Location: Agroecosystems Management Research2010 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
This research quantifies effects of farming practices and conservation practices on water quality at the watershed scale and evaluates new practices at the plot and field scale. Field-scale research provided a fourth year of data on nitrate losses and crop yield when water table management is used with sub-surface drainage systems. A final project report for a multi-location Conservation Innovation Grant with the United States Department of Agriculture (USDA) Natural Resources Conservation Service described a model that quantified how nitrate losses would decrease under water table management across the Midwest. In another report, the performance of woodchip-wall denitrification bioreactors installed in 1999 was tracked, showing nitrate passing the bioreactors to tile drains was reduced on average by 49%. Denitrification, nitrous oxide production, and wood consumption rates were also measured in these bioreactors. A new "edge of field" type of bioreactor was installed in the fall 2009 to better determine the kinetics of the denitrification process as a function of nitrate concentration, flow rate, and temperature, and to improve our capacity to model this new practice. Cover crops are another practice that decreases nitrate loss: a long term data set with winter rye and fall oat cover crop treatments was appended with current data, with nitrate loads decreased by 48% under rye and 39% under oat cover crops. There has also been progress towards accurately simulating cover crop effects on water quality using the Root Zone Water Quality Model. Field-scale research on soil hydrology was also conducted to evaluate new methods to measure soil water contents, and new models described variation in water table fluctuation across glaciated landscapes and how soil water can be replenished from the water table. Under watershed-scale research, a multi-location Conservation Effects Assessment Project (CEAP) database, known as Sustaining the Earth's Watersheds - Agricultural Research Data System (STEWARDS) was expanded and updated to include a geographic (map-based) interface for accessing hydrologic and water quality data. Monitoring of discharge and water quality was continued in three Iowa watersheds; the South Fork Iowa River, Walnut Creek, and Walnut/Squaw Creeks. Topographic data obtained from a Light Detection and Ranging (LiDAR) survey was subjected to a series of analyses to map potential sites for nutrient removal wetlands, two-stage ditches, and practices to reduce concentrated flow erosion in watershed in Illinois. This new technology may lead to improved targeting of conservation practices.
1. Denitrifying bioreactors - an approach for reducing nitrate loads in stream waters. Low-cost and simple technologies are needed to reduce export of excess nitrogen in subsurface drainage water to sensitive aquatic ecosystems. Denitrifying bioreactors are an approach where solid carbon substrates are added into the flow path of contaminated water. ARS scientists in Ames, Iowa, summarized design alternatives for bioreactors, their effectiveness, and factors limiting performance. Two common designs have proven successful in field settings, with nitrate removal rates ranging up to 22.0 g N m-3 day-1, depending on the type of design, rate of water flow, and incoming nitrate concentration. Additionally, bioreactors may reduce transport of veterinary antibiotics (sulfamethazine, enrofloxacin) applied to soil in manure, and transport of the herbicide atrazine, commonly applied to corn, without affecting denitrification rates. Denitrifying bioreactors are cost effective and complementary to other practices that can decrease nitrate loads to surface waters. This information will be 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. Potential water quality impact of drainage water management in the Midwest USA. Drainage water management (DWM) is a promising technology for reducing nitrate losses from artificially drained (or “tiled”) fields. While there is an extensive history for the practice in North Carolina, little is known about the efficacy or cost effectiveness of the practice under Midwest conditions where artificial drainage is widely used. ARS scientists in Ames, Iowa, used soil and land cover databases with modeling to estimate that 4.8 million ha of land used to grow corn within the Midwest would be suitable for DWM, potentially reducing nitrate loss by approximately 83,000 metric tons (91,300 tons) per year. Considering the cost of control structures, redesign of new drainage systems, and payments to farmers to adjust the control structures to reduce nitrate losses, the cost per kg of nitrate reduced in drainage water for DWM was estimated at $2.71 ($1.23/lb). This information will be of use to farmers and state and federal action agencies in setting priorities for the expenditure of conservation monies to improve surface water quality.
3. Potential water quality impact of cover crop use in corn-soybean rotations in the Midwest. Cover crops are a promising management practice for reducing nitrate losses from agricultural fields in the Midwest, because they in effect lengthen the growing season with living plants in fields with a corn-soybean rotation. Oat and rye cover crops are potential cover crops for the Midwest because they grow well in cool weather. Oat does not overwinter, whereas rye is extremely winter-hardy and overwinters easily. Over four years an oat cover crop reduced nitrate losses in tile drainage by 39% and the rye cover crop reduced it by 48%. Additionally, cover crops have the potential to increase soil carbon and improve long-term soil productivity. This information will be of use to farmers and state and federal action agencies in setting priorities for the expenditure of conservation monies to improve surface water quality.
