Presentation on theme: "Introduction The export of nutrients (nitrate and phosphate) from intensively-managed agro-ecosystems of the US Midwest has been blamed for degradation."— Presentation transcript:
Introduction The export of nutrients (nitrate and phosphate) from intensively-managed agro-ecosystems of the US Midwest has been blamed for degradation of water quality throughout the Mississippi River Basin and the expansion of the so-called dead zone in the Gulf of Mexico. Wetlands are effective for reducing nutrient export in agricultural landscapes due to their highly active soil and plant biological communities. Constructed wetlands located down slope from croplands can redirect agricultural runoff, retain some of these nutrients, and thus provide a way of reducing nutrient discharge to streams. In the Midwest, nutrient export is generally episodic with the largest storms (10% of the time) producing >50% of the nitrate and >90% of the phosphate load. Several climate models have predicted that the region’s climate will become more variable resulting in more intense storms and more frequent dry/wet periods. A the present, we have limited knowledge of the response of constructed wetlands to these frequent wet/dry cycles. Objectives Investigate the biological, physical, and chemical processes controlling N and P cycling in wetlands constructed in agricultural landscapes. Examine how different hydrologic conditions (dry/wet) influence nutrient release in agricultural wetland soils. Figure 3. Van Wert Co, Ohio Wetland Figure 2. Whitley Co, Indiana Wetland Hypothesis For wetlands constructed on former croplands, antecedent soil conditions will directly affect P retention and release depending on the availability of P adsorbing minerals and soil P levels. During dry periods, a portion of the organic P fractions will undergo mineralization, resulting in greater availability of mineral P (PO 4 -3 ) for export and biological uptake. Background soil properties 1.ST 0-20 cm = Starkey Farm topsoil 2.ST 40-60 cm = Starkey Farm subsoil 3.VW = Van Wet Co., Ohio wetland 4.WC = Whitley Co., Indiana wetland Nutrient Flux Study Laboratory mesocosm study simulating dry/wet periods was conducted. 6 cores (PVC pipe: 50 X 10 cm) were collected from each site. At all sites, surface (0-20 cm) cores were extracted; however, at the Starkey farm a second set of subsurface cores (40-60 cm) were also collected. Half of the cores from each site were allowed to dry for 5 weeks while the remaining half was drained to field capacity and kept at 4° C. Materials and Methods Study Locations Soil cores were collected from several sites across Indiana and Northwest Ohio. Soils in the area developed in Wisconsinan glacial till. Sites included two constructed wetlands and one cropland slated for future wetland construction. Figure 4. Hendricks Co, Indiana Cropland Northeast Indiana Purdue Agricultural Center Whitley Co., IN Wetland constructed in 2007 Tile and surface water fed Van Wert Co., OH Wetland constructed in 2002 Surface water fed Starkey Farm, Hendricks Co., IN Proposed site for future wetland construction Figure 1 – Site location map DryMoist Flood SitepH% Clay% Silt Bulk density (g/cm 3 ) % OM Total P (mg P kg -1 ) 16.7 ± 0.127 ± 125 ± 11.3 ± 0.16.6 ± 0.8237 ± 23 26.7 ± 0.0428 ± 322 ± 01.5 ± 0.044.9 ± 0.5176 ± 23 35.4 ± 0.0136 ± 325 ± 11.7 ± 0.17.6 ± 0.4205 ± 42 46.5 ± 0.0141 ± 427 ± 71.7 ± 0.0410.7 ± 0.7249 ± 14 Figure 5. Background soil properties All cores were flooded with stream water (7 cm overlying water). Water samples were collected periodically and analyzed for NO 3, PO 4, DOC, pH, ORP, and dissolved Fe.
Soil Core Removal Process Digging around to remove core 40 – 60 cm depth sampling Removing vegetation around base of core Results - Nutrient Concentrations Figure 7. Phosphate concentrations during the first 12 days of experiment. Values with no error bars fall below the detection limit of the instrument (DL=0.05 mg L -1 ). Checking depth for subsurface sampling Summary of Results Additional sampling events remain in this project; therefore, the results shown here represent only the beginning portion of this experiment. Overall, these results showed that flooding of the dried soil cores caused greater release of phosphate, nitrate and DOC compared to the moist cores. However, regardless of dry/moist treatments, there was no release of PO 4 -3 from cores collected from existing wetlands. After 2 days of flooding, high concentration of NO 3 - was recorded. Concentration decreased rapid thereafter suggesting active denitrification. The response of DOC release to flooding was delayed compared to that of NO 3 -. As expected, nutrient release was lower with the sub-surface (ST 40-60) compared to the surface (ST 0-20 topsoil) cores. Unlike the other cores, the dried subsurface cores also exhibited a steady (although small) release of nutrients. Acknowledgements The authors would like to thank the Center for Earth and Environmental Science (CEES) at Indiana University~Purdue University at Indianapolis for financial support. Specific appreciation is owed to Vince Hernly, Katelin Fisher, Anchal Bangar, and Andrea Shilling for their assistance in the field, Bob Hall for technical support, and to Adam Ahmadi for laboratory assistance. Figure 9. DOC concentrations during the first 12 days of flooding Results - Dissolved organic carbon (DOC) References Aldous, A., P. McCormick, et al. (2005). "Hydrologic regime controls soil phosphorus fluxes in restoration and undisturbed wetlands." Restoration Ecology 13(2): 341-347 Corstanje, R. and K. R. Reddy (2004). "Response of biogeochemical indicators to a drawdown and subsequent reflood." Journal of Environmental Quality 33(6): 2357-2366. Figure 8. Nitrate concentrations during the first 12 days of experiment. Values with no error bars fall below the detection limit of the instrument (DL=0.5 mg L -1 ). Inserting core to a depth of 20 cm