Runoff water quality from manured riparian grasslands with contrasting drainage and simulated grazing pressure

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Abstract

Globally, management of grazed riparian areas is critically important to agricultural sustainability and environmental quality. However, the potential impacts of riparian grazing management on water quality are not well-documented, particularly in the southeastern USA. The objective of this work was to determine sediment and nutrient export under simulated rainfall from poorly drained and well-drained riparian soils where heavy or light grazing pressure by cattle was simulated. Plots were established on stands of existing vegetation to create grazing pressure treatments of (a) light-use (full ground cover, uncompacted), and with stands modified to establish (b) heavy-use (bare ground, compacted) treatments. Vegetation on poorly drained soils consisted of several typical wetland species (e.g., Pontederia cordata L., Juncus coriaceus Mackenzie) in the southeastern USA, whereas mixed tall fescue (Festuca arundinacea Schreb.)–dallisgrass (Paspalum dilatatum Poir.) stands were the dominant vegetation on well-drained soils. Runoff volume was generally greater from heavy-use than from light-use for poorly drained soils and for well-drained soils. Greater runoff volume was also observed from poorly drained soils compared to well-drained soils for both light-use and for heavy-use treatments. Light-use plots were remarkably effective at minimizing export of total suspended solids (TSS) on both soils (<30 kg ha−1). Mean total Kjeldahl P (TKP) export was fourfold greater from heavy-use plots than from light-use plots on both soils. While export of nitrate-nitrogen (NO3-N) was unaffected by grazing pressure and soil drainage, mean ammonium-nitrogen (NH4-N) and total N (TN) export from poorly drained heavy-use plots was greater than fivefold that from well-drained light-use plots. Results indicate that livestock heavy-use areas in the riparian zone may export substantial TSS and nutrients, especially on poorly drained soils. However, when full ground cover is maintained on well-drained soils, TSS and nutrient losses may be limited.

Introduction

Grazing cattle (Bos spp.) and other livestock can be a source of sediment and nutrients, such as nitrogen (N) and phosphorus (P), to surface waters (White et al., 1980, Kurz et al., 2006). These agricultural non-point source contaminants of N and P can contribute to eutrophication of surface waters (Carpenter et al., 1998), whereas increased sediment loads have potential to reduce populations of benthic organisms and fish, as well as overall primary productivity of aquatic ecosystems (Cooper, 1993). However, the impact of grazing management on nutrient and sediment export to surface waters is not well-quantified.

Several studies have reported the effectiveness of grass buffer strips (Bingham et al., 1980, Chaubey et al., 1995, Daniels and Gilliam, 1996, Blanco-Canqui et al., 2006) at controlling non-point sources of pollution from agriculture. Additionally, studies have also reported that grass buffers can be an important component of riparian buffer systems (Osborne and Kovacic, 1993, Sovell et al., 2000, Lowrance and Sheridan, 2005). Given that riparian buffers serve as the final buffer between potential agricultural contaminants and surface waterways, management of grassed areas of riparian zones is critical to preserve the water quality functions of riparian areas and to prevent these areas from becoming a significant source of sediment and nutrients to surface waters. In this manuscript, these grassed areas of riparian zones will be referred to as “riparian grasslands”, a term which refers to grassed areas which are in close proximity (<30 m) to surface waters and prone to seasonal flooding.

Riparian zones on farms with cattle or other grazing livestock are often managed as grazing areas because they are typically unsuitable for row crop production due to topography, presence of woody vegetation, and seasonal flooding. However, forages in these areas can be relatively productive because of favorable moisture conditions during drier periods of the year. The incentive to graze riparian grasslands may be even larger for small-scale livestock producers, who may have limited grazing land and forage resource availability. However, early reports have documented that poor grazing management can lead to reduced stand density and forage ground cover (Alderfer and Robinson, 1947). More recent reports have indicated that poor grazing management can negatively influence infiltration, runoff, erosion, and sediment deposition (McGinty et al., 1979, Loch, 2000) and limit the ecosystem services provided by the riparian area.

