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 T excoco (Mexico) as part of a scheme to vegetate the land and prevent erosio n by wind. However, its effect on the microbial functioning of the soil is unknown. We investigated under laboratory conditions the decomposition of s ewage sludge and its effect on carbon and nitrogen mineralization and ammon ia (NH3) volatilization. Soil was sampled from four locations, one undraine d and three drained, for 1, 5 and 8 years. The samples were divided into tw o groups, one of which was amended with sewage sludge, and were aerobically incubated for 70 days at 22 +/- 1 degrees C. The microbial biomass activit y 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 co ncentrations. Addition of 1.5 g of dry sewage sludge kg(-1) dry soil, equiv alent to 200 mg C kg(-1), increased CO2 production significantly in the soi l 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 t reatments even when 1000 mg sewage sludge C kg(-1) was added. Volatilizatio n of NH3 increased with soil pH and application of sludge. Concentrations o f 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 n ot 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 t hus unavailable for microorganisms.