4. Improving nitrogen (N) management using simulation models. N fertilizer rates determined using the Late Spring Nitrate Test (LSNT) differ each year because variable spring weather influences the amount of soil N available to the crop. Previous work showed the LSNT was effective in reducing nitrate losses, but adoption of this practice is limited because of the time requirement for soil sampling and fertilizer application. ARS researchers in Ames, Iowa, used the ARS Root Zone Water Quality Model (RZWQM) to simulate how weather impacts nitrogen in the soil profile and subsequent N fertilizer rates. The model simulated lower nitrate leached when the LSNT was used to determine N application rates, compared to fall N fertilizer applications, as observed in a paired watershed study. Early season precipitation and temperature accounted for 90% of the yearly variation in spring N requirements. This research will help us better understand how weather patterns should be considered in managing N. This will help us develop simple tools to optimize N fertilizer recommendations with less reliance on soil sampling and analysis that can delay critical crop-management decisions. This would benefit producers by making it easier for farmers to adopt this effective conservation practice.
5. Persistence and leaching of the veterinary antibiotic sulfamethazine in soil evaluated in laboratory studies. Degradation occurred in two phases and was described using an availability adjusted first-order model. Under anerobic conditions sulfamethazine was more persistent than under aerobic conditions. The formation of soil-bound (non-extractable) residues was the primary mechanism of sulfamethazine removal from soil. Leaching studies showed that sulfamethazine is highly mobile in soil. This information is of interest to the public and policy makers who need to understand how antibiotics applied to soil in manure may affect water quality and aquatic ecosystems.
6. Improving landscape scale descriptions of soil water variability and movement using topographic data. Crop growth often depends on water supplied to the soil from shallow groundwater, through a process commonly called sub-irrigation. However, in glacial landscapes with "pothole" depressions, this water source becomes variable spatially and difficult to predict. Water table depths depend upon landscape position and time of year. The water table is roughly parallel with the soil surface after spring snowmelt, but not during the growing season when the water table gradients are reduced and are influenced by the positioning of subsurface drains. Scientists in Ames, Iowa, found upland positions have the greatest variation in water table depth, whereas lower sites dominated by tile drains have less variation. These descriptions will help scientists estimate groundwater recharge following rainfall, and how much groundwater is taken up to replenish growing crops. This information, obtained using a novel technology to monitor soil water storage and movement, is of interest to scientists who want to describe soil water patterns and better understand crop water use and potential leaching of nutrients and agricultural chemicals at the landscape scale.
7. Groundwater quality during conversion of cropland to a prairie reconstruction. How would water quality change if agricultural land were converted back to native prairie? Opportunities to answer this question rarely occur. ARS scientists in Ames, Iowa, tracked nitrate and phosphorus (P) in groundwater during establishment of tall-grass prairie vegetation. We found nitrate and P contrasted one another. Nitrate decreased within three years, when the prairie first became well established. Thereafter, nitrate was seldom detected in shallow groundwater beneath low-lying drainageways. In groundwater beneath higher parts of the landscape, nitrate took three years to begin to decrease, but did not fully diminish, and averaged 2 ppm nitrate-N after five years. The landscape differences resulted from greater amounts of leachable N in upland subsoils. Phosphorus showed a different story, and did not change with time during the study. The largest P concentrations in groundwater occurred in low-lying positions, where sediments were deposited that resulted from upslope soil erosion under past tillage. These transported topsoils provide a source of P, and shallow groundwater conditions in these areas help make this P more mobile. Phosphorus concentrations in these drainageways indicated risk to surface water quality, especially when shallow groundwater rises to the surface and contributes to runoff. The conclusion that in-field deposition of eroded soil can pose a long-term risk for water quality is of concern to stakeholders with interests in agricultural water quality and ecosystem restoration.
8. Managing soils as a part of urban construction projects. During urban construction, topsoil is removed, subsoil is compacted, and only a thin layer of topsoil is returned to the site. Compaction results in poor growth of lawns and trees, increased runoff of nutrients and pesticides, and increased soil erosion. ARS scientists in Ames, Iowa, showed that compost combined with prairie grasses improved water holding capacity and reduced soil erosion compared with a control lawn site. While runoff was not measurably decreased in this short-term experiment, increased water storage should be of long term benefit for restoring the hydrologic functioning of soils disturbed by urban development. This information is of interest to urban planners who desire to remediate urban soil after construction projects are done.Tomer, M.D., Schilling, K.E. 2009. A Simple Approach to Distinguish Land-use and Climate-change Effects on Watershed Hydrology. Journal of Hydrology. 376(1-2):24-33.