163 8 Net Ecosystem Carbon Dioxide Exchange over a Temperate, Short-Season Grassland: Transition from Cereal to Perennial Forage V.S. Baron, D.G. Young, W.A. Dugas, P.C. Mielnick, C. La Bine, R.H. Skinner, and J. Casson CONTENTS 8.1 Introduction 164 8.2 Methods 165 8.2.1 Net Ecosystem CO 2 Exchange 165 8.2.2 Soil CO 2 Flux 166 8.3 Results and Discussion 166 8.3.1 Climate 166 8.3.2 BREB CO 2 Flux 166 8.3.2.1 Initial Growth Period 167 8.3.2.2 Regrowth 168 8.3.2.3 Grazing 168 8.3.2.4 Dormant Period 168 8.3.2.5 Spring 168 8.3.2.6 Diurnal CO 2 Flux 169 8.3.3 Soil Respiration 170 8.3.4 Ecosystem Sink or Source 172 8.4 Conclusion 172 Acknowledgment 173 References 173 © 2006 by Taylor & Francis Group, LLC 164 Climate Change and Managed Ecosystems 8.1 INTRODUCTION North American grasslands may be part of a terrestrial carbon (C) sink. 1–3 The terrestrial or “missing” sink, which includes cropland and forests, may play a role in offsetting CO 2 emissions generated from fossil fuel use and global changes in land management (e.g., deforestation) through C sequestration in soils, vegetation, and residues. 1,2,4 Fan et al. 5 estimated the northern boundary of the terrestrial sink to be approximately 51° N lat. The potential sink size of grasslands may be significant because they cover a large area. 4 World grasslands represent 32% of global vegetation on an area basis. 1 Estimated area of U.S. pasture and rangeland is 51 and 161 million ha, respectively, compared to cropland at 155 million ha. Area of public grazing land in the U.S. is 124 million ha. 4 Canadian pastureland was estimated at 10 million ha, of which 9 million ha is located in western Canada. 6–9 Entz et al. 10 estimated an area of 44 million ha of range in the Northern Great Plains region of Manitoba, Saskatchewan, Alberta, North Dakota, South Dakota, and Montana. There are approximately 6.5 million ha of native rangeland mostly south of 52° N lat in the semi-arid region of the Prairie Provinces. 11 Rangelands have relatively low potential C sequestration rates per hectare, but cover large areas of the North American continent and, thus, could act as a large potential C sink. Improved pastures have larger potential rates of C sequestration as they are located in humid and subhumid regions and receive greater management inputs; most are managed below production potential, 4,12 but are also subject to greater removal of biomass-C as conserved ruminant feed. Most pastureland in the U.S. is located east of 98° W long., 12 where annual precipitation balances or exceeds evapotranspiration. 13 In Canada, the majority of pasture is located on the black and gray wooded soils in the subhumid Aspen Parkland and Boreal Transition zone of Western Canada 8 and in all regions of eastern Canada. 6–9 Alberta contains the largest area of pasture of the Prairie Provinces at 2.2 million ha as well as 6.6 million ha of rangeland consisting of naturalized and native species. 14 Micrometeorological studies carried out by the USDA-Agricultural Research Ser- vice Rangeland Carbon Dioxide Project 15 documented ecosystem CO 2 balance for various grasslands. 16–23 These studies provided ecological insights into fundamental processes that affect C sequestration. All CO 2 uptake occurs as a result of photosyn- thesis during the growing season. Uptake is countered by respiration, resulting in CO 2 emission from the crop canopy, root, and soil microbial degradation of litter and crop organic residues. 24,25 Bremer et al. 25 estimated that CO 2 respired from a tall-grass prairie ecosystem in Kansas was three to four times that accumulated in new biomass during the season; a fraction of the CO 2 respired by the system is re-assimilated. Most of this research, involving aspects of grassland CO 2 flux, was conducted during the growing season in the Great Plains region. While several short-term studies indicate that grassland ecosystems act as small sinks, there are only a few studies that evaluated ecosystems annually. Climate, season, species, phenology, and management (e.g., grazing, fire, etc.) influence both CO 2 uptake and ecosystem respiration. 