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

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 (NO₃-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 NO₃-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 NO₃-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 NO₃N 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 NO₃-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.