William A. Dugas

Soil CO2 flux in a tallgrass prairie.

Abstract Soils are an important component of the global carbon budget due to their large C storage capacity and the ability to replenish atmospheric C via soil surface CO2 flux. Our objectives were to quantify year-round soil CO2–C fluxes in a tallgrass prairie and to develop an equation to predict flux using soil temperature and soil water content. Soil CO2–C flux, soil temperature and soil water were measured on selected days throughout the year from 1993 through 1998, n=216, at the Blackland Research Center, Temple, Texas, USA. On any date, there was little difference in average daily soil temperatures among years, but there were large differences in soil water content among years that often were related to differences in precipitation totals. Soil CO2–C flux had a seasonal pattern that was more similar to soil temperature than soil water (minimum in the winter and maximum in the early summer). Average annual soil CO2–C fluxes, which were 1.6, 1.3, 1.2, 1.0, 2.1 and 1.5 kg CO2–C m−2 yr−1 in 1993 through 1998, respectively, increased with annual precipitation. Regressed separately, the exponential relationship between soil CO2–C flux and soil temperature accounted for approximately 46% of flux variability while a quadratic relationship between flux and soil water content accounted for 26% of the variability. Both terms were combined into one equation that explained about 52% of the flux variance. Predicted and measured fluxes showed similar patterns throughout the year, there was little bias between predicted and measured fluxes, averages were essentially equal and the root mean square error between measured and predicted fluxes was about 38% of the average flux. The equation accounted for 76% of flux variability of an independent data set from the Konza Prairie in Kansas. The relationship between flux, soil temperature and soil water content should provide accurate predictions of soil CO2 flux in tallgrass prairies in the midwestern US.

Bowen ratio, eddy correlation, and portable chamber measurements of sensible and latent heat flux over irrigated spring wheat*

Measurements of the latent (LE) and sensible (H) heat flux density in the atmospheric boundary layer of irrigated crops have applications for understanding processes in agriculture and meteorology and for water management. The objective of this research was to compare measured Bowen ratios and calculated LE and H from four Bowen ratio systems (BR1–BR4) of different design with each other and with fluxes measured by three sets of eddy correlation instrumentation (H and LE) and a portable chamber (LE). Measurements were made on 9 and 10 April 1989 in an irrigated wheat field at the Maricopa Agricultural Center near Maricopa, Arizona. The Bowen ratio system designs varied in terms of temperature and humidity sensors and measurement arm movement. Bowen ratios were lower (more negative) on 9 April for all of the systems. The range of the four Bowen ratios was greatest in the early morning and late afternoon (±0.1) and least around noon (+0.02). Measured net radiation and soil heat flux density were constant in the Bowen ratio LE calculations. The range of daytime LE from the four systems on 9 April and from three on 10 April was 11% and 1% of the mean LE, respectively. The three eddy correlation H measurements were essentially equal to each other. The average eddy correlation H was 82% and 69% of Bowen ratio H on 9 and 10 April, respectively whilst the eddy correlation LE was 77% and 67% of Bowen ratio LE on the two days. On 9 and 10 April, portable chamber LE was greater than Bowen ratio LE during periods of southerly winds owing to the effect of advected energy to the southern field edge where chamber measurements were made. On 10 April, portable chamber LE was 125% of Bowen ratio LE. This study has shown that: (1) Bowen ratios from instrumentation of different designs were similar; (2) eddy correlation H from three systems were similar to each other and were slightly less than Bowen ratio H; (3) eddy correlation LE was consistently and significantly less than Bowen ratio LE; (4) measurements of portable chamber LE on the edge of a field were affected by surrounding conditions.