16,22,24–26 In particular, drought 20,24 and the dormant season 19,20 are periods of net CO 2 loss. Thus, © 2006 by Taylor & Francis Group, LLC Net Ecosystem Carbon Dioxide Exchange 165 in areas with short growing seasons, intermittent drought, and long dormant periods such as the Aspen Parkland region, 27 C sequestration may be limited. Conversion of cropland to grassland and intensification of grassland management are key methods to enhance rate of C sequestration in soils. 4,12 Studies about the ecosystem CO 2 exchange process are needed to assess the Aspen Parkland region for potential as a C sink. Our objective was to evaluate net ecosystem CO 2 exchange, within season and annually, during the establishment or seedling year and during the first production year of a forage stand in transition from a cereal to forage sequence to be used for pasture in a mixed farm crop rotation. 8.2 METHODS Net CO 2 ecosystem exchange measurements were made on a black chernozemic sandy loam soil in transition from cereal to perennial pasture, at Lacombe, Alberta, Canada (52° 26′ N: 113° 45′ W). The site had been in a cereal-forage rotation over the past 20 years. Farming practices prevalent in the region were used — silage and hay were removed in the seedling and first production years and grazing of the forage species mixture occurred in September or October. For the seedling year the field was sown (May 15, 2002) to barley (Hordeum vulgare L.), a nurse crop, and under sown to a mixture of meadow bromegrass (Bromus riparius Rehm.) and alfalfa (Medicaigo sativa L.). The nurse crop was removed as silage on August 1, 2002. In 2002, the seedling forage stand was allowed to regrow until fall when it was grazed severely (4.2 Animal Unit Months, AUM, over 19 days) between September 27 and October 15. For the first production year, in 2003, the field was harvested as hay on July 16, allowed to regrow, and was grazed lightly (0.63 AUM over 14 days) between September 2 and September 15. Hereafter reference to heavy and light grazing are stocking rates of 4.2 and 0.63 AUM ha –1 , respectively, over 19 and 14 days during the seedling and first production years, respectively. Each year 100 kg ha –1 N was applied in the spring. 8.2.1 N ET E COSYSTEM CO 2 E XCHANGE A tower with Bowen ratio/energy balance (BREB) instrumentation (Model 023/CO2 Bowen ratio system, Campbell Scientific, Inc., Logan, UT) was placed on a 2% northwest-facing slope after seeding in spring 2002. Borders of the field provided a minimum 200-m fetch from the tower. Bowen ratios were calculated as described previously. 16,18,19,28 Measurements began on May 26, 2002. When the BREB method was not valid for calculating turbulent diffusivity, because of differences in the sign of the sensible heat flux and the temperature/humidity gradient, it was calculated using wind speed, atmospheric stability, and canopy height. 16 This occurred 12% of the time. Daily net CO 2 flux measurements between May 15 (seeding date) and May 26 were estimated from the average daily flux for the first week of measurements. Carbon dioxide and water vapor concentration gradients were measured at 1.4 and 2.4 m above the soil surface with an infrared gas analyzer (Model 6262, LICOR, Inc., Lincoln, NE). Methods were described previously by Frank and Dugas 18 and Frank et al. 19 © 2006 by Taylor & Francis Group, LLC 166 Climate Change and Managed Ecosystems The seedling year includes year-round CO 2 exchange data from May 15, 2002 to May 14, 2003 and the first production year includes data from May 15, 2003 to May 14, 2004. 8.2.2 S OIL CO 2 F LUX Soil respiration measurements were made with a vented closed system using a LICOR 6200 portable infrared gas analyzer fitted with a soil respiration chamber (LICOR 6000-09) with a volume of 1000.3 cm 3 and a diameter of 10.3 cm allowing an area of exposed soil of 71.5 cm 2 . Rings were inserted 25 mm into bare soil in three positions around the BREB tower and were kept free of vegetation. Flux measurements were made by placing the chamber over the rings for periods of 3 to 5 min. Measurements were made at approximately 2-week intervals during the growing season, spring and fall from 1100 to 1300 hours, but not in winter. BREB measurements are presented as a daily total of net CO 2 flux (g m 2 /d –1 ). Soil respiration measurements are presented in the same units, but are averaged over at least three subsamples. Net CO 2 uptake into the ecosystem is indicated by a positive sign and net efflux by a negative sign. Linear regression was used to relate midday soil respiration (independent variable) to BREB nighttime CO 2 flux (dependent variable). A significant linear regression coefficient (P ≤ 0.05) was indicative of a relationship between the fluxes of different origins. The coefficient of determination (R 2 ) was used to quantify the extent to which the variation in BREB nighttime CO 2 flux was explained by soil respiration. 