Nitrate-nitrogen patterns in the Raccoon River Basin related to agricultural practices

Abstract

Nitrate-N concentrations in the Raccoon River have increased beginning in the early 1970s. Since this river is the predominant water supply for the City of Des Moines in Iowa, there is concern about the potential long-term impacts of these trends. Improvements in water quality from agricultural watersheds are critical to protect the water supply, and understanding the factors affecting water quality will lead to potential changes in agricultural management to improve water quality. The historical database of nitrate-nitrogen (NO 3 -N) concentrations sampled at the Des Moines Water Works were combined with observations on N fertilizer use, animal production, crop yields, land-use changes, and precipitation patterns to evaluate these interrelationships. Mean annual NO 3 -N concentrations in the Raccoon River watershed have been increasing since 1970 in spite of no significant change in N fertilizer use for the past 15 years. There have been three years with maximum NO 3 -N concentrations above 18 mg L -1 . However, these spikes occurred throughout the past 30 years and are not isolated to the last 10 years of record. Nitrate-N loads from the Raccoon River watershed have shown a slight decrease in the past ten years because of the increased crop yields and increased removal of N in the corn (Zea mays L.) and soybean (Gylcine max [L.] Merr.) grains. Production numbers for cattle have decreased by 63% since the early 1980s, while hogs have shown a 20% decrease over the same time period. Therefore, N available for application into the basin has decreased by 25%. Annual variations in NO 3 -N loads are significantly related to precipitation in the first five months of the year. A significant correlation was found between the land area within the watershed cropped to small grains and hay crops and the increase of NO 3 -N since 1970 (r = -0.76). This relationship was caused by alteration in the seasonal water-use patterns and loss of N during the fall or early spring in the water movement in contrast to corn or soybean, which have a limited N uptake pattern concentrated between June and early September. Changes in the water-use patterns caused by shifts in cropping patterns provide an explanation for the positive correlation between precipitation and flow during the early part of the year. Development of agricultural management practices that can potentially affect water quality will have to be more inclusive of all components in agricultural systems, rather than only changing fertilizer rate or timing.

Phosphorus source—sink relationships of stream sediments in the Rathbun Lake watershed in southern Iowa, USA

“…Nitrate-N concentrations greater than 6.0 mg L −1 are common in surface waters in Iowa, although many watersheds have significantly more row crop production than present in the Rathbun Lake watershed (Jaynes et al 1999;Tomer et al 2008;Hatfield et al 2009 , the streams in the watershed carry significant amounts of sediment, likely a result of stream bank erosion (Zaimes et al 2008;Tufekcioglu et al 2012). Seasonal sediment and nutrient loads were not calculated as sufficient stream flow data are not available.…”

Navigating the Socio-Bio-Geo-Chemistry and Engineering of Nitrogen Management in Two Illinois Tile-Drained Watersheds

“…Extensive hydrologic modifications, including channelization and subsurface tile drainage, are common in watersheds dominated by intensive corn-soybean production in the midwestern United States (Baker et al, 2008). This is a "leaky" system, particularly for nitrate N, and large nutrient loads are carried downstream even when farmers follow best management practice recommendations (Royer et al, 2006;Baker et al, 2008;Hatfield et al, 2009;David et al, 2010). Policies and plans to address the loss of nutrients from agricultural watersheds have been relatively ineffective.…”

Convergence of agricultural intensification and climate change in the Midwestern United States: implications for soil and water conservation

“…However, the corn-soybean rotation became dominant in Midwestern agriculture during the same time and is also associated with water quality changes. Hatfield et al (2009) found that increased nitrate-N concentrations in the Raccoon River since 1970 were related to shifts in crop production that replaced small grains and hay with corn and soybean crops. Combined, these changes result in increased discharge rates and nitrate-N concentrations that are correlated with one another, particularly where agricultural subsurface drainage is extensive (Schilling and Lutz 2007;Tomer et al 2008).…”