Effect of Municipal Wastewater as a Wetland Water Source on Soil Microbial Activity

Authors: FINOCCHIARO, RAYMOND - UNIVERISTY OF MISSOURI; Kremer, Robert

Submitted to: Communications in Soil Science and Plant Analysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2009
Publication Date: 9/10/2010
Citation: Finocchiaro, R.G., Kremer, R.J. 2010. Effect of Municipal Wastewater as a Wetland Water Source on Soil Microbial Activity. Communications in Soil Science and Plant Analysis. 41(16):1974-1985.

Interpretive Summary: Constructed wetlands are important for restoring wildlife habitat and reducing flooding in river floodplains. The Eagle Bluffs Conservation Area (EBCA) was established in the Missouri River floodplain near Columbia, MO, to serve as a wildlife refuge and public access for observing and hunting wildlife. Because it is a seasonal wetland, EBCA is flooded during autumn by irrigation with treated wastewater effluent from the nearby Columbia City Wastewater Treatment Facility. Rather than discharged into the river, wastewater effluent serves as a valuable source of water during the flooded stage of the wetland (hydroperiod). Although treated wastewater effluent poses little or no human health concern, its impact on soil biology and productivity, and on subsequent growth and health of wetland plants has received little attention. Wastewater effluent can contain high salt contents after treatment, which may accumulate in flooded soils and detrimentally affect important biological processes carried out by soil microorganisms during the plant growing season when soils are not saturated. Our objective was to determine the effects on soil microbial activity by wastewater effluent used to flood constructed wetlands at EBCA. We found that soil respiration (carbon dioxide released from soil) decreased in two different soils flooded with wastewater effluent. The reduction in microbial activity detected in soils after drainage was related to high salt content of the wastewater effluent (four times higher than river water), which included a high concentration of sodium, the major substance contributing to high salinity. When the wastewater-treated soil was leached with water, the salt and sodium contents were reduced and soil microbial activity increased. Our study suggests that wastewater effluent used for constructed wetlands should be monitored for high salt content and that remedial practices such as integrating periodic flooding with a non-saline water source may be necessary to avoid detrimental effects on biological processes in wetland soils. This information has important implications for scientists, wildlife conservation personnel, and wetland managers because it helps to focus investigative research aimed at anticipating potential problems with using novel water sources such as wastewater effluent for wetland flooding; and in developing water management systems to avoiding potential salt buildup that can disrupt soil biological processes.

Technical Abstract: Microbial activity, as determined by CO2 evolution, was compared between two soils irrigated with either municipal wastewater effluent or Missouri River water. Irrigation of soils was conducted in greenhouse microcosms with irrigation timing and quantity designed to simulate wetland moist-soil management techniques. The wastewater effluent had twice the electrical conductivity (EC) and 4 times the sodium concentration as Missouri River water. Increased salinity and sodicity may reduce soil microbial activity, biomass, and diversity. We assayed microbial activity (respiration) on soil samples when soil salinity and sodicity were relatively high (EC = 5.3 – 45.6 mS cm-1, exchangeable sodium percentage (ESP) = 19 - 53%), and after reducing salinity and sodicity by leaching soils with de-ionized water (EC= 2.5 - 3.2 mS cm-1, ESP = 10 - 27%). An additional assay was performed approximately 100 d after leaching when plants that germinated from the seed bank in the microcosms were harvested. Gas chromatography was used to measure CO2 evolved in soil samples that were incubated for 24 h, 48 h, 72 h, and 7 d. Evolved CO2 as a measure of microbial activity was significantly reduced in preleached, wastewater effluent-irrigated soils compared with Missouri River water irrigation. However, immediately after leaching, activity significantly increased and was similar to Missouri River water-irrigated soils. Microbial activity in nearly all treatments remained greater than preleached levels despite decreasing after 100 d of post-leaching treatments. Increased activity after leaching appears to be related to decreased soil salinity and sodicity, which may have lowered osmotic pressure in the soil matrix and facilitating suitable conditions for microbially-mediated metabolic processes. This study demonstrated that high soil salinity and sodicity induced by irrigation with wastewater effluent decreased soil microbial activity, which may impact nutrient cycling in constructed wetland ecosystems.