Published online 13 September 2006
Published in J Environ Qual 35:1960-1965 (2006)
DOI: 10.2134/jeq2005.0308
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
Sorption of Atmospheric Ammonia by Soil and Perennial Grass Downwind From Two Large Cattle Feedlots
Xiying Haoa,*,
Chi Changa,
H. Henry Janzena,
George Claytonb and
Brett R. Hilla
a Agricultural and Agri-Food Canada, Lethbridge Research Center, 5403 1st Ave S., Lethbridge, Alberta, T1J 4B1, Canada
b Agricultural and Agri-Food Canada, Lacombe Research Centre, 6000 C and E Trail, Lacombe, Alberta, T4L 1W1, Canada
* Corresponding author (haoxy{at}agr.gc.ca)
Received for publication August 10, 2005.
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ABSTRACT
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Livestock manure in feedlots releases ammonia (NH3), which can be sorbed by nearby soil and plants. Ammonia sorption by soil and its effects on soil and perennial grass N contents downwind from two large cattle feedlots in Alberta, Canada were investigated from June to October 2002. Atmospheric NH3 sorption was measured weekly by exposing air-dried soil at sampling points downwind along 1700-m transects. The amount of NH3 sorbed by soil was 2.60 to 3.16 kg N ha–1 wk–1 near the source, declining to about 0.25 kg N ha–1 wk–1 1700 m downwind, reflecting diminishing atmospheric NH3 concentrations. Ammonia sorption at a control site away from NH3 sources was much lower: 0.085 kg N ha–1 wk–1. Based on these rates, about 19% of emitted NH3 is sorbed by soil within 1700 m downwind of feedlots. Field soil and grass samples from the transect lines were analyzed for total N (TN) and KCl-extractable N content (soil only). Nitrate N content in field soil followed a trend similar to that of atmospheric NH3 sorption. Soil TN contents, because of high background levels, showed no clear pattern. The TN content of grass, downwind of the newer feedlot, followed a pattern similar to that of NH3 sorption; downwind of the older feedlot, grass TN was correlated to soil TN. Our results suggest that atmospheric NH3 from livestock operations can contribute N to local soil and vegetation, and may need to be considered when determining fertilizer rates and assessing environmental impact.
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INTRODUCTION
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LIVESTOCK OPERATIONS release substantial ammonia (NH3) into the atmosphere. Ammonia deposition is a concern since it contributes to eutrophication of surface water and acidification of semi-natural ecosystems (Fangmeier et al., 1994; Bull and Sutton, 1998; Erisman and Monteny, 1998; Krupa, 2003). Elevated atmospheric NH3 levels can have phytotoxic as well as growth-stimulating effects on vegetation (Nasholm, 1998), with the risk of NH3 damage related to the distance from the source, landscape characteristics and plant sensitivity (van der Eerben et al., 1998). Goulding et al. (1998) reported that the deposited N itself, and the soil acidification associated with it, has reduced the number of plant species in a 140-yr-old park grass hay meadow. Visible injuries to pine (Pinus spp.) and spruce (Picea spp.) needles was reported immediately downwind of livestock buildings (Pitcairn et al., 1998). Total vegetation tissue N concentration is broadly correlated with atmospheric N deposition (Baddeley et al., 1994; Pitcairn et al., 1995; Pitcairn et al., 1998; Pitcairn et al., 2002) because plant leaves can absorb significant quantities of NH3 from the air (Hutchinson et al., 1972). Thus, increased atmospheric NH3 input to agro-ecosystems can contribute a major part of total N requirements for crop production.
Soil can sorb NH3 emitted from surrounding intensive livestock operations (Hao et al., 2005) to provide additional N throughout the growing season. This could further complicate fertilizer and manure application recommendations for crop production. Very few studies have measured the rate of atmospheric NH3 sorption by soil and vegetation near intensive livestock operations. Asman and Van Jaarsveld (1990) estimated that 20% of total emitted NH3 and NH4+ is redeposited within 1 km of the source (animal housing), with 80 to 90% returning to earth within 10 km. The remainder is dispersed into the atmosphere, sometimes traveling hundreds of kilometers. Information about rates of atmospheric NH3 dry deposition and its impact on vegetation is scarce for North American conditions where livestock management and landscape conditions may be different from those studied elsewhere. The objective of this study is to investigate the rate of atmospheric NH3 sorption and its effect on soil and perennial grass N content downwind from two large intensive cattle feedlots.
