Elsevier

Ecological Engineering

Volume 75, February 2015, Pages 441-448
Ecological Engineering

Evaluating the potential role of denitrifying bioreactors in reducing watershed-scale nitrate loads: A case study comparing three Midwestern (USA) watersheds

https://doi.org/10.1016/j.ecoleng.2014.11.062Get rights and content

Abstract

The transport of nitrate (NO3-N) from agricultural lands to surface waters is a complex and recalcitrant problem. Subsurface drainage systems that are especially prevalent in the corn-growing regions of the Midwestern USA facilitate NO3-N transport. Several conservation practices, including fertilizer and manure management, cover crops, natural and installed wetlands, and wood-chip denitrification bioreactors are options that can mitigate NO3-N losses from agricultural lands. Using simple methods of estimation we examine the cumulative volume of denitrification bioreactors required to treat various amounts of NO3-N in base flow, a proxy for tile drainage, at the watershed scale. The use of load duration curves from three different watersheds shows that NO3-N transport is disproportionately skewed toward larger daily base flows. Approximately 50% of the annual NO3-N is transported in largest 30% of daily base flows. Using previous estimates of NO3-N removal by wood-chip bioreactors, we calculated cumulative bioreactor volumes needed to achieve a range of hydraulic residence times (HRT) given rates of base flow observed in three agricultural watersheds. These analyses suggest that cumulative watershed bioreactor volumes sufficient to achieve an HRT of 0.5 days will reduce at least 20% of the total annual NO3-N loss in one watershed and 30% in the other two watersheds. The area required for wood-chip bioreactors would be at most 0.27% of the watershed area.

Introduction

Excess nitrate (NO3-N) in tile drainage water is commonly observed in Midwestern USA watersheds. The transport of this N through the Mississippi River Basin has contributed to seasonal hypoxia in the Gulf of Mexico (Rabalais et al., 1996, Goolsby et al., 2001, Mitsch et al., 2001). Causes of NO3-N loss include subsurface (tile) drainage, nitrogen additions from fertilizer or manure, and seasonal decoupling in the timing of N mineralization in soil and plant uptake (Jaynes et al., 2001, Dinnes et al., 2002, Gentry et al., 2009). Conservation practices for reducing NO3-N losses in drainage water include management of fertilizer timing and amount, cover crops, drainage water management (also known as controlled drainage), wetlands, saturated buffers (Jaynes and Isenhart, 2014), and denitrification bioreactors (Interim Conservation Practice Standard 747; Schipper et al., 2010). Additionally, in-stream denitrification removes NO3-N and this process may be enhanced by riparian buffers, or two stage ditches.

In both wetlands and denitrification bioreactors, NO3-N removal is dependent upon the hydraulic residence time in systems and the inherent denitrification capacity of the microbial community (Kadlec, 2005, Schipper et al., 2010). The exact kinetics of NO3-N removal in these systems is complex, depending upon the NO3-N concentration, the biomass of the denitrifier community, temperature, and carbon bioavailability (Schipper et al., 2010). At high NO3-N concentrations or loads, the microbial community may be saturated with respect to NO3-N and the kinetics appear to be zero-order. At lower concentrations, NO3-N is limiting and denitrification rates are proportional to NO3-N concentration and the kinetics are first-order (Schipper et al., 2010, Chun et al., 2010).

Previous research in many Midwestern watersheds shows that NO3-N loads are maximal in the March-June period which coincides with the greatest tile drainage and stream flows (Tomer et al., 2003, Tomer et al., 2008, Royer et al., 2006, Schilling et al., 2012). In this paper we use data from three Midwestern agricultural watersheds to evaluate the relationship between NO3-N loads and tile flow and use these relationships to estimate the effectiveness of bioreactors for attenuating stream nitrate loads. Measured data from the watersheds are combined with previously published denitrification rate constants to estimate nitrate removal. Thereby, our knowledge of the kinetics of denitrification in bioreactors is leveraged in a modeling exercise to evaluate the potential role of bioreactors in treating dynamic tile flows as observed at the watershed scale.

Denitrifying bioreactors can be designed to intercept artificial drainage (tile drainage), or to intercept lateral flow of groundwater (Schipper et al., 2010). To determine the total volume of bioreactors needed within a field or a watershed, estimates of subsurface drainage and NO3-N removal are required. A 45% reduction in the total N load entering the Gulf of Mexico was the goal articulated by the Hypoxia Task Force (Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, 2001, Christianson and Helmers, 2011). At the watershed scale, artificial subsurface drainage (tile drainage) flows are rarely known, but base flow can serve as a reasonable proxy (Tomer et al., 2008, Tomer et al., 2010, King et al., 2014). In this study, we used load duration curves to estimate NO3-N loads at various base flows and then calculated the cumulative size of the reactors based on the hydraulic residence time required to achieve 50% N removal at the watershed scale.

Section snippets

Watersheds

The South Fork of the Iowa River (SFIR) lies in north central Iowa in Hamilton and Hardin Counties. It is primarily agricultural (85%) with corn (Zea mays L.) and soybeans (Glycine max L.) accounting for the majority of the production (Tomer et al., 2008). For this analysis, the monitoring data from station SF450 were used, which had a drainage area of 58,054 ha. Tomer et al. (2008) estimated that on average, 30% of the watershed received swine manure applications each year. Details of the

Summary and conclusions

These analyses suggest that NO3-N removal by wood-chip bioreactors can approach 30% of total NO3-N transported at the watershed scale with a modest amount of land being converted into bioreactors. These CEAP watersheds provide robust data sets to test this concept in terms of moderate to large seasonal NO3-N concentrations and significant areas of tile drained lands. We do show that the seasonally skewed distribution of N transport toward greater losses in higher base flows needs to be

Acknowledgements

The authors thank Kevin Cole for his assistance with the hydrologic and water quality data.

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    Current address: USDA, Agricultural Research Service, Grassland Soil and Water Research Laboratory, 808 East Blackland Road, Temple, TX 76502, USA.

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