Principal Investigator: Adam Birr and Jeff Strock
Organization(s): Minnesota Corn Growers Association; University of Minnesota Southwest Research and Outreach Station
Sponsor: MPCA 319 Funds
Award Amount: $183,766
Start Date: 4/26/2010 | End Date: 8/29/2014
FINAL REPORT is available in the Minnesota Water Research Digital Library
Quantify the soil properties, hydrology, and water quality of perennial vegetation on undisturbed soil with no history of row crop production, and measure the changes in the first two years of row crop production.
Background and Introduction
Data quantifying the soil and water characteristics of perennial vegetation on undisturbed soils are very limited in the Midwest. This study will document the soil properties, hydrology, and water quality of perennial vegetation on undisturbed soil, with no history of row crop production, using two different research designs.
Why is this important?
Many Total Maximum Daily Load (TMDL) implementation plans include the establishment of perennial vegetation, or the use of land set aside programs, to remediate pollution in the agricultural landscapes of Minnesota; however, the quantification of the water quality benefits of such programs as the Conservation Reserve Program (CRP) at the field scale is lacking. Best management practices designed to reduce sediment and nutrient export from agricultural watersheds will be more effective with a better understanding of the vegetation, soil, management and hydrologic controls that link spatially variable sediment and nutrient sources and sinks to transport processes at the field scale.
Location of the Study
The Hick’s Farm, located in Redwood County, includes a 160 acres comprised entirely of perennial vegetation that has been part of the family farm since 1857. The area has no history of row crop production, and no subsurface drainage has been installed. Grazing has not occurred since the 1980’s; however, forage is harvested once annually. The perennial vegetation is composed primary of smooth bromegrass with a Kentucky bluegrass understory. Native forbs and grasses are also present; however, the native species make up a small amount of the overall plant diversity.
A hillslope located at the southern edge of the field was subdivided into three discrete drainage areas referred to as “watersheds” for the purposes of this study. Two of the watersheds are composed entirely of perennial vegetation (0.79 and 0.98 acres in size). A third watershed (1.7 acres in size) has approximately 0.67 acres utilized for long-term conventional row crop production that drains onto the perennial vegetation on the hillslope.
The Hick’s Farm is located within the Cottonwood River Watershed, a tributary of the Minnesota River. Soils in the study area are representative of the well-drained loams and clay loams of the region. The undisturbed soil at the site is classified as a Storden loam.
Analysis was completed to further examine how much of the greater Cottonwood River Watershed was composed of lands with similar slope that may serve as potential treatment areas to mitigate non-point source pollution between agricultural areas and floodplains. This analysis provided context for our results in terms of applying findings to a broader area. The majority of the land (88.2%) in the Cottonwood River Watershed has slopes under six percent; approximately nine percent of the land in the Cottonwood River Watershed was similar to the project site (slopes of 6 to 12%). The proper placement of perennial vegetation for water quality benefits within a watershed will be critical for maximizing the return of such programs.
Research Methods and Data Collection
The watersheds were monitored using H-flumes equipped with automated water monitoring stations (see picture below). Monitoring was conducted year-round with flow apportioned discrete samples collected during snowmelt and storm events to characterize nutrient and sediment loads, and bacteria concentrations in addition to the hydrologic characteristics of each event. In addition:
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Water samples were collected automatically whenever runoff occurred. Each of the H-flumes were outfitted with a bubble water level sensor and datalogger to continually monitor flow. Instrument shelters located near the H-flumes contained equipment for measuring water level and collecting samples.
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Water samples were analyzed for: pH, conductivity, temperature, total suspended solids (TSS), total phosphorus, dissolved reactive phosphorus, nitrate-nitrogen, ammonium-nitrogen, total nitrogen, and E. coli.
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Soil samples were collected and analyzed for organic matter, pH, cation exchange capacity (CEC), total nitrogen, total carbon, total phosphorus and textual analysis.
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Soil physical properties, including in-situ infiltration and soil bulk density measurements, were collected in 2012 to quantify the undisturbed soils under perennial vegetation, and again in 2014 to quantify changes after conversion of the perennial vegetation to row crop production.
The two watersheds composed entirely of perennial vegetation will use a paired watershed study design to evaluate differences in hydrology and water quality between perennial vegetation on undisturbed soils and corn-soybean row crop agriculture on the study hillslope. The basic premise of a paired watershed design is that there is a relationship between paired hydrology and water quality data for the two watersheds. The relationship must be consistent over time except for the influence of changes imposed on the treatment watershed. The approach used two watersheds (control and treatment) and two periods of study referred to as calibration and treatment. During the control period (February 2011 through May 2013), each of the watersheds was composed entirely of perennial vegetation on undisturbed soils and treated identically. In late May 2013, the treatment watershed (NVe) was converted to row crop production while the control watershed (NVw) remained as perennial vegetation on undisturbed soils.