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MATERIALS AND METHODS
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The experiment was conducted during the growing season from 6 June to 16 Oct. 2002 in southern Alberta, a region with over 700 000 head of beef cattle. Three locations were chosen for this study, a control site and two 25 000-head-capacity feedlots (referred to here as the old lot and new lot) operated by the same owner. The control site is located at the northeast end of Keho Lake and northwest of the intensive livestock operation area, upwind from all NH3-emitting sources. The nearest NH3-emitting source is 4 km downwind. The two lots were chosen because they have the same management practices, capacity, and size (800 by 800 m), but have been operating for different lengths of time. The old lot development permit was issued in February 1977 (25 yr before this study) whereas the new lot permit was issued in April 1997. The old lot is about 16 km east and 3.3 km south of the control site. There are various other livestock operations nearby. The closest is a 400-head feeder hog operation 0.9 km south (upwind). There are also six other livestock operators with a total of 400 dairy, 2500 beef cattle, and 4400 broiler chickens within a 1700-m distance. Within an extended 3.2-km radius, there are an additional 750 head of dairy, 33 000 head of beef cattle, 750 broilers, and 560 farrow and feeder hogs. A large 25 000-head beef cattle operation is located 3.2 km east and 2 km north (downwind) from the old lot. The new lot is 4 km east (downwind) of the control site, and is situated at the irrigation-rain fed crop production boundary with only one other livestock operation (mixed-livestock with 1900 beef cattle and 400 farrow/finish hogs) nearby, 3.3 km east (downwind).
The vegetation immediately adjacent to the feedlots and along the county roads is perennial grass. Smooth brome (Bromus inermis Leyss.) is the dominant species at the old lot and crested wheatgrass (Agropyron cristatum L. Gaertn.) is dominant at the new lot. The perennial grass species reflect what was seeded as ground cover at the time the county roads were constructed.
Weather data were obtained from an Alberta Agriculture, Food, and Rural Development weather station within 1 km of the old lot (except for wind direction and pan evaporation data, which came from the Lethbridge Research Center weather station, 20 km south of the experimental sites). Over the 5-mo study period, the average daily temperature varied as much as 9°C above and 12°C below the long-term average. The precipitation was more than double the long-term average whereas pan evaporation was close to normal. The maximum pan evaporation values occurred in early July. The wind was predominantly from the south and southwest (average wind direction was 212.3°) with wind speed higher and more variable in the months of June and July than August to October.
The total amount of NH3 emitted from the two large livestock operations during the study period was estimated at 645 kg d–1 for each feedlot, based on the emission factor of 9.41 kg NH3-N per head of cattle per year (USEPA, 2004). Both feedlots operated at full capacity for most of the study period.
Ten sampling points were selected for each feedlot, one inside the feedlot premises but outside the feeding pen (P1, which was assigned distance 0 m) and the other nine (P2 to P10) downwind along county roads (Fig. 1
). In the old lot, P2 is 100 m north of the northeast corner and, in the new lot, P2 is 50 m northeast of the northeast corner. There were 200 m between all remaining adjacent points. The distance downwind is defined as the distance between the sampling point and the northeast corner of the feedlot. All sampling points outside livestock operations were located along county roadsides to minimize interference with farming operations and provide easy access.
The rate of atmospheric NH3 sorbed by soil was determined using the method of Hao et al. (2005). Briefly, at each sampling point, a stand was installed that holds two straight-sided plastic vials containing 5 g soil each. The soil used was Brown Chernozemic sandy loam (an Aridic Haplustoll in the U.S. Soil Taxonomy and an Aridic Kastanozem in the FAO Soil Classification System). This soil had low total carbon (TC) (8397 ± 144 mg C kg–1) and TN contents (905 ± 13 mg N kg–1) with typical NO3–-N (35.48 ± 0.77 mg N kg–1), NH4+-N (3.67 ± 0.39 mg N kg–1), and pH values (7.43 ± 0.05).
The soil in the sampling vial was protected from rain by a plastic cover so that only the atmospheric NH3 sorption was measured. For all points, the height of the soil sample vials was about 1 m above the ground. Soil sample vials were collected and replaced weekly at each point. The exposure time varied between 5 and 9 d due to adverse weather conditions, but results were adjusted and expressed on a weekly basis. After collection, soil samples were first weighed to obtain the weight after exposure (4.954 ± 0.008 g) then extracted with 50 mL of 2 M KCl. The NH4+ concentration in the extract was determined using a Bran Luebbe Auto-Analyzer III (the concentration of NO3– in the air-dried soil was not affected by the 1-wk exposure treatment).
