Potential geographic distribution of atmospheric nitrogen deposition from intensive livestock production in North Carolina, USA
Introduction
Agricultural waste is an inevitable byproduct of raising livestock. Historically, farmers have managed this problem by using nutrient-rich livestock wastes as a crop fertilizer. Yet, as small independent farms are increasingly converted to large-scale, industrialized, confined animal feeding operations (“CAFOs”), livestock waste management and disposal becomes more challenging. Here, we address the regional consequences of industrialized livestock production in North Carolina, where the number of animals produced has greatly increased while the number of farms is declining (Furuseth, 1997).
North Carolina is one of the leading states in livestock production. Here, we use the term “livestock” to include swine, cattle, and poultry. The state ranks second in the nation in both swine and turkey production, accounting for approximately one-sixth of the U.S. total (North Carolina Department of Agriculture and Consumer Services [NCDA & CS], 2007). Currently, the retention of liquid waste in large anaerobic storage reservoirs (“lagoons”) is the most widespread and least costly treatment method for livestock waste (Barker, 1996). In North Carolina there are over 2500 CAFO lagoons, 85% of which are located in the Coastal Plain (NC Division of Environment and Natural Resources, Division of Water Quality [NCDENR DWQ], 2002a; also see Fig. 1). Unlike the municipal treatment of human waste, these waste management facilities employ few wastewater treatment processes. Recent studies report that CAFOs in the Coastal Plain of North Carolina are major sources of nitrogen to the region's nutrient-sensitive estuarine and coastal water (Cahoon et al., 1999, Walker et al., 2000, Whitall and Paerl, 2001, Mallin and Cahoon, 2003).
Nitrogen limitation is widespread worldwide, and occurs commonly in terrestrial, freshwater, and marine systems (Elser et al., 2007). Excessive nutrient loading to nitrogen limited waters can create eutrophic conditions, which are linked to noxious phytoplankton blooms and hypoxia (Paerl and Whitall, 1999, Paerl, 2002), changes in fish and benthic macroinvertebrate communities (Burkholder and Glasgow, 1997, Glasgow and Burkholder, 2000, Alderman et al., 2005), and outbreaks of harmful aquatic organisms (e.g. Burkholder et al., 1992).
Atmospheric transport is a major pathway by which nitrogen is delivered to riverine, estuarine and coastal ecosystems (Walker et al., 2000, Whitall et al., 2004). Ammonia (NH3) volatilizes from lagoons and terrestrial systems to which the waste is applied (Barker et al., 2006, Shaffer and Walls, 2005). In the atmosphere, it is transformed to other forms of reduced nitrogen, transported via air movement and atmospheric patterns, and deposited to other ecosystems through both wet and dry deposition. Approximately 80% of NH3 emissions in the U.S. are generated by livestock waste (Battye et al., 1994).
Atmospheric deposition of nitrogen emissions now accounts for up to 40% of new nitrogen inputs to coastal ecosystems (Paerl et al., 2002). In North Carolina, direct deposition of reduced N is likely to have a large impact on the large estuarine complex including the Albemarle and Pamlico Sounds (Fig. 1), collectively called the Albemarle-Pamlico Sound. Together with its subestuaries, the Sound is the second largest estuarine system in the U.S. It is chronically N-limited, and direct deposition to open water can bypass the mesohaline zone of the estuary, where biological uptake removes much of the N from riverine inputs (Paerl et al., 2002).
National Atmospheric Deposition Program (NADP) data indicate that wet deposition of NH4+ has increased in the past decade in southeastern North Carolina, and rates are particularly high in portions of the state's Piedmont and Coastal Plain (NADP, 2006; see Fig. 2). The spatial aggregation of CAFOs in the state's Coastal Plain suggests that nitrogen loading from these operations may be greatest in this region. While CAFO lagoons are considered to be the largest contributors to NH3 emissions (EPA, 2003), the observed NADP rates (Whitall and Paerl, 2001, Walker et al., 2004) have not been directly linked to CAFO nitrogen emissions.
To better understand the sources of nitrogen deposition, we address two questions. First, how much nitrogen do North Carolina livestock produce in manure, and how much is emitted as NH3? Second, what is the potential spatial distribution of atmospheric nitrogen deposition from CAFO lagoons? Although management decisions are often made on a statewide basis, few studies have made such assessments at spatial scales larger than single watersheds (but see Paerl et al., 2002, Mallin and Cahoon, 2003, Walker et al., 2004). For this reason, and because North Carolina makes a significant contribution to national livestock production (NCDA & CS, 2007), we seek to address these two questions at the state level.
Section snippets
Nitrogen production and ammonia emissions from livestock manure
We calculated total annual nitrogen production and ammonia (NH3) emission rates (in units of metric tons (t) NH3 yr− 1) for three categories of livestock: swine (breeders and growers), poultry (broilers, other chickens and turkeys), and cattle (beef and dairy). We estimated total statewide annual nitrogen production for the Mountains, Piedmont, and Coastal Plain physiographic provinces of North Carolina (Fig. 1) following Kellogg et al. (2006). In order to determine nutrient content of manure,
Nitrogen production and ammonia emissions from livestock waste
We calculated total nitrogen production and NH3 emissions for each physiographic region across the state of North Carolina for cattle, swine, and poultry (Table 1). An examination of 2006 county livestock census data for North Carolina shows that there were 860,000 cattle, 9,500,000 swine and 805,701,000 poultry, including broiler chickens, other chickens, and turkeys (NCDA & CS, 2007). This census data highlights the disparity among population sizes of livestock — they are orders of magnitude
Discussion
Industrialized farming and CAFOs have replaced traditional farming practices by increasing livestock populations per farm, which also increases the amount and concentration of animal waste. Our results emphasize (a) the amount of nitrogen produced and emitted by livestock manure in industrialized farms in North Carolina, and (b) the significance of the spatial aggregation of CAFOs in the eastern portion of the state for patterns of atmospheric nitrogen deposition.
Much of the livestock feed
Acknowledgements
The authors would like to gratefully acknowledge Michael Aitken, JoAnn Burkholder, Larry Cahoon, Kim Colson, Evan Hammer, Hans Paerl, Gary Phillips, and Mansoureh Tajik for their helpful comments and guidance. This manuscript resulted from an interdisciplinary, project-based course at the University of North Carolina-Chapel Hill. C. Mitchell was the instructor and J. Costanza, A. Goewert, and S. Marcinko were students. Elizabeth Selig was also a member of the course, and helped with GIS
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