Jay M. Ham

Fluxes of CO2, water vapor, and energy from a prairie ecosystem during the seasonal transition from carbon sink to carbon source

Abstract In many temperate-zone ecosystems, seasonal changes in environmental and biological factors influence the dynamics and magnitude of surface-atmosphere exchange. Research was conducted to measure surface-layer fluxes of CO 2 , water vapor, and energy in a C 4 -dominated tallgrass prairie during the autumnal transition from carbon sink to carbon source. Data were collected between DOY 220 and 320, 1996 on the Konza Prairie Research Natural Area near Manhattan, KS, USA. Mass fluxes were measured with a tower-based conditional sampling (CS) system, and the surface energy balance was measured with Bowen ratio (BR) methods. Soil-surface CO 2 fluxes were measured with a closed-chamber system. Carbon and energy fluxes decreased over the study period as the canopy senesced. When skies were clear, daily net CO 2 exchange (NCE) varied from a maximum gain of 17.8 g CO 2 m −2 day −1 on DOY 226 to a maximum loss of −10.3 g CO 2 m −2 day −1 on DOY 290. Over the 100-day study period, the ecosystem had a net loss of −217 g CO 2 m −2 , with the change from sink to source occurring on about DOY 255. Soil-surface CO 2 fluxes were −0.4 mg CO 2 m −2 s −1 at the start of the study but declined to −0.04 mg CO 2 m −2 s −1 on DOY 320. The Bowen ratio increased from 0.5 to 4 over the study period. The seasonal trend in NCE was governed by the senescence of the canopy and not abrupt changes in weather. Senescence also influenced canopy conductance, which caused a seasonal transformation in the surface energy balance. Data suggest that any climatic or management factors that affect the rate and timing of the autumnal sink-source transition can have a strong influence, on the carbon and water balance in the ecosystem.

Fluxes of CO2 and water vapor from a prairie ecosystem exposed to ambient and elevated atmospheric CO2

Increasing concentrations of atmospheric CO2 may alter the carbon and water relations of prairie ecosystems. A C4-dominated tallgrass prairie near Manhattan, KS, was exposed to 2 × ambient CO2 concentrations using 4.5 m-diameter open-top chambers. Whole-chamber net CO2 exchange (NCE) and evapotranspiration (ET) were continuously monitored in CO2-enriched and ambient (no enrichment) plots over a 34-d period encompassing the time of peak biomass in July and August, 1993. Soil-surface CO2 fluxes were measured with a portable surface chamber, and sap flow (water transport in xylem) in individual grass culms was monitored with heat balance techniques. Environmental measurements were used to determine the effect of CO2 on the surface energy balance and canopy resistances to vapor flux. In 1993, frequent rainfall kept soil water near field capacity and minimized plant water stress. Over the 34-d measurement period, average daily NCE (canopy photosynthesis — soil and canopy respiration) was 9.3 g CO2 m−2 in the ambient treatment adn 11.4 g CO2 m−2 under CO2 enrichment. However, differences in NCE were caused mainly by delayed senescence in the CO2-enriched plots at the end of the growing season. At earlier stages of growth, elevated CO2 had no effect on NCE. Soil-surface CO2 fluxes typically ranged from 0.4 to 0.66 mg CO2 m−2 s−1, but were slightly greater in the CO2_enriched chambers. CO2 enrichment reduced daily ET by 22%, reduced sap flow by 18%, and increased canopy resistance to vapor flux by 24 s m−1. Greater NCE and lower ET resulted in higher daytime water use efficiency (WUE) under CO2 enrichment vs. ambient (9.84 vs. 7.26 g CO2 kg−1 H2O). However, record high precipitation during the 1993 season moderated the effect of WUE on plant growth, and elevated CO2 had no effect on peak aboveground biomass. CO2-induced stomatal closure also affected the energy balance of the surface by reducing latent heat flux (LE), thereby causing a consequent change in sensible heat flux (H). The daytime Bowen ratio (H/LE) for the study period was near zero for the ambient treatment and 0.21 under CO2 enrichment.