Soil samples from depths 0 to 15 and 15 to 30 cm at each sampling point were taken on 17 June for the old lot and 27 June 2002 for the new lot and the control site. In addition, soil samples were also collected in nearby crop fields (<10 m apart for each sampling point). Soil samples were air-dried and ground to pass through a 2-mm sieve and subsamples were finely ground to pass a 0.150-mm sieve for total N (TN) analysis. The soil TN contents were determined using a NA 1500 Series 2 Carlo Erba Instrument (Rodano, MI, Italy). Nitrate N (NO3–-N) was extracted with 2 M KCl and the NO3–-N concentration in the extract was determined using a Bran Luebbe Auto-Analyzer III.
Plant tissue (grass) samples were taken on 11, 18, and 29 Sept. 2002 at each sampling trap point, except inside the feedlots and at the control site. Methods used to determine plant TN were similar to those used for soil.
Correlation analysis was used to relate the rate (or amount) of NH3 sorption by soil to weather conditions in the study area using procedures Reg and Univariate in SAS (SAS Institute, 2001). The relationship between the amount of atmospheric NH3 sorption by soil and the distance downwind was also investigated using segmented linear regression in SAS. The point where the two linear regression lines meet is termed the distance of local impact from the known emitting source. Similarly, the relationship between the soil and plant N contents and the distance downwind was investigated using linear regression in SAS.
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RESULTS AND DISCUSSION
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Ammonia Sorption Rates
The weekly atmospheric NH3 sorption by soil varied widely from June to October 2002 (Fig. 2a
) with similar NH3 sorption for both locations. The average weekly NH3 sorption by soil over the study period ranged from 0.38 to 2.06 with an overall average of 0.826 kg N ha–1 wk–1 (for all 10 sampling points at both feedlots), significantly above the background level of 0.085 kg N ha–1 wk–1 at the control site. The high sorption rate in early July may partly reflect high air temperatures, pan evaporation, and wind speed during this period, which favors the volatilization of NH3 from livestock manure.
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Fig. 2. Weekly rate of atmospheric NH3 absorption by soil downwind from 25 000-head cattle feedlots (June-October 2002).
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Atmospheric NH3 sorption by soil was positively related to air temperature, wind speed, and evaporation potential, but negatively related to rainfall amount (Table 1). Higher air temperature, wind speed and evaporation potential will promote the emission of NH3 from manure. Rainfall, on the other hand, will increase the amount of NH3 being held in manure as the manure water content increases. As well, rainfall will remove dissolved NH3 from the atmosphere (wet deposition, not measured in this study) decreasing the atmospheric NH3 concentration. For the sampling points farthest from the feedlot operations (P7-P10 for the old lot and P8-P10 for the new lot), NH3 sorption by soil was only positively affected by wind speed (Table 1). When wind speed is high, the air with elevated NH3 above the feedlot (NH3-emitting source) would be carried further downwind to these sites (1000 to 1700 m away from the feedlots).
The weekly rate of NH3 sorption by soil decreased rapidly as the distance of the sampling points from the feedlot increased. The highest rates occurred inside the feedlot operations (3.17 kg N ha–1 wk–1 for the old lot and 2.59 kg N ha–1 wk–1 for the new lot), and leveled off to about 0.50 kg N ha–1 wk–1 at a distance of about 700 m from the corner of the feedlots (Fig. 2b). The amount of NH3 sorbed by soil inside the feedlots (2.59 and 3.17 kg N ha–1 wk–1) and values at 100 and 200 m downwind (1.20 to 1.60 kg N ha–1 wk–1) were similar to those reported by McGinn et al. (2003) for southern Alberta feedlot operations. Although the rate of NH3 sorption by soil levels off at 700 to 1700 m distance, the sorption rate of 0.25 kg N ha–1 wk–1 was still about 3 to 4 times higher than at the control site (0.085 kg N ha–1 wk–1).