Micrometeorological and chamber measurements of CO2 flux from bare soil

Accurate measurements of carbon dioxide (CO2) flux from soil are important because this flux is an important component of the surface carbon budget, and a good indicator of the level of soil microbial and root activity. Half-hour CO2 fluxes from bare soil were measured using soil chamber and Bowen ratio/energy balance (BREB) methods for 4 days in December 1992, at the Blackland Research Center, Temple, TX. Soil chamber CO2 measurements were made sequentially at nine positions in the field. Three CO2 BREB systems were used. The CO2 flux was ≈ 0 in the early morning and after sunset and was maximum (slightly less than 0.1 mg m−2 s−1 (2.3 μmol m−2 s−1)) near midday. The coefficient of variation (CV) of chamber CO2 fluxes across the nine positions averaged 40% throughout the day, indicating the need for a large number of chamber measurements to obtain a representative CO2 flux measurement. The CV of the three daily BREB CO2 fluxes was less than 2%, indicating the BREB CO2 fluxes from the three systems were equal. There was good agreement between fluxes from the two methods. The average chamber and BREB CO2 fluxes for the entire period of measurements were 0.039 and 0.042 mg m−2 s−1, respectively, while the root mean square difference between half-hour fluxes from the two methods was 0.017 mg m−2 s−1. The methods are complementary and can both be used for soil CO2 flux measurements. The chamber method is low cost and easy to use, and offers the possibility of replicated measurements over space. The BREB method integrates over a large spatial area and is thus less affected by the high spatial variability of soil CO2 flux.

Carbon fluxes on North American rangelands

Abstract Rangelands account for almost half of the earths land surface and may play an important role in the global carbon (C) cycle. We studied net ecosystem exchange (NEE) of C on eight North American rangeland sites over a 6-yr period. Management practices and disturbance regimes can influence NEE; for consistency, we compared ungrazed and undisturbed rangelands including four Great Plains sites from Texas to North Dakota, two Southwestern hot desert sites in New Mexico and Arizona, and two Northwestern sagebrush steppe sites in Idaho and Oregon. We used the Bowen ratio-energy balance system for continuous measurements of energy, water vapor, and carbon dioxide (CO2) fluxes at each study site during the measurement period (1996 to 2001 for most sites). Data were processed and screened using standardized procedures, which facilitated across-location comparisons. Although almost any site could be either a sink or source for C depending on yearly weather patterns, five of the eight native rangelands typically were sinks for atmospheric CO2 during the study period. Both sagebrush steppe sites were sinks and three of four Great Plains grasslands were sinks, but the two Southwest hot desert sites were sources of C on an annual basis. Most rangelands were characterized by short periods of high C uptake (2 mo to 3 mo) and long periods of C balance or small respiratory losses of C. Weather patterns during the measurement period strongly influenced conclusions about NEE on any given rangeland site. Droughts tended to limit periods of high C uptake and thus cause even the most productive sites to become sources of C on an annual basis. Our results show that native rangelands are a potentially important terrestrial sink for atmospheric CO2, and maintaining the period of active C uptake will be critical if we are to manage rangelands for C sequestration.

Effects of free-air CO2 enrichment on energy balance and evapotranspiration of cotton

Abstract The effects of free-air CO2 enrichment (FACE) at 550 μmol mol−1 on the energy balance and evapotranspiration, ET, of cotton (Gossypium hirsutum L.) were investigated. Latent heat flux, λET was calculated as the residual in an energy balance approach from determinations of net radiation, Rn minus surface soil heat flux, G0, minus sensible heat flux, H. Rn was directly measured. G0 was determined from measurements with soil heat flux plates at 10 mm depth, corrected for temperature changes in the soil above. H was determined from measurements of air temperature with aspirated psychrometers, of foliage temperature with IR thermometers, and of wind speed with cup anemometers. Under ambient CO2 (control) conditions (about 370 μmol mol−1), the λET from the energy balance approach agreed fairly well with values from several other methods, including the Bowen ratio method, lending credence to the technique. However, the results had an uncertainty of the order of 20% associated with the Rn measurements. Therefore, an apparent increase in ET of about 13% in the FACE plots was judged insignificant. The conclusion that any effects of CO2 enrichment to 550 μmol mol−1 on the ET of cotton were too small to be detected was consistent with the results of other investigators who determined ET in the same experiment using stem flow gauges and the soil water balance.