Determination of soil water evaporation and transpiration from energy balance and stem flow measurements

Frequent measurements of soil water evaporation (E) and transpiration (T) are needed to quantify energy and water balances of sparse crops. Field experiments were conducted in Lubbock, TX to examine the feasibility of partitioning evapotranspiration (ET) from a cotton crop (Gossypium hirsutum L.) during periods of partial cover. The Bowen ratio energy balance method and heat balance stem flow measurements were used to make near-instantaneous measurements of ET and T, respectively. Transpiration on a unit land area basis was determined by normalizing stem flow measurements by leaf area or plant density. Soil water evaporation was computed as the difference between ET and T. The accuracy of the method was evaluated by comparing calculated values of E with measured values obtained from soil microlysimeters. Measurements over an 8-day period following an irrigation indicated that daily values of calculated E were within 0.5 mm of measured values in six out of seven comparisons when stem flow measurements were normalized on a leaf area basis. On average, daily calculated E was within ±11% of measured values. Calculated and measured cumulative E agreed to within 0.6 mm at the end of the evaluation period. Computing T by normalizing stem flow on a plant density basis resulted in overestimates of T and underestimates of E. Error analysis indicates that the precision of the E estimate decreases rapidly as evaporation becomes a smaller fraction of ET, and is influenced equally by the resolution of the stem flow and leaf area measurements. This study demonstrates that high frequency, independent measurements of soil and canopy evaporation can be obtained by measurement of ET and stem flow.

Fluxes of CO2 From Grazed and Ungrazed Tallgrass Prairie

Abstract To determine the impact of seasonal steer grazing on annual CO2 fluxes of annually burned native tallgrass prairie, we used relaxed eddy accumulation on adjacent pastures of grazed and ungrazed tallgrass prairie from 1998 to 2001. Fluxes of CO2 were measured almost continuously from immediately following burning through the burn date the following year. Aboveground biomass and leaf area were determined by clipping biweekly during the growing season. Carbon lost because of burning was estimated by clipping immediately prior to burning. Soil CO2 flux was measured biweekly each year using portable chambers. Steers were stocked at twice the normal season-long stocking rate (0.81 ha steer−1) for the first half of the grazing season (∼ May 1 to July 15) and the area was left ungrazed the remainder of the year. That system of grazing is termed “intensive-early stocking.” During the early growing season, grazing reduced net carbon exchange relative to the reduction in green leaf area, but as the growing season progressed on the grazed area, regrowth produced younger leaves that had an apparent higher photosynthetic efficiency. Despite a substantially greater green leaf area on the ungrazed area, greater positive net carbon flux occurred on the grazed area during the late season. Net CO2 exchange efficiency was greatest when grazing utilization was highest. We conclude that with grazing the reduced ecosystem respiration, the open canopy architecture, and the presence of young, highly photosynthetic leaves are responsible for the increased net carbon exchange efficiency. Both GR and UG tallgrass prairie appeared to be carbon-storage neutral for the 3 years of data collection (1998 ungrazed: −31 g C·m−2, 1998 grazed: −5 g C·m−2; 1999 ungrazed: −40 g C·m−2, 1999 grazed: −11 g C·m−2; 2000 ungrazed: +66 g C·m−2, 2000 grazed: 0 g C·m−2).

Aerodynamic and surface resistances affecting energy transport in a sparse crop

Aerodynamic and surface properties of the soil and canopy affect energy transport within sparse crops. A study was conducted with cotton (Gossypium hirsutum L.) to evaluate the feasibility of using both surface and within-canopy resistances to describe heat and water vapor transport from a row crop at partial cover. Bowen ratio measurements of the field energy balance and stem flow measurements of transpiration were coupled with detailed radiation measurements to determine sensible and latent heat flux from the soil and canopy separately. Flux measurements were combined with point measurements of temperature and vapor density to calculate the surface and aerodynamic resistances to heat and vapor transport using a one-dimensional model. The characteristics of the soil and canopy were described with bulk parameters, and a hypothetical within-canopy airstream was employed to model transport between the surface and the within-canopy air. Micrometeorological estimates of canopy surface resistance to vapor transport were similar in magnitude and behavior to data reported for full canopies. Computed values of soil surface resistance to vapor transport increased as the soil dried, reaching a maximum value of 1578 s m−1. However, results suggest that a sophisticated soil surface model will be needed to accurately quantify a diffusive soil resistance. An alternate approach for describing soil evaporation yielded more favorable results, in which soil water potential and temperature were used to estimate the vapor concentration at the immediate soil surface, eliminating the need for a diffusive soil resistance. Within-canopy aerodynamic and surface resistances had equal influence on vapor transport from the canopy. Aerodynamic resistances to transport from the soil surface were greater than those for the canopy at low wind speeds. All aerodynamic resistances tended to decrease as wind speed increased. However, calculated within-canopy aerodynamic resistances were highly variable and could not be adequately described by average wind speed, wind direction or canopy size. This study indicates that within-canopy transport from the soil and canopy cannot be partitioned or quantified using simple gradient diffusion relationships in combination with standard meteorological data.

