Assessments to Reduce Nitrogen and Phosphorus Nonpoint Source Pollution of Iowa's Surface Waters

The quality of Iowa’s water resources is an important issue to the state’s citizens and it has received much attention in recent years.  Long ago, nitrogen (N) and phosphorus (P) plant nutrients were identified as primary factors in causing water quality impairments.  Agriculture dominates land use in Iowa with over 90% of its land area currently devoted to crop and livestock production.  It is not surprising then that agriculture is widely considered to be the dominant contributor of N and P nonpoint source (NPS) water pollution within the state.

For compliance with the Clean Water Act, the U.S. EPA has required all states to establish N and P nutrient management strategies to reduce NPS pollution of these plant nutrients in surface waters.  In 2003, the Iowa Department of Natural Resources entered into a cooperative agreement with the USDA/ARS National Soil Tilth Laboratory (NSTL) to generate a report that determines the impacts of various conservation practices on Iowa’s surface water quality.  Dana Dinnes, an NSTL Support Scientist, conducted many reviews of research publications over an 18-month period to write this extensive report that is to serve as a foundation for developing the State of Iowa’s N and P nutrient management strategy.  (To view a copy and/or download this report, please click here.  This file is in .pdf format and requires Adobe Acrobat Reader to view and/or download. The file size is approximately 6.4 mg and the report is 376 pages in length.)

The first section of the report presents background information for the general public that explains the natural and human-induced factors that affect NPS water pollution.  Next are water quality impact assessments of conservation practices.  For each conservation practice the assessments identify mechanisms of nutrient reduction and/or removal, current documented degrees of success, applicable and limiting conditions, and sources of variability in its performance.  A summary presents a compilation of the conservation practices’ estimated long-term water quality impacts, plus recommendations for the state to consider for programs aimed at reducing the problems.  Most notable among the assessed practices are those that function to considerably reduce both N and P losses, which are cover crops, diversified cropping systems, in-field vegetative buffers, livestock exclusion from stream areas, and stream buffers.

Mr. Dinnes concluded that although a number of conservation practices may substantially decrease NPS nutrient loss, a single practice alone in a given area may not be able to reduce nutrient losses to the extent necessary to meet water quality standards.  Therefore, comprehensive conservation management plans may often require the adoption of multiple conservation practices.  For instance, to not overload a field-edge conservation practice beyond its functional capacity it may be necessary to implement in-field practices that will increase water storage and reduce N and P transport.  He also states that to determine what and where to enact changes, one must choose practices that are suitable for the area and have shown the greatest potential for achieving success.

All of Iowa’s citizens will directly benefit from this report’s information and guidance when it is put into action in the state’s nutrient management strategy.  Downstream states will indirectly benefit because they will be receiving less N and P pollution in the surface water they receive from Iowa, making it easier for them to also meet water quality standards.