Chamber and micrometeorological measurements of CO2 and H2O fluxes for three C4 grasses

Accurate measurements of surface fluxes of carbon dioxide (CO2) and water (H2O) are important for several reasons and can be made using several types of instrumentation. For three C4 grasses—bermudagrass (Cynodon dactylon (L.) Pers.), a mixed species native tallgrass prairie, and sorghum (Sorghum bicolor (L.) Moench.)—we measured evapotranspiration (ET) using a canopy chamber (CC) and Bowen ratio/energy balance (BREB) instrumentation and we measured leaf CO2 uptake using a leaf chamber (LC), and, after accounting for soil CO2 fluxes, we calculated leaf uptake using a CC and BREB instrumentation. In addition, soil CO2 fluxes from bare soil were measured using a CC and soil chamber (SC). Measurements were made on 4 and 5 May 1994 at the Blackland Research Center, Temple, TX. Flux of CO2 into the leaf was considered positive and was expressed per unit ground area. Half-hour CC ET measurements were consistently and substantially greater than BREB measurements for all grasses, perhaps because of increased soil evaporation due to greater turbulence inside the CC. Leaf CO2 uptake measured using the three methods showed similar diurnal trends for all grasses (responding, primarily, to changes in photosynthetic photon flux density), but consistently tended to be greatest for BREB measurements. The regression equation for LC CO2 uptake as a function of BREB uptake had a slope not statistically different from 1.0, with large scatter likely because of limited leaf area sampled. CC CO2 uptake was consistently the least, partly because we may have underestimated soil CO2 flux in the CC. Half-hour soil CO2 fluxes from the CC were significantly greater (P < 0.05) than those from the SC for about two-thirds of the day on bare soil, perhaps because of large chamber ventilation rates. Differences of daytime soil CO2 fluxes averaged 0.07 mg m−2 s−1 (1.0 mg m−2 s−1 ≈ 22.7 μ mol m−2 s−1). These results show the consistency, repeatability and, we believe, accuracy of leaf CO2 uptake and soil CO2 flux measurements made using all methods.

Sap flow measurements of transpiration from cotton grown under ambient and enriched CO2 concentrations

Increasing atmospheric CO2 concentration has many implications for agriculture and forestry, one of which is the effect it will have on transpiration (T). The objective of this work was to quantify T of cotton (Gossypium hirsutum L.) grown in the field under ambient (370 μmol mol−1) and enriched (550 μmol mol−1) CO2 concentrations. Measurements were made in 1990 and 1991 at the Maricopa Agricultural Center, Arizona. Constant-power sap flow gauges were used to measure T. In 1990, three plants and in 1991, 10 plants were simultaneously instrumented with gauges in each of the CO2 treatments. Leaf area of plants with gauges was measured. T measured by sap flow was compared with evapotranspiration (ET) calculated by water balance in 1990 and with T calculated by water balance in 1991. Soil evaporation was measured using microlysimeters in 1991, and was found to be essentially equal (approximately 0.8 mm day−1, or about 10% of T) in the two CO2 treatments. There were no consistent differences in leaf area of plants with gauges between the two CO2 treatments. Sap flow, for periods from 15 min to 2 weeks, was not significantly different between the two CO2 treatments in either year, except for a few days in 1990. In 1991, the coefficient of variation of daily sap flow across plants was the same (about 30%) for both CO2 treatments throughout the year. The water balance ET (1990) and T (1991) were similar to sap flow in both years, and also showed no effect of CO2 treatment. These results show that for this crop, grown under well-watered and high-fertility conditions, there was no effect of CO2 on T, on a per unit ground area or per plant basis. These results are relevant for assessing the effects of increasing atmospheric CO2 concentrations on transpiration by cotton.