The third watershed quantified the effect of perennial vegetation on undisturbed soils located on the hillslope to treat water leaving the portion of the watershed with a long history of row crop production situated at the top of the hillslope (NVm-field). This watershed was evaluated using an above-and-below design consisting of two nested watersheds. Hydrology and water quality differences were assessed for individual events between the monitoring stations capturing the row cropped portion of the watershed and the monitoring station located at the base of the hillslope below the perennial vegetation treatment area (NVm). The site will be used to quantify the water quality benefits from the targeted placement of perennial vegetation in landscape positions that may serve as treatment areas for water leaving row crop agriculture.
Results
Summary results are below.
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Precipitation was extremely variable throughout the study period. Wet springs were followed by dry summers and falls. With this said, many months were above normal including the wettest May on record in 2012.
Paired Watershed Design
Note: The treatment watershed (NVe) was converted to row crop production while the control watershed
(NVw) remained as perennial vegetation on undisturbed soils.
Hydrology
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No run-off was observed from the perennial vegetation on undisturbed soils when the soils were non-frozen. Minimal run-off was observed during snowmelt in 2012 (0.09 inches of run-off per acre) and 2014 (0.23 inches of run-off per acre) from the perennial vegetation on undisturbed soils.
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Following the conversion of perennial vegetation to cropland, four run-off events occurred from the treatment watershed in 2013 resulting in 0.73 inches of run-off per acre.
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Run-off was infrequent and of short duration: the average run-off event on frozen soils lasted 5.4 hours; the average run-off event on non-frozen soils (after NVe converted to cropland, only) was 42 minutes.
Sediment
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Annual sediment losses were minimal from the perennial vegetation on undisturbed soils ranging from 0.39 to 1.08 pounds per acre (run-off only occurred on frozen soils).
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Sediment losses in 2013 and 2014 were 945 and 1.95 pounds per acre after conversion of perennial vegetation to cropland. Most of these losses occurred four run-off events in June 2013.
Nitrogen
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Annual total nitrogen losses were minimal from the perennial vegetation on undisturbed soils ranging from 0.13 to 0.70 pounds per acre (run-off only occurred on frozen soils).
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The largest total nitrogen losses were associated with the four non-frozen soil run-off events in the treatment watershed (NVe) after conversion to cropland, totaling 1.8 pounds per acre.
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Large nitrogen losses through surface run-off were not anticipated as most nitrogen losses occur through leaching or through artificial drainage (i.e. tile).
Phosphorus
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The watersheds managed in perennial vegetation did have elevated total phosphorus concentrations, ranging from 0.66 to 1.80 mg/L; however, the annual total phosphorus export loads were low when combined with run-off volumes, ranging from 0.02 to 0.09 pounds per acre.
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The watersheds composed of perennial vegetation had a higher fraction of the total phosphorus in the dissolved (61%) form compared to the treatment watershed (NVe) after conversion to cropland (20%).
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The events with the largest total phosphorus export loads occurred in the treatment watershed (NVe) in 2013 after conversion to cropland.
Soil Bulk Density
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Following the conversion to agricultural row crop production, there was a statistical significant (p-value= 0.05) increase in soil bulk density in the treatment watershed (NVe) at the 0-10 cm, 40-60 cm, 60-80 cm, 80-100 cm, and 100-120 cm soil profile depths.
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Soil bulk density in the lower 40-100 cm depth was similar for the recently converted cropland and an adjoining field that has been in row crop production for many decades.
Infiltration
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Following the conversion to agricultural row crop production, infiltration rates were reduced between 39 and 80 percent, respectively, and hydraulic conductivity rates were reduced between 41 and 67 percent, respectively, at 4 different soil moisture levels.
Above and Below Watershed Design
Hydrology
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The watershed with perennial vegetation captured more snow in the winter than the adjacent cropland and generally had a snowmelt of longer duration that had a higher run-off value for frozen soil conditions.
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There was a 72% reduction in mean event runoff volume from the below watershed compared to the above watershed.
Sediment
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Runoff that occurred from the above watershed in June accounted for 35% of the overall runoff, and 89% of the sediment yield.
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Runoff that occurred from the below watershed on frozen soil accounted for 95% of the overall runoff, and 81% of the sediment yield.
Phosphorus
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The mean runoff event in the below watershed had 77% less total phosphorus and 76% less dissolved ortho-phosphorus yield compared to the above watershed (p-value= 0.05).
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In the below watershed, events on frozen soil were responsible for 95% of the runoff and 82% of the total phosphorus yield.
Nitrogen
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The mean runoff event yield in the below watershed had 75% less total nitrogen and 87% less nitrate-nitrogen yield compared to the above watershed (p-value= 0.05).
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There was a significant reduction (7%) for the nitrate-nitrogen runoff event flow weighted mean concentration from the below watershed compared to the above watershed (p-value= 0.05).
