Potential geographic distribution of atmospheric nitrogen deposition from intensive livestock production in North Carolina, USA

Abstract

To examine the consequences of increased spatial aggregation of livestock production facilities, we estimated the annual production of nitrogen in livestock waste in North Carolina, USA, and analyzed the potential distribution of atmospheric nitrogen deposition from confined animal feeding operations (“CAFO”) lagoons. North Carolina is a national center for industrial livestock production. Livestock is increasingly being raised in CAFOs, where waste is frequently held, essentially untreated, in open-air lagoons. Reduced nitrogen in lagoons is volatilized as ammonia (NH₃), transported atmospherically, and deposited to other ecosystems. The Albemarle-Pamlico Sound, NC, is representative of nitrogen-sensitive coastal waters, and is a major component of the second largest estuarine complex in the U.S. We used GIS to model the area of water in the Sound within deposition range of CAFOs. We also evaluated the number of lagoons within deposition range of each 1 km² grid cell of the state. We considered multiple scenarios of atmospheric transport by varying distance and directionality.

Modeled nitrogen deposition rates were particularly elevated for the Coastal Plain. This pattern matches empirical data, suggesting that observed regional patterns of reduced nitrogen deposition can be largely explained by two factors: limited atmospheric transport distance, and spatial aggregation of CAFOs. Under our medium-distance scenario, a small portion (roughly 22%) of livestock production facilities contributes disproportionately to atmospheric deposition of nitrogen to the Albemarle-Pamlico Sound. Furthermore, we estimated that between 14–37% of the state receives 50% of the state's atmospheric nitrogen deposition from CAFO lagoons. The estimated total emission from livestock is 134,000 t NH₃ yr^− 1, 73% of which originates from the Coastal Plain. Stronger waste management and emission standards for CAFOs, particularly those on the Coastal Plain nearest to sensitive water bodies, may help mitigate negative impacts on aquatic ecosystems.

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.