Evaluating canopy temperature-based indices for irrigation scheduling

SummarySince the development of commercial versions of infrared sensors, they have been increasingly used to determine canopy temperature and schedule irrigations. However, some shortcomings of the technique have been identified, among them the sensitivity of canopy temperature measurements to weather fluctuations. Based on field and computer simulated data, an analysis of the suitability of crop water stress indices (CWSIs) developed from canopy temperature under variable weather conditions was done. Important day to day fluctuations of CWSI values determined using an empirical baseline (empirical CWSI) appeared common for nonstressed crops, particularly under low vapor pressure deficit conditions. These fluctuations generate uncertainty in the use of this empirical index to determine needs for irrigation. The use of an improved index (theoretical CWSI) requiring measurements of net radiation, soil heat flux and wind speed, and estimates of aerodynamic and canopy resistances reduced but did not eliminate these fluctuations. Results using a simulation model showed that the empirical CWSI provided late indication of irrigation needs, after some water stress has developed, which may limit its application for crops sensitive to water stress. These simulations also indicated that the theoretical CWSI was able to track the development of water stress and provide reasonable indication of irrigation needs. However, this result may not be fully realized in field applications where the determination of CWSI may be affected by various sources of variability which are not accounted for by the model.

Heat balance, porometer, and deuterium estimates of transpiration from potted trees

Abstract The objective of this study was to quantify the accuracy of transpiration estimated using heat balance gauges, porometry, and deuterium tracers methods. Measurements were made on one Eucalyptus tree and three Prunus trees during three measurement periods (MPs) in the summer of 1991 at the Institute of Hydrology, Wallingford, UK. Gravimetric measurements of transpiration ( T g ) of the potted trees were used as the standard for comparison. Continuous estimates of transpiration were made using constant-power heat balance gauges ( T h ). A stomatal conductance-based transpiration ( T s ) was calculated using the Penman-Monteith equation. Deuterium oxide was used as a tracer for calculating transpiration ( T d ) for 5-day periods. There were no systematic differences between daily T g and T h for the one Prunus tree for which daily T g could be accurately measured, or, for all trees, between T g and T h for the entire MP. The root mean square difference (RMSD) between daily T g and T h was 0.26 kg per tree day −1 for the one Prunus tree. There was a consistent underestimation of daily T g by T s , while T h on these days was closer and not consistently different. The RMSD between daily T g and T d was 1.0 kg per tree day −1 , more than twice the error for T h . For daily and 5-day periods, the T h RMSD was lower than the RMSD from T s , and T d , respectively. Positive and negative aspects of each method are discussed.

Transpiration from sorghum and soybean growing under ambient and elevated CO2 concentrations

The increasing concentration of carbon dioxide in the atmosphere ([CO,]) has several direct effects on plants and these effects may be different for C, and C, plants. Our objective was to measure hourly and daily whole-plant transpiration rates from the C, plant grain sorghum (Sorghum bicolor (L.) Moench) and the C, plant soybean (Glycine max (L.) Merr.) grown under ambient (359 pmolC0, mol-’ dry atmospheric air) and elevated (705 ymolmol- ‘> [CO,] values. Transpiration measurements were made for 22 days in August 1994 at Auburn, Alabama, USA, using stem flow gauges on plants growing in open top chambers, n = 8 for each [CO,] and species. Leaf area averaged slightly more than 0.1 m* per plant for sorghum and about 0.2 m* per plant for soybean. Averages (15 min and daily) of transpiration, per unit leaf area, were consistently greater from plants growing under the ambient [CO,] for both sorghum and soybean. Average daily transpiration from plants growing under the elevated [CO,] was significantly smaller (P = 0.05) on all but 2 days for soybean and on 9 of the 22 days of measurements for sorghum. Average daily sorghum transpiration was 1 128gm-2 day-’ and 772gm-* day-’ from plants growing under an ambient and elevated [CO,], respectively. Corresponding soybean averages were 731 gm-* day-’ and 416gm-* day ‘. The transpiration reduction under elevated [CO,] was greater for the C, plant soybean than for the C, plant sorghum. These results support previous studies showing that transpiration, per unit leaf area, from sorghum and soybean will both be reduced if atmospheric [CO,] continues to increase, although the reduction may be greater for C, plants.