A significantly higher rate of NH3 sorption by soil inside the old lot than the new lot could be due to a higher rate of NH3 emission, although both lots had 25 000-head cattle capacity. Another possibility is that this difference reflects higher NH3 emissions from soils in the feeding pens of the old lot. The soil N content was higher in the old lot (Fig. 3a
and 4a
), which has been used for livestock production for 20 yr longer than the new lot, possibly leading to higher emission of soil-derived NH3.
The relationship between NH3 sorption by soil and distance downwind from the source was further investigated using segmented linear regression, which showed that NH3 emitted from the feedlot has a strong potential influence on NH3 sorption by soil up to a distance of 426 m for the old lot and 266 m for the new lot. Beyond this point, the effect of the feedlots in this study cannot be differentiated from other potential NH3 emitting sources. These NH3 impact distances are within the ranges reported elsewhere (Dragosts et al., 2002). Besides the stronger source of NH3 in the old lot, the feedlot's location in relation to the sampling point layout and prevailing wind direction (southwest, 212.3°) may also have influenced the observed differences. As NH3 moves away from the emitting surface in the feedlot, its concentration decreases from diffusion and mixing with air. Because the old lot is located farther south of its downwind sampling points, these points were closer to the center of the NH3 diffusion plume than the points downwind of the new lot (Fig. 1). Thus, a greater distance of influence for the old lot would be expected. The average of 31 to 35 mg N m–2 wk–1 (or 18.2 kg ha–1 yr–1) sorbed by soil at distances beyond the impact distance were similar to the average values earlier reported for the whole north County of Lethbridge (Hao et al., 2005).
Based on measured rates of NH3 sorption, and assuming NH3 emitted was sorbed by soil downwind in a half-circle shape to the 1700-m distance, an estimated 19% of the total emitted would be sorbed by soil in this area. Out of the total amount of NH3 sorbed, 2% went directly back to the feedlot operations (in the 800 by 800 m area, of which about half was covered by NH3-emitting manure and soil, the other half by bare NH3 absorbing soil), 12% within 300 m, 16% within 1000 m, and 19% within 1700 m of feedlot operations.
The estimated fraction of emitted NH3 sorbed by soil from this study is higher than the 3.3% of emissions within 300 m of poultry buildings reported by Fowler et al. (1998). The higher rates in our study may be partly due to differences in the NH3-emitting sources (25000 head of cattle vs. 120 000 broilers). In addition to greater emissions from manure, some NH3 in our study may also have been emitted from the soil. The NH3 source in Fowler et al. (1998) study was poultry housing, which releases NH3 from a well-defined point higher aboveground level. Since NH3 emissions from higher elevations generally travel greater distances (Asman, 1998), a smaller fraction of NH3 would be sorbed locally by soil than in our study. Also, under semi-arid conditions as in our study, more atmospheric NH3 would be in gaseous than dissolved form, leading to greater dry deposition.
Effect of Atmospheric Ammonia on Soil and Grass
Nitrate contents in field soil samples were highest inside the feedlots and decreased with distance away from the feedlots (Fig. 3). While soil NO3– content was higher at the lower 15- to 30-cm depth than in the 0- to 15-cm depth inside the feedlot, similar NO3– values for both depths were observed outside the feedlot. Precipitation on 2 June (32 mm), 8 to 11 June (102 mm) and 18 to 19 June (48 mm) might have leached NO3– downward inside the feedlot because there was no vegetation. For sampling points outside the feedlot, the perennial vegetation cover may act as buffer and intercept/sorb some rainfall water, reducing NO3– leaching. Also, most roadsides have some slope, so any excessive rain water is more likely to run off into the ditch (so less water infiltrates into the soil). Vegetation uptake of NO3– from soil could also reduce the differences in NO3– concentration between the two soil depths. At the new lot, the NH3-N sorption by soil in the traps was significantly correlated with KCl-extractable NO3– content in field soils (r = 0.911***, n = 9). Padgett et al. (1999) also reported significantly higher soil NO3-N content near the NH3 emitting source. However, at the old lot sites, there was no significant correlation (r = 0.199, n = 9) between NH3 sorption by soil in the traps and NO3-N content of field soil. The soil NO3-N content at the old lot was probably controlled by soil TN mineralization and the long history of livestock operations rather than the current rate of NH3 sorption by soil over the 5-mo study period. Indeed, the soil NO3-N was strongly correlated with soil TN content at the old lot (r = 0.865**, n = 9) but not at the new lot (r = –0.212, n = 9).
