Location: Agroecosystems Management Research2011 Annual Report
1a. Objectives (from AD-416)
This project describes Watershed Assessment Studies (WAS) to be conducted in two Iowa watersheds that are benchmark watersheds of ARS’s Conservation Effects Assessment Project (CEAP). This project consists of three objectives that are to: 1) Develop and implement a data system to organize, document, manipulate, and compile water, soil, management, and socio-economic data for assessment of conservation practices at field, farm, and watershed scales for the South Fork of the Iowa River and Walnut Creek, Story County watersheds. 2) Measure and quantify water quality, water quantity, and soil quality effects of conservation practices at the field, farm, and watershed scale for the South Fork of the Iowa River and Walnut Creek (Story County) watersheds. Two sub-objectives are: a) Quantify extent and placement of conservation practices in the South Fork watershed and impacts of those practices on water and soil quality. b) Relate contaminant sources to transport paths and processes for pathogens, antibiotics and nutrients using hydrologic and land use data with isotope- and DNA-based methods. 3)Assess and evaluate watershed and river basin responses to current and improved management practices for water quality by comparing observed to model-predicted results for the South Fork of the Iowa River and Walnut Creek (Story County) watersheds.
1b. Approach (from AD-416)
The work will take place in the Iowa River’s South Fork watershed (78,000 ha), and in Walnut Creek watershed, Story County (5,200 ha). Both watersheds are within the area of most recent glaciation in Iowa (about 10,000 years B.P.), known as the Des Moines lobe. Walnut Creek has a water quality database dating to 1991, and a history of watershed modeling and nutrient-management research. The South Fork watershed also has challenges associated with intensive livestock production. Its water quality database dates back to 2001, and information on conservation practices have been gathered and targeting methods explored. This research will leverage these assets towards attaining CEAP goals through database development, watershed assessments and modeling studies. Watershed assessment studies for the South Fork will include combined geographic analyses of soil survey, topographic, crop cover, and conservation-practices inventory data to improve our ability to assess the targeting of conservation practices towards sensitive lands. Combined hydrologic and water quality data will be used to evaluate effects of practices on runoff water quality and better understand how different pathways of water movement impact water quality as measured at the watershed scale. Source tracking methods for fecal-contaminant indicator bacteria will be developed and tested. Finally watershed models will be evaluated to improve our ability to predict the impact of changes in conservation systems that are reasonable future scenarios. Thereby, the project will develop information that can increase the effectiveness of USDA’s conservation programs in tile-drained watersheds.
3. Progress Report
A field study was begun to measure transport of tylosin and tylosin-resistant bacteria in drainage water after swine manure application. Elevated levels of the fecal bacterium, Enterococcus, and tylosin-resistant Enterococcus were observed after manure application and these populations persisted to the following spring. Tylosin was detected at trace levels (<1 ppb) in tile drainage water. Methods were developed to measure erm genes, which confer tylosin-resistance, in soil and drainage water by qPCR. An eleven-year study to compare loads of phosphorus and bacteria in field-edge runoff from a manured and a non-manured field was concluded. Preliminary results show the ephemeral nature of runoff requires well-maintained equipment and vigilant efforts to monitor at the field edge. Stream monitoring efforts were continued in three Iowa watersheds to document long-term trends in hydrology and water quality in Iowa watersheds. New Iowa watershed data were included in the Sustaining the Earth's Watersheds–Agricultural Research Data System (STEWARDS) database, and efforts to expand the data included from other Conservation Effects Assessment Project (CEAP) watersheds were continued. The Root Zone Water Quality Model (RZWQM) was tested with scenarios developed to simulate winter cover crops across the U.S. Midwest. Preliminary results indicate that winter cover crops potentially could reduce nitrate losses from drained fields by at least 89 million kg yr-1, or about 11% of the total nitrate load in the Mississippi River. We completed field mapping of high-carbonate soils around closed depressions (potholes) commonly found in glacial landscapes. Carbonate accumulations indicate where water has moved out and up from the depressions. Understanding water and chemical flow pathways in these depressions are needed to understand water quality and soil productivity in glacial landscapes of the Midwest. Finally, a review of the Conservation Effects Assessment Project (CEAP) watershed studies was written, co-authored with an ARS scientist in Oxford, Mississippi. Substantial progress has been made in assessing new management practices for reducing nitrate contamination of surface waters due to farming activities. All of this research was instrumental in the planning of a new project plan for the Ames location's contribution to NP211.