8.3 RESULTS AND DISCUSSION 8.3.1 C LIMATE The seedling year season could be described as a dry spring and summer with a moist fall (Figure 8.1); April to October precipitation was 277 mm and the long- term average for this period was 363 mm. The first production year season had a wet spring with a dry summer; April to October precipitation was 262 mm. Tem- peratures during the June to August period were generally above average. Mean winter temperatures (October to March) were above average for the seedling year and average to below average for the first production year. Spring time air temper- atures were average. 8.3.2 BREB CO 2 F LUX Periods of initial growth, regrowth, grazing, dormancy, and transition between dor- mancy and growth could be identified from daily net CO 2 exchange in both seedling and first production years (Figure 8.1). From May 15 to September 30 (entire growth period), net uptake occurred on only 65 and 61% of the 140 days for seedling and first production years, respectively (Figure 8.1A and B). Daily flux values were highly variable due to effects of variation in radiation, temperature, and precipitation on plant and soil processes. Occasional spikes of soil CO 2 efflux often occurred immediately after rainfall events that flush CO 2 from soil pores. 24,29 © 2006 by Taylor & Francis Group, LLC Net Ecosystem Carbon Dioxide Exchange 167 8.3.2.1 Initial Growth Period For both seedling and first production year, maximum daily BREB CO 2 flux coin- cided with periods of initial growth in June and July (Figure 8.1A and B). From May 15 until Aug. 1, 2002 seedling year average daily net flux was 6.7 g m 2 d –1 FIGURE 8.1 Net daily CO 2 flux and precipitation for (A) May 15, 2002 to May 14, 2003 (seedling year) and (B) May 15, 2003 to May 14, 2004 (first production year) for meadow bromegrass–alfalfa. A B Seedling Year 2002-03 Harvest Regrowth Grazing Harvest Regrowth Grazing g m -2 d -1 mm d -1 g m -2 d -1 mm d -1 40 30 20 10 0 -10 -20 -30 -40 30 20 10 0 -10 -20 -30 -40 100 90 80 70 60 50 40 30 20 10 0 90 80 70 60 50 40 30 20 10 0 1361 66 1962 26 2562 86 3163 46 10 40 70 1001 30 May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May First Production Year 2003-04 © 2006 by Taylor & Francis Group, LLC 168 Climate Change and Managed Ecosystems and from May 15 until July 16, 2003 the average flux in the first production year was 1.7 g m 2 d –1 . 8.3.2.2 Regrowth Harvest was followed immediately by a period of net CO 2 loss that lasted 25 days in the seedling year and 14 days in the first production year. McGinn and King 30 observed 12 days of ecosystem respiration after cutting alfalfa during midsummer in Ontario. Mean daily CO 2 flux during these periods was –4.9 and –5.5 g m 2 d –1 during seedling and first production year, respectively. Once a plant canopy was reestablished, net CO 2 uptake into the ecosystem occurred well into September of both years. In the seedling year, August and September rainfall resulted in regrowth of forage, but the perennial stand had to establish a canopy after the barley was removed in spite of dry early summer conditions. Regrowth was slow in the first production year due to drought, but generally net CO 2 uptake occurred (Figure 8.1B). Average net CO 2 flux after harvest until September 30 was –0.75 and 0.41 g m 2 d –1 during seedling and first production years, respectively. Kim et al. 24 observed net CO 2 release at a rate of –3 g m 2 d –1 from a tall-grass prairie ecosystem in Kansas during drought conditions. 