Responses in stomatal conductance to elevated CO2 in 12 grassland species that differ in growth form

Responses in stomatal conductance (g st ) and leaf xylem pressure potential (ψ leaf ) to elevated CO2 (2x ambient) were compared among 12 tallgrass prairie species that differed in growth form and growth rate. Open-top chambers (OTCs, 4.5 m diameter, 4.0 m in height) were used to expose plants to ambient and elevated CO2 concentrations from April through November in undisturbed tallgrass prairie in NE Kansas (USA). In June and August, ψ leaf was usually higher in all species at elevated CO2 and was lowest in adjacent field plots (without OTCs). During June, when water availability was high, elevated CO2 resulted in decreased g st in 10 of the 12 species measured. Greatest decreases in g st (ca. 50%) occurred in growth forms with the highest potential growth rates (C3 and C4 grasses, and C3 ruderals). In contrast, no significant decrease in g st was measured in the two C3 shrubs. During a dry period in September, reductions in g st at elevated CO2 were measured in only two species (a C3 ruderal and a C4 grass) whereas increased g st at elevated CO2 was measured in the shrubs and a C3 forb. These increases in g st were attributed to enhanced ψ leaf in the elevated CO2 plants resulting from increased soil water availability and/or greater root biomass. During a wet period in September, only reductions in g st were measured in response to elevated CO2. Thus, there was significant interspecific variability in stomatal responses to CO2 that may be related to growth form or growth rate and plant water relations. The effect of growth in the OTCs, relative to field plants, was usually positive for g st and was greatest (>30%) when water availability was low, but only 6–12% when ψ leaf was high. The results of this study confirm the importance of considering interactions between indirect effects of high CO2 of plant water relations and direct effects of elevated CO2 on g st , particularly in ecosystems such as grasslands where water availability often limits productivity. A product of this interaction is that the potential exists for either positive or negative responses in g st to be measured at elevated levels of CO2.

Modeling the effect of mulch optical properties and mulch-soil contact resistance on soil heating under plastic mulch culture☆

Abstract A numerical model was developed to simulate the effect of plastic mulches on the field energy balance and soil temperature regime. Newton-Raphson methods were used to solve the energy balances of the soil and mulch simultaneously. The resultant temperature of the soil surface was used as a boundary condition for an implicit finite difference model of soil heat flow. The model contained a detailed radiation section to account for mulch optical properties in shortwave and longwave bands. Heat transfer between the mulch and soil surface was modeled using a thermal contact resistance, and transport in the boundary layer was quantified using flux profile theory. The effect of evaporation and condensation on the underside of the mulch was not considered. The accuracy of the model was verified by comparing simulated soil temperatures with field data collected under five commercially available mulches. Simulated average daily temperatures were within ±1.3°C of the observed values and root mean square errors were less than 2.5°C in all cases. The model showed that mulch optical properties strongly influenced the way energy was partitioned at the surface. Complete characterization of the mulches in the longwave spectrum was required for realistic simulation. Changing longwave reflectance by only 0.3, for example, caused maximum soil temperature to change by 5°C under some mulches. Thermal contact resistance between the mulch and soil affected temperatures by up to 20°C beneath mulches that strongly transmitted or absorbed radiation. However, contact resistance was not an important factor for mulches that had nearly equal shortwave transmittance and absorptance. Results demonstrate that a complex relationship exists between mulch optical properties and thermal contact resistance. Modification of mulch longwave properties during manufacturing and control of thermal contact resistance during mulch installation appear to be two promising areas for research. An alternative numerical scheme was evaluated that utilized linear forms of the energy balance equations. The linearized model produced results almost identical to the original model, but required less computing time and could be adapted easily to simulate multiple mulch layers.