1. Trace contaminants in woodchip bioreactors evaluated. Woodchip bioreactors are a promising new technology to remove nitrate from the Mississippi River Basin and reduce hypoxia in the Gulf of Mexico. The woodchips support populations of bacteria which convert nitrate to nitrogen gas, a process called denitrification. Agricultural Research Service (ARS) and Iowa State University scientists, in Ames, Iowa, examined the potential of these bioreactors to remove agricultural chemicals from field drainage water. Two antibiotics and a herbicide were rapidly removed from water in a laboratory-scale woodchip bioreactor. The mechanism of removal appears to be binding to the woodchips. Two antibiotics, enrofloxacin and sulfamethazine, temporarily suppressed denitrification activity and reduced populations of denitrifying bacteria. The herbicide atrazine did not affect denitrification. Woodchip bioreactors, which are effective in nitrate removal, will also remove pesticides and veterinary antibiotics from drainage water. This information is of use to farmers and state and federal action agencies in setting priorities for the expenditure of conservation monies to improve surface waters affected by excess nitrate.
2. Examining the role of wetlands in watershed water quality management. Wetlands can reduce watershed nitrate loads and provide other ecosystem services, but their placement and contributions to nutrient reduction will need to be determined on a watershed specific basis. Agricultural Research Service (ARS) scientists in Ames, Iowa, and Oxford, Mississippi, collaborated with researchers at Iowa State University and The Wetlands Initiative to build detailed topographic data for a 16,000-acre watershed in Illinois through a laser altimetry (LiDAR) survey. Applying conservative selection criteria, they identified 11 sites that could be converted to wetlands with minimal loss of productive cropland. These wetlands could intercept and treat tile drainage from 30% of the watershed. A modeling analysis showed that these wetlands could reduce nitrate loads from the watershed by as much as 16%. However, load reductions among the wetland locations varied considerably, depending on watershed-to-wetland area ratios and nitrate loads generated above each wetland. Land use patterns affect both the watershed-wetland ratio and nitrate loads. This research provides a framework that could be used by policy makers interested in developing incentive structures that encourage wetlands, including the establishment of nutrient trading schemes.
3. Conservation practices and water quality evaluated. United States Department of Agriculture (USDA)-Agricultural Research Service's (ARS's) Conservation Effects Assessment Project (CEAP) conducted watershed research at 14 ARS locations, including Ames, Iowa. Watershed modeling, field studies to assess practices, and evaluation of practice placement in watersheds were used to identify water quality impacts from conservation practices in large agricultural watersheds. Field studies show conservation practices improve water quality, but water quality problems have persisted in larger watersheds. This apparent dissociation between practice-focused assessment and water quality status occurred because: 1) Conservation practices were not targeted according to critical sources and pathways of contaminants; 2) Sediment in streams often originated more from channel and bank erosion than from erosion of soil in fields; 3) Timing lags, historical legacies, and shifting climate combined to mask effects of practice implementation; and 4) Water quality management strategies that address single contaminants do not consider inherent trade-offs among multiple contaminants. These lessons can be leveraged to improve strategies for implementing conservation programs and to set water quality goals with realistic timelines.
4. Water table control, nitrogen (N) rate, and weather affect nitrate loss to subsurface drainage. Control of subsurface drainage can reduce nitrate loss to drain tile flow, but the effects may vary with different N application rates and weather conditions. Agricultural Research Service (ARS) scientists in Ames, Iowa, and Fort Collins, Colorado, used the Root Zone Water Quality Model (RZWQM2) to investigate long-term effects of controlled drainage. Changing from free to controlled drainage reduced measured annual N loss in tile flow by 12%. Long-term RZWQM2 simulations suggest that N loss might be reduced by 39% through controlled drainage and decreased N rates, with minimal decreases in corn yield. This research will help agricultural scientists better understand effects of controlled drainage towards (and its potential role in) reducing N losses from tile drainage.
5. Determination of economically optimal nitrogen (N) rates. Managing N fertilizer is one of the many challenges facing farmers today. Establishing the economically optimal N rate (EONR) typically involves fitting an equation to yield versus N rate data and finding the point on the fitted curve where the profit from an incremental increase in yield just pays for the incremental increase in the cost of the added N fertilizer. While different types of equations have been fitted to yield-N rate data, a measure of the statistical reliability in the computed EONR due is almost always lacking. Agricultural Research Servie (ARS) scientists in Ames, Iowa, demonstrated a new statistical method to compute EONR and its relative error applicable to most yield response equations. The approach was demonstrated for a range of published yield response data. It is easy to apply and should be useful for all researchers examining various farming practices and EONR.
6. Economically optimal nitrogen (N) rates for corn management zones delineated from soil and terrain attributes. Agricultural Research Service (ARS) scientists in Ames, Iowa, compared economically optimal N fertilizer rates for corn in a corn/soybean rotation among yield zones that were based on soil and landscape characteristics. The yield zones were generally demarked by slope position. Maximum yield was generally lower on the upper slope positions, but was a poor predictor of optimal N rate, which varied from 20 to 220 pounds per acre. Six years of central Iowa data indicate N rates could be reduced by applying relatively less N fertilizer to the lower slope positions and relatively more to upper slope positions. Dividing fields into yield zones may be a viable method for improving N fertilizer rates to corn and potentially reducing nitrate losses to surface and ground waters. These results help researchers design and interpret N fertilizer use efficiency experiments, resulting in improved recommendations for farmers.
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