8.3.2.3 Grazing A season-end grazing removed most of the live vegetative material from the seedling year stand during early October. A very light grazing in September of the first production year left residual biomass for light interception. During hard grazing (September 27 to October 30) in the seedling year an average net CO 2 release of –7.9 g m 2 d –1 was observed. During the light grazing (August 27 to September 15) in first production year an average net CO 2 loss of –0.8 g m 2 d –1 was observed. Fall residual dry matter was less than 50 and approximately 300 g m –2 in seedling and first production years, respectively. 8.3.2.4 Dormant Period Almost no growth occurred during the dormant period, October 1 to March 31, although 6 and 17 days of net uptake occurred in the seedling and first production years, respectively, mostly in October. Over the October 1 to March 31 period, average daily CO 2 release was –3.6 and –1.6 g m 2 d –1 during seedling and first production years, respectively. Relatively large losses from the system occurred during October, November, and March of the seedling year. Over the entire dormant period the range in CO 2 flux was 1.5 to –14.7 g m 2 d –1 in the seedling year (Figure 8.1A) and 7.2 to –8.2 g m 2 d –1 in the first production year (Figure 8.1B). 8.3.2.5 Spring The daily net CO 2 loss from the ecosystem in March of the seedling year (Figure 8.1A) lasting until late May 2003 (Figure 8.1B) was large. From April 1 until May 15 there were only 4 days of net uptake. This is partly due to the lack of leaf area © 2006 by Taylor & Francis Group, LLC Net Ecosystem Carbon Dioxide Exchange 169 as a result of heavy grazing the previous fall. Thus, net CO 2 loss occurred from respiration required to generate new leaf material just as it did after cutting. Pasture dry matter yields on April 18, May 1, and May 15 were 0.0, 100, and 500 kg ha –1 , respectively, in 2003 (seedling year). By contrast there was net uptake during 80% of the days between April 1 and May 14, 2004 for the first production year (Figure 8.1B). Dry matter yield on May 15, 2004 was 1600 kg ha –1 . The CO 2 flux averaged –12.7 and 3.3 g m 2 d –1 from April 1 to May 14 during the seedling year (2003) and first production year (2004), respectively. 8.3.2.6 Diurnal CO 2 Flux Diurnal CO 2 fluxes were representative of the contrasting periods within years (Figure 8.2). The 20 min average CO 2 fluxes averaged over a 14-day period for winter (Figure 8.2A) showed small CO 2 flux loss during day and night in early January for both seedling and first production years. During spring (Figure 8.2B), CO 2 fluxes were negative during day and night for the seedling year, reflecting high soil respiration rates (Table 8.1) and lack of a crop canopy, following intense grazing during the fall of 2002. By contrast, spring diurnal CO 2 fluxes during the first production year resembled those of the initial growth period (Figure 8.2C), although the peak values were smaller. For the examples given (Figure 8.2B and C), average daytime CO 2 flux for the first production year-spring was approximately 39% of the FIGURE 8.2 The 20-minute average CO 2 flux for 24-h periods of seedling and first produc- tion year. (A) Winter, averaged over 14 days, (B) spring, averaged over 14 days, (C) initial growth, averaged over 7 days prior to harvest, and (D) regrowth, averaged over 7 days after harvest. mg m –2 s –1 A 0.8 0.6 0.4 0.2 0.0 –0.2 –0.4 –0.6 –0.8 mg m –2 s –1 B 0.8 0.6 0.4 0.2 0.0 –0.2 –0.4 –0.6 –0.8 mg m –2 s –1 C 0.8 0.6 0.4 0.2 0.0 –0.2 –0.4 –0.6 –0.8 mg m –2 s –1 D 0.8 0.6 0.4 0.2 0.0 –0.2 –0.4 –0.6 –0.8 Seedling Year Production Year Winter, Jan. 2–15 Initial Growth 1 wk Preharvest Spring, April 1–14 Regrowth 1 wk Postharvest 0 600 1200 1800 2400 Time of Day (MST – h) 0 600 1200 1800 2400 Time of Day (MST – h) © 2006 by Taylor & Francis Group, LLC 170 Climate Change and Managed Ecosystems average of both years’ initial growth daytime flux; spring nighttime flux (Figure 8.2B) was 53% of the nighttime CO 2 flux for the initial growth period (Figure 8.2C). Diurnal CO 2 flux for the initial growing period (averaged over 7 days prior to harvest) were comparable to those from other studies (e.g., Frank et al. 19 and Sims and Bradford 20 ) for periods of active growth. Diurnal