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