Soil TN contents were more variable and showed no clear pattern downwind from both feedlot operations. The level of soil TN was apparently affected by other factors, such as climate, vegetation, C input, and C/N ratio. In addition, because soil TN content is very high in the 0 to 30 cm depth (68 864 kg N ha–1 near the old lot and 50 255 kg N ha–1 near the new lot), the comparatively small additions from atmospheric deposition are unlikely to be measurable except after very long time periods. The atmospheric NH3 input, even over 20+ yr, was less than 10% of TN initially present in top 0 to 15 cm soil.
The soil TN content across the sampling points generally showed higher values in the top 0 to 15 cm than in the 15 to 30 cm depth for both feedlot operations (Fig. 4a). The TN values in the 0 to 15 cm depth were much higher near the old than new lot and were similar to values in nearby crop land. The higher TN values near the old lot (3.0 g kg–1 in 0 to 15 cm soil vs. 1.67 g kg–1 near the new lot) may reflect its longer history of livestock operations (and associated manure deposition), allowing more time for soil and vegetation to sorb the NH3, and greater N input directly (soil sorption) and indirectly (as root systems and grass die back in the winter) to soil under N-enriched atmospheric conditions.
Plant tissue N data from all three sampling dates were pooled together (Fig. 4b), since differences in grass tissue N content among the three sampling dates were not significant. The TN content of crested wheatgrass near the new lot was affected by the elevated atmospheric NH3 near the feedlot operation and decreased as the distance increased. The TN content in crested wheatgrass at P4 (434 m) was 1.6% but dropped to 0.7% at points P8 to P10 (1233-1632 m). Pitcairn et al. (2002) reported the N content of mosses in a woodland ground flora decreased from 4% at 30 m to 1.6% at 650 m downwind from a poultry operation. Similarly, foliar N content of trees also decreased as the distance between the tree and poultry building increased (Pitcairn et al., 1998). In contrast, the TN level in smooth bromegrass was largely unaffected by distance downwind from the old lot. As with soil NO3– content, smooth bromegrass TN content was more closely correlated to soil N content (r = 0.675*, n = 9) rather than current soil NH3 sorption rates (r = 0.275, n = 9). This study clearly shows that the soil and vegetation N content was affected not only by the rate of atmospheric NH3– sorption by soil (distance downwind) but, more importantly, the history or length of feedlot operations since both feedlots under investigation in this study had the same operating capacity and management practices.
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CONCLUSIONS
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The amount of NH3 sorbed by soil was high near the feedlots and decreased downwind along the 1700-m transect lines. The amount of NH3 sorbed by soil was 2.60 to 3.16 kg N ha–1 wk–1 near the source, declining to about 0.25 kg N ha–1 wk–1 1700 m downwind, reflecting diminishing atmospheric NH3 concentrations. All are higher than the background level of 0.085 kg N ha–1 wk–1 observed at the control site. Of the total amount of NH3 emitted from 25 000 head cattle, an estimated 19% was sorbed by soil within 1700 m downwind from the feedlot operations. Field soil NO3– and perennial grass TN content also decreased downwind from the newly established feedlot (5 yr old), but there is no clear pattern in soil TN content.
Canada currently has no set limit for NH3 deposition. This study clearly shows higher rates of NH3 input near livestock operations and these levels affect soil and vegetation N nutrients cycling in the agro-ecosystem. A small fraction of the NH3 volatilized from livestock operations could substantially reduce crop requirements for N fertilizer in nearby soils. Current fertilizer recommendations in North America, based on soil test results, give no consideration to the amount of atmospheric NH3 potentially sorbed by crops and soil during the growing season. But in an area with intensive livestock operations, over-applying N fertilizer based on traditional soil testing is possible. Further research in this area is needed to maintain the long-term environmental sustainability of concentrated intensive livestock operations. Research is also needed to assess the potential impact of increased N input on water pollution, aquatic life, and the ecosystem as a whole.
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ACKNOWLEDGMENTS
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We acknowledge the Agricultural & Agri-Food Canada Matching Investment Initiative (MII) for funding and the collaboration of Troy Ormann and the County of Lethbridge in supporting this research. We thank G. Travis and P. Caffyn for laboratory and field assistance, C. Gilbertson for TN analysis, and T. Entz for statistical advice. This is Lethbridge Research Center Contribution 387-04039.
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ABSTRACT
INTRODUCTION