CO 2 flux for the regrowth period (averaged over a 7-day period immediately after harvest) showed negative daytime and nighttime flux for the seedling year (Figure 8.2C). Regrowth daytime CO 2 flux for the first production year averaged near zero. The low daytime CO 2 flux for both years reflected a small leaf area during regrowth. However, average nighttime regrowth CO 2 flux for both years was approximately 82% of those observed during initial growth, explaining the tendency for relatively large net losses from the eco- system during regrowth periods of both years. A decline in afternoon CO 2 flux rates for seedling and first production year initial growth (Figure 8.2C) and first production year regrowth (Figure 8.2D) was likely due to tissue water deficit, resulting in stomatal closure, as suggested in a similar example by Sims and Bradford. 20 The growing seasons used in this study were drier than normal. 8.3.3 SOIL RESPIRATION Our measurements of respiration are the sum of root and soil microbial respiration. 25 During the night, the BREB system measures aboveground (including canopy and litter) and belowground respiration. As expected, nighttime BREB CO 2 flux was highest during periods of maximum plant growth. Averaged over initial growth periods BREB nighttime respiration averaged –4.96 and –8.58 CO 2 flux g m 2 d –1 for seedling (May 15 to August 1) and first production years (May 15 to July 16), respectively, compared to –2.5 and –2.0 g m 2 d –1 , respectively, during dormant periods. TABLE 8.1 Midday Soil CO 2 Flux Measurements Taken during Selected Periods from 2002 to 2004 from Seedling Year and First Production Year Stands of Meadow Bromegrass–Alfalfa Mixtures at Lacombe, Alberta Period Sample No., n Mean, g m 2 d –1 Range 2002 June 6 to Sept. 30 27 –11.8 –5.5 to –19.8 Oct. 1 to Dec. 4 18 –3.7 –0.1 to –7.9 2003 April 1 to May 15 9 –13.6 –6.5 to –25.9 May 16 to Sept. 30 30 –8.9 –4.6 to –21.0 Oct. 1 to May 15 6 –4.6 –2.6 to –6.7 2004 April 1 to May 15 12 –6.5 –3.8 to –8.5 © 2006 by Taylor & Francis Group, LLC Net Ecosystem Carbon Dioxide Exchange 171 Mean soil CO 2 flux varied depending upon on time of year and stage of crop development (Table 8.1). Soil CO 2 flux during the growing season (May 16 to September 30) was approximately three times greater than the dormant season (November 1 to March 31), but growing season fluxes were more variable. Spring 2003 (April 1 to September 15) soil respiration losses were highest, supporting net BREB losses for the seedling year during April and early May (Figure 8.1A). Losses during spring 2004 (first production year) were about half the CO 2 flux values of spring 2003. Similar results were shown by De Jong et al. 31 in southern Saskatchewan, who concluded that high soil respiration rates following a drought were due to wetting and drying cycles, which stimulated soil microflora to accelerate respiration rate. Dormant season daily mean soil CO 2 flux were in agreement with Kim et al. 24 Frank and Dugas 18 determined daily average soil flux to be –1.7 g CO 2 m –2 from measurements made throughout fall and winter in North Dakota. Linear regression analyses between soil respiration CO 2 flux rates and BREB night time fluxes indicated that soil CO 2 fluxes did not predict BREB nighttime fluxes well (Table 8.2). Frank et al. 22 found that a single flux rate taken at midday overestimated the average of five sequential measurements taken at 3-hour intervals by 9%. However, they 22 determined that single, midday soil CO 2 flux rates were most representative of daily soil flux CO 2 compared to soil flux rates taken at other times of the day. TABLE 8.2 Linear Regression Relationship between Midday Soil (independent variable) and BREB Nighttime CO 2 Flux for Selected Periods in a Meadow Bromegrass–Alfalfa Stand during Seedling Year and First Production Year Combined at Lacombe, Alberta Relationship P Sig. a R 2 RMSE b n Entire Growing Season (May 15 to Sept. 30) –4.60 + 0.22x 0.09 0.11 3.3 26 Initial Growth Period c –5.99 + 0.10x 0.67 0.02 4.0 13 Re-growth Period c –2.19 + 0.60x 0.01 0.46 2.3 8 Entire Dormant Period (Oct. 1 to March 31) –1.45 + 0.86x 0.01 0.56 2.2 9 Spring Period (April 1 to May 15) –2.64 + 0.18x 0.01 