Effects of wastewater discharge on microbial populations and enzyme activities in mangrove soils

Abstract

Soil leaching experiments were conducted to assess the impact of discharging wastewater, of two salinities, on bacterial population sizes and microbial activities in mangrove soils. Synthetic wastewater, prepared in either deionized water or half-strength seawater, was applied to trays containing mangrove soils three times a week for a total of 12 weeks. Results show that the mangrove soils were capable of removing pollutants such as Cu, Zn, Cd, P and N from the wastewater. The total counts of both aerobic and anaerobic heterotrophic bacteria, nitrifiers and denitrifiers in mangrove soils receiving wastewater were significantly higher than those found in the control soils. The adenosine triphosphate (ATP) content and the activity of dehydrogenase and alkaline phosphatase in soils were also not affected by wastewater treatment. This indicates that the organic matter and nutrients present in wastewater did cause a large increase in bacterial growth in soils. The retention of heavy metals did not show any harmful effect in the first 10 weeks of this study as any inhibitory effect due to heavy metals might have been overshadowed by the stimulatory effects of the nutrients. The mangrove soils receiving wastewater prepared in deionized water (fresh water) generally had higher population sizes of various bacterial functional groups and supported more microbial activities than that treated with saline wastewater. These results suggest that the wastewater inputs (in particular, organic matter and nutrient) and salinity would have more significant effects on the microbial activities in mangrove soils than heavy metals.