentrations of NH4+ and NO3- decreased in the first 7 days and tended to increase again after 28-42 days. We conclude that the microbial activity of the soil was not affected by high pH and salt concentration but the soil was deficient in N. The organic material added with the sewage sludge was sequestered and thus unavailable for microorganisms.