0.39 1.7 8 a Probability of a significant regression. b RMSE is root of the mean square error of the linear regression. c Initial growth period for 2002–2003 is May 15 to Aug. 1 and for 2003–2004 is May 15 to July 16; regrowth period for 2002–2003 is Aug. 2 to Sept. 30 and 2003–2004 is 17 July 17 to Sept. 30. © 2006 by Taylor & Francis Group, LLC 172 Climate Change and Managed Ecosystems During the growing season, soil respiration did not explain variation in ecosystem BREB respiration well, as indicated by low R 2 values and regression equations with low probability of significance (Table 8.2). A large proportion of ecosystem respiration is derived from canopy dark respiration when dry matter yield is close to maximum. 32 However, during the regrowth phase, which included periods of net loss after harvest (Figure 8.1A and B) and drought, soil respiration explained 46% of the variation in BREB nighttime CO 2 flux. During the dormant period (essentially October and November measurements) soil CO 2 flux explained 56% of the BREB nighttime CO 2 flux. During the spring period there was a transition from dormancy to growth; both plant and soil microorganism metabolism should be high, but the crop leaf area is relatively low compared to mid-June. Over the spring period soil respiration explained 39% of the variability in BREB nighttime respiration (Table 8.2). Soil CO 2 flux should underestimate CO 2 flux from ecosystem respiration as it does not account for litter decomposition and dark respiration of live vegetative material. 8.3.4 ECOSYSTEM SINK OR SOURCE During the seedling year there was an average annual net daily CO 2 flux of –2.0 g m –2 d –1 for a net loss from the ecosystem of 730 g CO 2 m –2 yr –1 . During first production year the average annual net daily CO 2 flux was –0.01 g m –2 d –1 for a net annual loss of 3.65 g CO 2 m –2 yr –1 . Year to year variation in annual CO 2 flux is likely to be high. Long- term annual ecosystem equilibrium should be bounded by a standard deviation of CO 2 flux. In most cases enough years of data have not been collected to determine what this statistical boundary might be. Studies such as the current one are often not replicated or numbers of replicates are very small. Statistical rigor improves with the number of years involved in the study. This may not be economically possible, so it is important to be aware that small annual net losses might be indicative of an equilibrium state. Dugas et al. 16 estimated that annual fluxes of 183 to 293 g CO 2 m –2 yr –1 at a Temple, TX site as in “approximate equilibrium.” Sims and Singh 33 as cited by Frank et al. 19 estimated that the CO 2 budget of native grasslands should be near equilibrium. Losses in the current study indicate that on the basis of annual BREB CO 2 flux data that the ecosystem acted as a CO 2 source during the seedling year and that it was close to equilibrium during the first production year. However, approximately 530 g m –2 yr –1 of dry matter was removed as silage and during grazing in the seedling year and approximately 450 g m –2 yr –1 was removed in the first production year, equating to an additional 805 and 684 g CO 2 m –2 of C loss from the ecosystem besides that determined by BREB CO 2 flux measurements. Thus, in both years the forage stand acted as a CO 2 source. The removal of C from the ecosystem as harvested dry matter does not complete the C accounting because return of materials as manure after feeding or as feces during grazing may bring a fraction of the original crop-C back to the ecosystem. 8.4 CONCLUSION More years of annual measurement of CO 2 flux are required to determine precise patterns for CO 2 -dynamics on cropland pasture in mixed cropping systems. This © 2006 by Taylor & Francis Group, LLC [...]