Nitrate-nitrogen patterns in engineered catchments in the upper Mississippi River basin

Abstract

Many extensive subsurface tile drainage networks in the Corn Belt region of the United States are organized into quasi-governmental drainage districts. Approximately 3000 of these engineered watersheds exist in the state of Iowa. Tile discharge is the source of many headwater streams and contributes to loss of nitrate-nitrogen (NO₃-N) from cultivated fields. Downstream water use by municipal suppliers is impaired by stream concentrations > 10 mg/l, and load reductions > 30% may be necessary to mitigate Gulf of Mexico hypoxia. The objectives of this study were to evaluate NO₃-N concentrations and loads discharged from three typical drainage districts in north-central Iowa and explore the relation of drainage district NO₃-N concentrations to the downstream drainage network. NO₃-N concentrations averaged approximately 13 mg/l over a two-year period and exceeded 10 mg/l (the standard for safe drinking water in the U.S.) nearly 90% of the time. NO₃-N yields from the studied drainage districts ranged from 33 to 77 kg/ha per year. NO₃-N concentrations and episodes > 10 mg/l were observed to decrease downstream in a linear manner with log drainage area. A load reduction of 55% would be needed at the tile discharge to meet downstream water quality objectives. In-stream NO₃-N processing was observed immediately downstream of the tile outlet, but would appear to offer little potential for meaningful downstream reductions because the time period for NO₃-N processing was poorly timed with seasonal loading patterns. Study results suggest that focusing on NO₃-N reductions at the drainage district scale using best management practices, such as in-field nitrogen management or edge of field treatment, with constructed wetlands, would achieve significant downstream reductions.

Introduction

Excessive nutrient losses from intensively cropped watersheds of the U.S. Corn Belt have been implicated in stream eutrophication (Dodds and Welch, 2000, Chambers et al., 2008, McDowell et al., 2009) and development of hypoxic conditions in the Gulf of Mexico (Burkart and James, 1999, Goolsby et al., 1999, Turner and Rabalais, 2003, National Research Council, 2008, Alexander et al., 2008, Costello et al., 2009, USEPA, 2008, David et al., 2010). Nitrate-nitrogen (NO₃-N) export from the Corn-Belt is among the highest in the United States (Aulenbach et al., 2007), with as much as 35% of the total nitrogen load delivered to the Gulf from the states of Iowa and Illinois alone (Goolsby et al., 2000). Stream NO₃-N concentrations that exceed the United States Environmental Protection Agency's (USEPA) maximum contaminant level (MCL) of 10 mg/l threaten public water supplies that utilize surface water intakes (Jha et al., 2010, Schilling and Wolter, 2009).

Some of the most intensively cropped lands in the Corn Belt occur in the recently glaciated region of north-central Iowa where annual crops of corn (Zea mays L.) and soybeans (Glysine max L.) dominate agricultural production. Much of the pre-settlement landscape of north-central Iowa was once dominated by poorly drained wetlands and swamps, considered by early settlers to be “unfit for human habitation” (Kanwar et al., 1983). However, by the late 1800s to early 1900s, artificial subsurface drainage systems were installed to drain excess water from the landscape for crop production (Kanwar et al., 1983, Zucker and Brown, 1998). Because installing artificial drainage systems was cost and labor expensive and required cooperation among many different landowners, many states enacted legislation to facilitate organization of drainage districts (McCorvie and Lant, 1993). Drainage districts were financed by drainage taxes applied to landowners in the area according to the benefits expected to be received for the purpose of “improving the lands for agriculture” (McCorvie and Lant, 1993). A drainage district is essentially a highly managed catchment draining to a constructed outlet, usually a maintained ditch or tile line. By the late 1920s over 2.5 million hectares of Iowa land were part of drainage districts; today they number more than 3000 (Fig. 1). Organized drainage districts within seven states (IL, IN, IA, MI, MN, OH and WI) account for more than 20 million hectares of land in the United States (McCorvie and Lant, 1993). The conventional wisdom in agriculture is that proper drainage is necessary to allow earlier field operations, reduce yield variability and maximize crop yields in wet soils (Nolte and Duvick, 2010).

Watersheds containing artificial drainage systems are susceptible to increased losses of many agricultural pollutants. Losses of NO₃-N, phosphorus and pesticides to drainage tiles can account for significant pollutant loading to surface waters (e.g., Baker et al., 1975, Baker and Johnson, 1981, Buhler et al., 1993, Cambardella et al., 1999, Tomer et al., 2003, Kalita et al., 2006, Schilling and Helmers, 2008a, Schilling and Helmers, 2008b). NO₃-N yields from tile-drained areas are typically greater than 20 kg/ha (Jha et al., 2010), and can exceed 40 kg/ha/year (David and Gentry, 2000) to more than 60 kg/ha/year (Kladivko et al., 1991, Jaynes et al., 1999) during wet years.

Artificial drainage discharge often serves as the headwater source for second- and third-order natural stream systems (Smith et al., 2008). Headwater streams are considered to be critical locations for NO₃-N processing in river systems (Alexander et al., 2000, Peterson et al., 2001) through biotic or abiotic processes (Newbold, 1992, Royer et al., 2004, Triska et al., 2007). The effectiveness of headwater streams for processing NO₃-N, however, is not well established. Some studies have shown substantial denitrification occurring in headwater streams (Alexander et al., 2000, David and Gentry, 2000) whereas others report little impact of denitrification on annual NO₃-N export (Hill, 1979, Royer et al., 2004). Royer et al. (2004) noted that in headwater streams that receive tile drainage, denitrification rates can be spatially and temporally variable. Nonetheless, a strong relationship exists between discharge and NO₃-N concentrations in agricultural headwater streams when most of the flow originates as tile drainage. This has been demonstrated in tiled, first order catchments by David et al. (1997) (catchments 25–40 ha); Mitchell et al. (2000) (catchments 3–21 ha); and Royer et al. (2004) (catchments 1300–2500 ha).

The objectives of this study were to (i) evaluate NO₃-N concentrations and loads discharged from three typical drainage districts in north-central Iowa and (ii) establish the relation of drainage district NO₃-N concentrations to the downstream drainage network. Although our work was conducted in north-central Iowa, drainage districts are common in many Midwestern states and results are expected to apply regionally. We hypothesize that drainage district tile discharge exerts dominant control on downstream NO₃-N concentrations.