Nutrient Sources and Transport in the Missouri River Basin, with Emphasis on the Effects of Irrigation and Reservoirs1

Abstract

SPAtially Referenced Regressions On Watershed attributes (SPARROW) models were used to relate instream nutrient loads to sources and factors influencing the transport of nutrients in the Missouri River Basin. Agricultural inputs from fertilizer and manure were the largest nutrient sources throughout a large part of the basin, although atmospheric and urban inputs were important sources in some areas. Sediment mobilized from stream channels was a source of phosphorus in medium and larger streams. Irrigation on agricultural land was estimated to decrease the nitrogen load reaching the Mississippi River by as much as 17%, likely as a result of increased anoxia and denitrification in the soil zone. Approximately 16% of the nitrogen load and 33% of the phosphorus load that would have otherwise reached the Mississippi River was retained in reservoirs and lakes throughout the basin. Nearly half of the total attenuation occurred in the eight largest water bodies. Unlike the other major tributary basins, nearly the entire instream nutrient load leaving the outlet of the Platte and Kansas River subbasins reached the Mississippi River. Most of the larger reservoirs and lakes in the Platte River subbasin are upstream of the major sources, whereas in the Kansas River subbasin, most of the source inputs are in the southeast part of the subbasin where characteristics of the area and proximity to the Missouri River facilitate delivery of nutrients to the Mississippi River.

The interactive effects of excess reactive nitrogen and climate change on aquatic ecosystems and water resources of the United States

“…More than 70 % of N delivered to the Gulf of Mexico is derived from agricultural sources in the Mississippi River Basin, where increased winter and spring precipitation on cultivated fields may enhance the amount of N that runs off or is leached into groundwater and ultimately downriver (Smith et al 1997;Alexander et al 2008;Karl and Melillo 2009;Brown et al 2011). The extensive use of tile drains in this region will increase N removal by reducing residence time of high N waters in soils where plants and microorganisms can assimilate or denitrify reactive N (Dubrovsky et al 2010).…”

“…Organic-rich sediments are capable of storing N as well as C. Streams, lakes and reservoirs have the light penetration, algal and macrophyte primary production, and interaction between water and benthic sediments that promote biologically-driven nutrient uptake, sedimentation and ultimately burial Mulholland et al 2008;Harrison et al 2009;Brown et al 2011). While Tranvik et al (2009) proposed a global value of 600 Tg C, there is great uncertainty in the estimate of how much N is buried as organic matter in lake and reservoir systems.…”

Control of nitrogen and phosphorus transport by reservoirs in agricultural landscapes

“…Efforts to explain the ''missing'' nutrients have emphasized the role of biogeochemical processes including denitrification in streams (e.g., Hedin et al 1998;Mulholland et al 2008) and wetlands (e.g., Hanson et al 1994;Jordan et al 2011;Kadlec and Wallace 2009) as well as P retention and burial in natural lakes (Dillon and Evans 1993). More recently, landscape modeling studies at basin-and global-levels have indicated substantial retention of N and P by reservoirs in aggregate (Brown et al 2011;Harrison et al 2009;Wollheim et al 2008), suggesting these ecosystems could be more influential to downstream nutrient delivery than previously thought.…”

Antecedent flow conditions and nitrate concentrations in the Mississippi River basin

“…Increased denitrification occurs with irrigation because elevated soil moisture conditions increase microbial activity (de Klein and van Logtestijn, 1996;Groves and Bailey, 1997). Which process dominates during a drought is debatable and may depend on soil properties, fertilizer application rates, and climate (Aulakh and Bijay-Singh, 1997;Brown et al, 2011). In the Missouri River basin, a recent modeling effort found that increases in irrigation relate to decreases in total nitrogen export on a regional scale (Brown et al, 2011).…”

“…Which process dominates during a drought is debatable and may depend on soil properties, fertilizer application rates, and climate (Aulakh and Bijay-Singh, 1997;Brown et al, 2011). In the Missouri River basin, a recent modeling effort found that increases in irrigation relate to decreases in total nitrogen export on a regional scale (Brown et al, 2011). Irrigation likely occurs at a higher rate when the weather is drier than average, according to a study in Illinois (Bowman and Collins, 1987), therefore, lower nitrate anomalies in the Missouri River (HERM) following a drought may occur because processes associated with irrigation do not allow for the accumulation of nitrate in soil during drier-than-average climatic conditions.…”