While several studies have examined the impact of cattle manure to nutrient export from grasslands (McLeod and Hegg, 1984, Kuykendall et al., 1999, Sauer et al., 1999, Edwards et al., 2000a, Kleinman and Sharpley, 2003), few studies have specifically examined the impact of riparian grazing management and cattle feces and urine deposited in riparian areas on nutrient export (Butler et al., 2006, Butler et al., 2007). However, recent reports have documented the impacts of cattle on stream water quality under differing riparian grazing regimens. In Minnesota, USA, Sovell et al. (2000) reported reduced stream turbidity and fecal coliform levels in stream segments with adjacent rotational cattle grazing compared to continuous grazing. In Alberta, Canada, Scrimgeour and Kendall (2002) reported that different grazing regimens did not significantly impact stream water chemistry. Nevertheless, in one of the three streams studied, greater total P concentrations were reported from full season grazing than from livestock exclusion, early season grazing, or late season grazing.

The dendritic drainage pattern of the North Carolina Piedmont is characterized by small to moderate sized streams with a moderate stream density (McNab and Avers, 1996). Slopes near streams can be relatively steep due to severe erosion that occurred in the late 19th and early 20th centuries due to historic agricultural practices (Williams et al., 2004), adding to the risk of nutrient and sediment losses from these areas. In this region, poorly drained soils in close proximity to surface waters are often dominated by wetland plants such as rushes (Juncus spp.) and sedges (Carex spp.), and do not support production of typical forage species. Because of the growth habits of many of these wetland plants, poorly drained soils, and the susceptibility of the soil surface to treading damage (Cooper et al., 1995, Drewry, 2006, Kurz et al., 2006), these areas may be easily degraded by grazing cattle and become significant sources of export of sediment and nutrients. Generally, cattle congregate in moist riparian areas for available forage and water, cooler temperatures, and shade from riparian trees (Belsky et al., 1999). Together, these factors can increase the likelihood of unplanned heavy-use areas in grazed riparian areas, which may degrade water quality. However, research is needed to determine if well-managed grazing in riparian areas that limits degradation of forage ground cover can be a significant contributor of nutrients and sediments to surface waters.

This paper examines the impact of cattle feces and urine in poorly drained and well-drained riparian soils where heavy or light-use by grazing cattle was simulated. Using five heavy simulated rainfall events, runoff export was examined for volume, total suspended solids (TSS), dissolved reactive phosphorus (DRP), total Kjeldahl phosphorus (TKP), nitrate-nitrogen (NO3-N), ammonium-N (NH4-N), and total N (TN).

Section snippets

Study site

In February 2003, research plots were established on existing vegetation at North Carolina State University's Lake Wheeler Road Field Laboratory, Raleigh, NC, USA (35°43′N; 78°41′W; elevation=100 m). A randomized complete block experiment was established on a slope of ∼10%. Soil profiles at the site were assessed prior to plot establishment to find areas that presented the most consistent and similar soils on which to locate plots and experimental blocks. On the poorly drained position, the soil

Soil nitrogen and phosphorus

Mean soil inorganic N was generally highest on heavy-use plots, though not consistently so throughout the season (Table 3). This was expected, as the growth of forages in the light-use plots utilized more N for growth, leaving less readily available N in the soil. Mehlich-3 soil P varied much less from month to month, and the mean was consistently greater on well-drained plots (59 mg P kg−1) than poorly drained plots (19 mg P kg−1; Fig. 2).

Runoff volume

Analysis of cumulative runoff volume 30 min after runoff

Discussion

Results of this study suggest that both grazing pressure and soil drainage are important factors controlling the impacts of riparian grazing management. Both factors exerted influence to control runoff volume, though soil drainage may be of greater importance in terms of environmental impact considering that runoff volume from light-use plots on poorly drained soils was similar to that from heavy-use plots on well-drained soils. This was not evident with export of TSS, as light-use plots were

Conclusions

Results of this study indicate that cattle heavy-use or “lounging” areas in the riparian zone can be a significant source of TSS, NH4-N, TN, and sediment-bound P. However, when full cover was maintained (light-use), TSS, DRP, TKP, and NO3-N losses were generally limited. The exception was NH4-N and TN export, which was elevated during rain events when feces and urine were applied regardless of grazing pressure and soil drainage, and minimal at other times. Management to limit manure deposited

Acknowledgements

Special thanks are extended to the following individuals for invaluable help with the completion of this project: Elizabeth Barton, Dr. Cavell Brownie, Sharon Freeman, Dr. Jerry Huntington, Justin Garrett, Heather Glennon, Ricardo Goulart, Everett Gupton, T.J. Holliday, Brenna Houchins, Aaron Maye, Nathan McClintock, Izzy Menius, Guillermo Ramirez, April Shaeffer, Lucie Smith, Dr. Bir Thapa, Pete Thompson, and Mike Weeks.

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