Effects of long-term waste water irrigation on soil organic matter, soil microbial biomass and its activities in central Mexico

The effect of long-term waste water irrigation (up to 80 years) on soil org anic matter, soil microbial biomass and its activities was studied in two a gricultural soils (Vertisols and Leptosols) irrigated for 25, 65 and 80 yea rs respectively at Irrigation District 03 in the Valley of Mezquital near M exico City. In the Vertisols, where larger amounts of water have been appli ed than in the Leptosols, total organic C (TOC) contents increased 2.5-fold after 80 years of irrigation. In the Leptosols, however, the degradability of the organic matter tended to increase with irrigation time. It appears that soil organic matter accumulation was not due to pollutants nor did mic robial biomass:TOC ratios and qCO(2) values indicate a pollutant effect. In creases in soil microbial biomass C and activities were presumably due to t he larger application of organic matter. However, changes in soil microbial communities occurred, as denitrification capacities increased greatly and adenylate energy charge (AEC) ratios were reduced after long-term irrigatio n. These changes were supposed to be due to the addition of surfactants, es pecially alkylbenzene sulfonates (effect on denitrification capacity) and t he addition of sodium and salts (effect on AEC) through waste water irrigat ion. Heavy metals contained in the sewage do not appear to be affecting soi l processes yet, due to their low availability. Detrimental effects on soil microbial communities can be expected, however, from further increases in pollutant concentrations due to prolonged application of untreated waste wa ter or an increase in mobility due to higher mineralization rates.