Ecohydrological Response to Integrating Perennial Vegetation into Midwestern Agricultural Landscapes
What We Are Doing
We will quantify the impacts of different locations and percentages of native perennial vegetation (NPV) within a corn-soybean agricultural system on water, nutrient, carbon and sediment movement using a replicated small watershed experimental design. Our working hypotheses are that small percentages of NPV strategically located within a corn-soybean system will 1) increase total evapotranspiration and soil water storage capacity, thereby reducing surface runoff from watersheds, and 2) increase sediment retention and the storage of carbon, nitogen, and phosphorus in the soil-plant matrix, thereby reducing nutrient and sediment losses. Additionally, we predict that 3) sediment retention and plant transpiration will be maximized along strip edges, due to greater surface area and advective energy, respectively; and 4) total nutrient, carbon, and water uptake by NPV will be greatest at the toe position due to interactions between the rhizosphere and groundwater.
Figure 1. Watersheds with flumes, access tubes, and lysimeters marked.
Each watershed has a distinct surface flow outlet point where an H-Flume was installed in 2005 or early 2006 (two perennial watersheds) to monitor volume and rate of surface runoff (Figure 1). Each of these locations is also instrumented with an automated water sampler (ISCO6712 with integrated pressure transducer) for obtaining flow measurements and discrete water samples based on flow intervals.
Figure 2. Flume station.
Soil moisture is being monitored bi-weekly at 30-cm intervals at three locations in each watershed (summit, side-slope, and toe), using a neutron probe (CPN, Inc; 2005 and 2006) and capacitance probe (PR2; 2007 and 2008) with access tubes placed to an approximate depth of 120 cm (100 cm for PR2). Suction lysimeters were installed at the three watershed positions to a depth of 100 cm to extract water samples when sufficient soil water is available for soil water chemistry analysis. Groundwater monitoring wells were installed at two watershed positions (summit and toe).
Preliminary Water Quality Results from 2008 and 2009
- Total amount of rainfall in 2008 & 2009 are close to the long term average. Runoff has been greatly reduced where 10-20% of the watershed is in perennial vegetation.
Figure 3. Rainfall in 2008 and 2009.
Figure 4. Surface runoff in 2008 and 2009.
- Sediment loss has been reduced by approximately 90% where 10-20% of the watershed is in perennial vegetation (Figure 5).
Figure 5. Sediment export in 2008 and 2009.
- Total phosphorus and total nitrogen loss is substantially reduced where 10-20% of the watershed is in perennial vegetation (Figure 6).
Figure 6. Total phosphorus export and total nitrogen export in 2008.
- Nitrate-nitrogen concentrations are less in areas with perennial vegetation (Figures 7 and 8).
Figure 7. NO3-N concentrations in lysimeters at (a) upslope and (b) toeslope positions. Error bars denote the standard deviation of the replicates. Statistical difference of mean nitrate concentration between treatments (grass filters vs. cropland) was indicated for each monitoring period using two significant levels (**p < 0.05, *p < 0.1).
Figure 8. NO3-N concentrations in shallow groundwater at (a) upslope and (b) toeslope positions. Error bars denote the standard deviation of the replicates. Statistical difference of mean nitrate concentration between treatments (grass filters vs. cropland) was indicated for each monitoring period using two significant levels (**p < 0.05, *p < 0.1).
Preliminary Results for Plant Water Use from 2008
To document differences in water use patterns by major crop (corn) and prairie species (coneflower and big bluestem) and understand the hydrologic functions of prairie strips, we used sapflow techniques to monitor plant transpiration during the 2008 growing season.
- Whole plant water use was greater for corn plants compared to prairie plants (Figure 9).
Figure 9. Weekly whole plant water use for corn and two prairie species (coneflower and big bluestem) the 2008 growing season.
- On a per unit leaf area basis, prairie plants had greater water use compared to corn during the abnormally wet year of 2008 (Figures 10 and 11).
Figure 10. Average accumulated monthly water use for corn, coneflower, and big bluestem.
Figure 11. Accumulated water use over the entire measurement period for corn, coneflower, and big bluestem.
- Higher water use rates by prairie plants during wet periods may help maintain soil water balance within landscapes dominated by annual crops.
- Future research should assess the response of prairie and crop plants to rainfall variability and drought.
For more information, please contact us directly:
- Matt Helmers: ; (515) 294-6717
- Heidi Asbjornsen: ; (207) 522-4261