Does Quantity and Quality of Tile Drainage Water Impact In-stream Eutrophication Potential? Evidence from a Long-term Biofuel Cropping Systems Experiment
The challenge with farming is balancing crop nutrient needs while minimizing losses. Fertilizer nitrogen (N) and phosphorus (P) are added to agricultural fields to increase yields, but these nutrients in excess can lead to eutrophication. Tile drainage is used extensively across the U.S. Midwest, and these tile drainage lines can directly transport nutrients to streams. The quantity (i.e. load) and quality (i.e. chemical composition of nutrients and carbon) of the tile drainage water likely plays a crucial role in stream water quality, but most tile drainage studies focus only on the quantity of total dissolved and inorganic forms of N and P. This may not provide the whole story of how tile drainage water can affect stream water quality.
This project will measure chemical composition and eutrophication potential of subsurface drainage (tile-flow) from a variety of management practices. The goal is to look at how management practices affect the quality of tile drainage water resulting from soil N and P cycling and losses, and how that quality directly relates to in-stream water eutrophication.
Tile drainage water samples will be collected and analyzed from an ongoing, long-term experiment in central Iowa called the Comparison of Biofuel Cropping Systems (COBS). The COBS experiment consists of five treatments: continuous corn, corn-soybean, corn with cover crop, fertilized prairie, and unfertilized prairie.
- Since last quarterly report a sixth measurement from the CBOS site found both prairie systems dramatically decreased nitrate concentrations. Dissolved organic carbon (DOC), again, showed little differences among treatments.
- Physico-chemical characteristics of the stream waters collected in spring 2019 were measured for the incubation study, and most of the N found in the stream waters was present as nitrate-N (NO3-N), followed by organic N. The NO3-N concentrations in the highly impacted stream was 7.3 and 2.2 times greater than the lower- and medium-impacted streams. Most of the organic N was present as dissolved organic N (DON) in all the streams. Dissolved organic carbon (DOC) and DO concentrations showed little differences among the streams.
- Preliminary dissolved oxygen (DO) measurements from the first eutrophication potential incubation indicated unclear results related to the history of human impact on streams (assessed as background nitrate). Tile water from more sustainable practices increased depletion of stream water DO when at 10% volume, but decreases DO depletion at 5% and 25%. This finding is peculiar and warrants further research.
Water quality measurements for an entire growing season were collected. During the growing season, so far (March to July), nitrate concentrations in tile water ranged from 0.04 to 11.48 mg NO3—N / L (highest was in the Corn-Soybean rotation). The Continuous Corn with Cover Crop treatment decreased nitrate in tile water by an average of 19 percent. Prairie treatment decreased nitrate by 86 percent (range 3-99 percent ) compared to the two corn-soybean treatments. There is relatively little difference in dissolved carbon among treatments. In the coming month, we will be incubating tile water with stream water and measuring the eutrophication potential.
So far, two weekly samples have been collected from the Comparison of Biofuel Systems (COBS) plot. One week of samples has been analyzed and data processed. In that first week, researchers found nitrate leaching through tile drainage was highest from the corn-soybean treatment and lowest from the unfertilized prairie. Next steps include incubating tile water with stream water and measuring more quality indices on tile water.
The team has selected possible streams (high, medium and low impacted by nutrients), and progress has been made on fine-tuning the methods to use for this study. Sample collection and analyses will begin in spring 2018.