Decreasing nitrate-N loads to coastal ecosystems with innovative drainage management strategies in agricultural landscapes: An experimental approach
Highlights
► Two agricultural drainage systems are tested for nitrate-N reductions. ► Mean percent nitrate-N load reductions were similar. ► Hydraulic residence time is crucial for enhancing nitrate-N reduction. ► Significant relationship between nitrate-N concentration and time and drainage treatment.
Introduction
Global population growth will necessitate agricultural expansion within the next 50 years, so much so that food and fiber demands will play a significant role in global environmental change (Tilman et al., 2002). Agricultural production is ubiquitous in its use of inorganic fertilizers to increase yields, which results often in high loads of nutrients delivered from agricultural soils to adjacent receiving waters (Donner, 2003). Environmental issues surrounding nutrient contamination, specifically nitrate-N (NO3−-N) are primarily linked to its impact on surface water eutrophication. However, causes of coastal ecosystem degradation and eutrophication are rooted in nutrient loads derived from non-point sources (e.g., agricultural) high in the associated catchments and watersheds. The relevance of this issue is no more prevalent than in coastal areas of Mississippi and Alabama in the Gulf of Mexico.
Nitrogen is probably the most complex element to characterize in aquatic biogeochemical processes (Keeney, 1973). Nitrogen as a non-point source pollutant from crop fields, typically applied as urea, occurs predominantly in the inorganic form (NH4+). The dominant aqueous N species of NH4+, NO3−-N, nitrite (NO2−), and dissolved organic N, undergo simultaneous complex interactions and transformations of mineralization, immobilization, nitrification, denitrification, and assimilation at variable spatial and temporal scales (Braskerud, 2002, Klopatek, 1978, Ryden et al., 1984). Net N concentrations in aquatic systems are a combination of these processes as well as the rate of decomposition and the rate of sedimentation (Keeney, 1973).
Biogeochemical properties of N can be manipulated through management. By managing primary aquatic systems associated with agricultural N sources, scientists and managers can greatly increase reduction effectiveness. Studies have shown managed drainage ditch systems in agricultural landscapes will result in decreased loads of nitrogen (Cooper et al., 2002, Cooper et al., 2004, Kröger et al., 2007) entering adjacent aquatic systems. Agricultural drainage ditches are integral components and ubiquitous features of the agricultural landscape and act as major conduits of surface and subsurface flow N from agricultural lands to receiving waters. Drainage ditches are wetlands that are the forgotten links between agricultural fields and receiving waters (Moore et al., 2001). They possess hydric soils, support a diverse community of hydrophytes, and are subject to the unpredictable changes in soil saturation as a result of hydrological variability. Controlled drainage practices such as flashboard risers (Evans et al., 1992, Evans et al., 1995, Gilliam and Skaggs, 1986, Gilliam et al., 1979), controlled sub-irrigation (Bonaiti and Borin, 2010, Borin et al., 2001) and low-grade weirs (Kröger et al., 2008, Kröger et al., 2011) within ditches have been proposed as best management practices primarily aimed at reducing nutrient concentrations and loads in ditches reaching receiving waters by reducing total outflows. A commonly used practice for controlled drainage involves the use of a variable height riser in the drain or ditch outlet (Lalonde et al., 1996, Madramootoo et al., 1993, Skaggs and Gilliam, 1981). This concept relies on the ability to control drainage intensity by determining the height of the riser and thus, control volume of outflow and load of solutes (Wesström et al., 2001). Kröger et al. (2011) documented that there were no statistically significant differences in N and P concentrations or loads when risers were compared to low-grade weirs. Nevertheless, both technologies increased hydraulic residence time and both significantly declined influent nutrient concentrations and loads (67–98% N and P).
Taking into consideration that certain surface drainage ditches are hundreds of meters long with variable slopes, the installation of low-grade weirs (henceforth referred to as weirs) within the drainage ditch at multiple spatial locations within the agricultural landscape would create continuous stepwise increase of water levels that improve retention and control drainage. This innovative concept provides drainage management on an annual and spatially gradated basis, rather than a single slotted riser occurring during the dormant season. Spatial allocation of weirs has some significant theoretical improvements over conventional drainage ditch systems. The very important service of first flush capture of non-point source contaminants would occur at multiple locations and entry points along the drainage ditch, rather than just at the outflow. Multiple weirs will increase chemical residence time within each drainage ditch, and provide multiple sites for magnified microbial transformations, nutrient adsorption, and improved sedimentation. Spatially orientated weirs show promise to significantly improve N reductions by expanding and creating synergistic aerobic and anaerobic soil conditions through decreased flow rates and increased water levels and volumes.
The current study, using an experimental outdoors setup, aimed to investigate changes in NO3−-N load and concentration as a result of controlled drainage practices (i.e. use of weirs) in artificially created drainage ditches. NO3−-N reductions were tested by using replicated ditch systems (1.8 m (width) × 58.7 m (length) × 0.3 m (diameter)) that compared weired versus traditional drainage systems (i.e., without weirs) at Arkansas State University (ASU) agricultural research facility in Jonesboro, Arkansas.
Section snippets
Experimental setup
The ASU agricultural research facility (35°50′32.92″N, 90°42′15.87″W) is located approximately 5 km west of the Arkansas State University campus in Jonesboro, AR. This site contains eight drainage ditches that were constructed in 2006. Each ditch was hydro seeded in 2007 and 2008 and now consists of well established vegetative communities (Typha latifolia L.). Ditches have a mean width and length of 1.8 m and 58.7 m, respectively, with 0.1% slopes along their length.
The ASU facility was used to
Results
Particle size analysis of sediments highlighted that ditch sediments consistently consisted of 97 ± 1% silt, with no statistical differences between ditch systems. HOBO® water level data from the detention basin showed a slight increase in water level throughout the duration of the runoff event (±6 cm). A linear correlation showed no significant statistical change between retention pond water depth and inflow rates through time for the duration of the runoff event (Fig. 3). Mean dissolved oxygen
Discussion
The observed differences in DO between the retention pond and ditches may have resulted from differences in measured DO values and inflow values resulting from pond stratification. The pipes used to fill the ditch were slightly lower in the water column than the datasonde, indicating that DO may have been lower. Differences in DO between treatments could be due to increased turbulence in the conventionally drained systems due to higher velocity, or due to higher oxygen depletion in the system
Conclusion
Agricultural water management is moving towards increasing environmental stewardship while improving nutrient reductions within captured runoff. Controlled drainage with low-grade weirs is a strategy for surface drainage ditches that will increase hydraulic retention capacity, potentially improve conditions for biogeochemical transformations and thus enhance NO3−-N concentration and load reductions over conventionally drained systems. Future research aims to provide field scaled nutrient
Acknowledgements
The authors would like to thank the Agricultural Research facility at Arkansas State University. Funding for this project was gratefully provided by the Mississippi Alabama Sea Grant Consortium Award #NA10OAR4170078. The authors would also like to acknowledge Mississippi Agricultural Forestry Experiment Station and the Forest Wildlife Research Center for support. For more information on this project please visit: www.fwrc.msstate.edu/water.
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