Conservation - Water

How do land management practices impact nitrate leaching?

Good land management practices can minimize nitrate leaching loss and reduce the potential of nitrogen (referred to herein as nitrate) contamination in groundwater. Studies in Iowa have shown that as the nitrogen application rate to corn is increased, the nitrate concentration in the tile lines increases. The nitrate concentrations in drainage water from row-crop systems commonly exceed the drinking water standard of 10 ppm. From a study at Gilmore City, nitrogen application rate to corn in a corn-soybean rotation had to be below 100 lb-N/ac for the average annual nitrate concentrations to be below 10 ppm.

Conservation practices, such as converting cropland to grassland, can help reduce nitrate amounts in groundwater but may not be economical. Research done throughout the Midwest Corn Belt has shown greater nitrate losses from row-crop systems than perennial vegetation systems. From a study in south-central Minnesota, nitrate concentration and the volume of subsurface drainage were reduced under the perennial vegetation systems which did not have nitrogen applied to them. Good timing of manure and fertilizer application can increase the nitrogen uptake by crops and reduce downward movement of nitrate. No-till can reduce surface runoff and nitrogen loss in runoff; on the other hand, it may increase nitrate leaching by increasing infiltration.

For further information:

How nitrogen enters groundwater:
http://extension.missouri.edu/explore/envqual/wq0256.htm

Nitrate leaching into tile system:
http://www.agry.purdue.edu/ext/pubs/AY-04-01.pdf

Agricultural nitrogen management for water quality protection in the Midwest:
http://www.oznet.ksu.edu/waterquality/nitrogen%20pub.pdf

How does crop residue impact water quality?

Residue on the soil surface can serve many functions and can impact water quality. Residue helps to dissipate raindrop energy so there is less detachment of soil and less potential soil loss. Also, residue can help provide greater resistance to water runoff, decreasing surface runoff velocity and sediment transport capacity.

Increasing residue cover often results in decreasing the amount of surface runoff and reduces the amount of soluble nutrients and pollutants (e.g. pesticides, herbicides) moving out of crop fields. Overall, residue cover is expected to decrease field-to-stream transport of sediment and sediment-bound contaminants.

For further information:

Conservation tillage and water quality:
http://www.ces.purdue.edu/extmedia/WQ/WQ-20.html

Soil erosion and water quality:
http://www.extension.iastate.edu/Publications/PM1901E.pdf

How well do buffers work?

Buffers have been found to be most effective in trapping particulate pollutants. Additionally, the export of soluble pollutants decreases when infiltration is maximized. Narrow buffers have also been shown to be effective in reducing the export of particulate pollutants when the integrity of the system is maintained. One of the primary functions of buffers is to slow surface water movement, which reduces the export of particulate pollutants. Narrow strips of dense grass can function in this capacity and provide water quality benefits. Narrow strips could also be used in-field as vegetative barriers to slow pollutant movement and control concentrated flow erosion. To maximize infiltration of runoff, wider buffers or a greater buffer area to source area ratio should be used.

While buffer performance varies depending on its location and climate, research has shown that buffers can have a positive impact on water quality. Buffers reduce concentrations of nitrogen, phosphorus, and sediment in surface water runoff. Also when the buffer's root zone intercepts shallow groundwater, buffers have been shown to reduce nitrate-nitrogen concentrations through plant uptake. The ranges in water quality improvement have been found to vary significantly, but when buffers are designed and maintained properly, they could trap about 50 percent of incoming sediment, somewhat less for sediment-bound nutrients, and much less for dissolved nutrients. Nitrate-removal efficiency in shallow groundwater that interacts with the root zone of the buffer varies, but the mean efficiency is usually greater than 50 percent. However, the percent of groundwater interacting with the root zone of the buffer depends on the geologic and hydrologic conditions of the site and may be limited in cases where subsurface drainage systems short-circuit subsurface flow through the buffer.

In designing a buffer system, the flow contact of surface water or groundwater with the buffer should be maximized, and the integrity of the buffer vegetation should be maintained. While buffers have the potential to provide significant water quality improvement, in-field and agricultural best management practices need to be considered, since buffers best serve as polishers of the water moving through them. Buffer systems need to be well-maintained to function effectively. Maintenance requirements include irrigation, mowing, weed control, and reseeding when necessary.

For further information:

Sediment trapping in vegetative filters:
Flow Pathways and Sediment Trapping in a field-scale Vegetative Filter

Vegetative barriers for erosion control: http://extension.missouri.edu/explore/agguides/agengin/g01653.htm

Vegetated filter strips for improved water quality:
http://www.ces.purdue.edu/extmedia/AY/AY-285.pdf

Vegetation buffer strips in agricultural areas:
http://files.dnr.state.mn.us/publications/waters/buffer_strips.pdf