... J., 94, 240, 2002 11 Willms, W.D and Jefferson, P.G., Production characteristics of the mixed prairie Constraints and potential, Can J Anim Sci., 73, 665, 1993 © 2006 by Taylor & Francis Group, LLC 174 Climate Change and Managed Ecosystems 12 Schnabel, R.R et al., The effects of pasture management practices In The Potential of U.S Grazing Lands to Sequester Carbon and Mitigate the Greenhouse Effect,... and climate, Can J Soil Sci., 54, 299, 1974 32 Pattey, E et al., Measuring nighttime CO2 flux over terrestrial ecosystems using eddy covariance and nocturnal boundary layer methods, Agric For Meteorol., 113, 145, 2002 33 Sims, P.L and Singh, J.S., The structure and function of ten western North American grasslands IV Compartmental transfers and energy flow within the ecosystem, J Ecol., 66, 983 , 19 78. .. present, Nature, 3 48, 711, 1990 2 Sundquist, E.T., The global carbon dioxide budget, Science, 259, 934, 1993 3 Houghton, R.A., Davidson, E.A., and Woodwell, G.M., Missing sinks, feedbacks, and understanding the role of terrestrial ecosystems in the global carbon source, Global Biogeochem Cycles, 12, 25, 19 98 4 Follet, R.F., Kimble, J.M., and Lal., R., The potential for U.S grazing lands to sequester... grazing in tall grass prairie, J Environ Qual., 27, 1539, 19 98 26 Haferkamp, M.R and MacNiel, M.D., Grazing effects on carbon dynamics in the northern mixed-grass prairie Environ Manage., 33, S462, 2004 27 Padbury, G et al., Agroecosystems and land resources of the Northern Great Plains, Agron J., 94, 251, 2002 28 Dugas, W.A., Micrometeorological and chamber measurements of CO2 flux from bare soil, Agric... McGinn, S.M., and McLean, H.D.J., Effects of soil temperature and moisture on soil respiration in barley and fallow plots, Can J Soil Sci., 79, 5, 1999 30 McGinn, S.M and King, K.M., Simultaneous measurements of heat, water vapour and CO2 fluxes above alfalfa and maize, Agric For Meteorol., 49, 331, 1990 31 De Jong, E., Schapperd, H.J.V., and MacDonald, R.B., Carbon dioxide evolution and cultivated... The potential for U.S grazing lands to sequester soil carbon, in The Potential of U.S Grazing Lands to Sequester Carbon and Mitigate the Greenhouse Effect, R.F Follet et al., Eds., Lewis Publishers, Boca Raton, FL, 2001, 401 5 Fan, S et al., Science, 282 , 442, 19 98 6 Clark, E.A., Buchanan-Smith, J.G., and Weise, F.S., Intensively managed pastures in the Great Lakes Basin A future oriented review, Can... Ecoregions The Ecosystem Geography of Oceans and Continents, Springer, New York, 19 98 14 MacAlpine, N.D et al., Resources for Beef Industry Expansion in Alberta, Alberta Agriculture Food and Rural Development, Edmonton, Alberta, 1997 15 Svejcar, T., Mayeux, H., and Angell, R., The rangeland carbon dioxide flux project, Rangelands, 19, 16, 1997 16 Dugas, W.A., Huer, M.L., and Mayeux, H.S., Carbon dioxide fluxes... fluxes over three Great Plains grasslands In The Potential of U.S Grazing Lands to Sequester Carbon and Mitigate the Greenhouse Effect, R.F Follet et al., Eds Lewis Publishers, Boca Raton, FL, 2001, 167 20 Sims, P.L and Bradford, J.L., Carbon dioxide fluxes in a southern plains prairie, Agric For Meteorol., 109, 117, 2001 21 Frank, A.B., Carbon dioxide fluxes over prairie and seeded pasture in the Northern... fluxes over bermudagrass, native prairie and sorghum, Agric For Meteorol., 93, 121, 1999 17 Angell, R.F et al., Bowen ratio and closed chamber carbon dioxide flux measurements over sagebrush steppe vegetation, Agric For Meteorol., 1 08, 153, 2001 18 Frank, A.B and Dugas, W.A., Carbon dioxide fluxes over a northern, semi arid mixed grass prairie, Agric For Meteorol., 1 08, 317, 2001 19 Frank, A.B et al., Carbon... than in the dormant period Periods of drought and lag phases of growth after cutting and hard grazing are periods when soil respiratory losses may be larger than net plant uptake on a daily basis These periods reduce the number of growing days further, seriously eroding potential ecosystem C sequestration To balance ecosystem CO2 exchange in this short-season temperate pasture system every effort must . 200 2-0 3 Harvest Regrowth Grazing Harvest Regrowth Grazing g m -2 d -1 mm d -1 g m -2 d -1 mm d -1 40 30 20 10 0 -1 0 -2 0 -3 0 -4 0 30 20 10 0 -1 0 -2 0 -3 0 -4 0 100 90 80 . Flux 166 8. 3.2.1 Initial Growth Period 167 8. 3.2.2 Regrowth 1 68 8.3.2.3 Grazing 1 68 8.3.2.4 Dormant Period 1 68 8.3.2.5 Spring 1 68 8.3.2.6 Diurnal CO 2 Flux 169 8. 3.3 Soil Respiration 170 8. 3.4. Skinner, and J. Casson CONTENTS 8. 1 Introduction 164 8. 2 Methods 165 8. 2.1 Net Ecosystem CO 2 Exchange 165 8. 2.2 Soil CO 2 Flux 166 8. 3 Results and Discussion 166 8. 3.1 Climate 166 8. 3.2 BREB