Positional variation in the soil energy balance beneath a row-crop canopy☆

When crops are grown in a row configuration, differential shading of the soil, coupled with other soil-canopy micrometeorological interactions, may result in large positional variations in the soil energy balance. Experiments were conducted near Manhattan, KS, to examine positional variations in the soil energy balance beneath a soybean (Glycine max) canopy during periods of partial cover. Continuous measurements of net radiation (Rn) and soil heat flux (G) were obtained at four equally spaced positions between one pair of plant rows. Net radiation was calculated from radiation, temperature, and emissivity measurements of the soil, canopy, and sky. Soil heat flux was determined at each location from detailed heat flux and subsurface temperature measurements. When the soil surface was dry, sensible heat flux (H) was estimated as a residual of the heat balance by assuming that latent heat flux was negligible. Measurements of H and soil-air temperature gradients were used to estimate aerodynamic transport coefficients at each position. Results indicate that the magnitude and pattern of the soil heat balances are strongly dependent on location beneath the canopy. The patterns of G, Rn, and surface temperature at each measurement position were associated with the patterns of shortwave soil irradiance, which were functions of sun-canopy geometry. Soil irradiance between the plant rows sometimes exceeded global irradiance because of reflected radiation from the canopy. Daily Rn between the rows was five to ten times greater than that measured directly beneath the canopy, depending on environmental conditions. Large positional variations in G were also documented. Temperature differences between the sunlit and shaded portions of the surface exceeded 25°C when the surface was dry. Air temperatures, measured at 5 cm above each position, were also dependent on position with respect to the plant rows. When the surface was dry, the majority of sensible heat flux occurred from the soil directly between the plant rows, exceeding 400 W m−2 under certain conditions. Estimates of local aerodynamic transfer coefficients for the soil surface ranged from 1 to 50 mm s−1, but were highly variable and not correlated with above-canopy windspeeds or position beneath the canopy. Results suggest that positional variation in aerodynamic transport from the soil cannot be discerned with the methods used in this study.

MEASUREMENT AND MODELING OF SOIL CO2 FLUX IN A TEMPERATE GRASSLAND UNDER MOWED AND BURNED REGIMES

Soil-surface CO2 flux (R), which is a large component of the carbon (C) budgets in grasslands, usually is measured infrequently using static or dynamic chambers. Therefore, to quantify annual C budgets, estimates of RS are required during days when no direct measurements of RS are available. Other researchers have developed empirical models based on soil temperature, soil volumetric water content (0), and leaf area index (LAI) that have provided reasonable estimates of RS during the growing season in ungrazed tallgrass prairie. However, the effects of mowing and grazing, which are common in grass- lands, on predictions of RS from those models are uncertain. Predictions of RS during dormancy (postsenescence to spring fire) also are uncertain. Data from a year-long mowing study, which simulated grazing, were used to refit these models. Output from the models then was compared to independent data collected from nearby prairie sites. Results showed that LAI must be included to accurately estimate RS in mowed prairie ecosystems. When LAI was not included in the model, predicted daily RS following mowing was nearly four times greater than measured Rs, and cumulative, annual RS was overestimated by 95-102%. When LAI was included in the model, predictions of RS were comparable to measured RS in the mowing study. Annual estimates of cumulative RS ranged from 3.93 to 4.92 kg C02/ M2. When comparing the model with independent chamber data from nearby sites, cumu- lative RS during those studies was within ?9% of cumulative estimates calculated from measured Rs. The model overestimated daily RS during a dry period, suggesting a nonlinear response of RS to soil water content; soil water matric potential may be more appropriate than Ov for modeling Rs. Data suggest that Rs, in addition to being dependent on soil temperature and soil water content, is dependent on the photosynthetic capacity of the canopy and the subsequent translocation of C belowground.

PHOTOSYNTHETIC GAS EXCHANGE AND WATER RELATION RESPONSES OF THREE TALLGRASS PRAIRIE SPECIES TO ELEVATED CARBON DIOXIDE AND MODERATE DROUGHT

Undisturbed tallgrass prairie was exposed to ambient and elevated (twice-ambient) levels of atmospheric CO2 and experimental dry periods. Seasonal and diurnal midday leaf water potential (Ψleaf), net photosynthesis Anet, and stomatal conductance (gs) responses of three tallgrass prairie growth forms—a C4 grass, Andropogon gerardii; a broad-leaved woody C3 shrub, Symphiocarpos orbiculatus; and a C3 perennial forb, Salvia pitcheri—were assessed. Ψleaf in A. gerardii and S. orbiculatus was higher under elevated CO2, regardless of soil moisture, while Ψleaf in S. pitcheri responded only to drought. Elevated CO2 always stimulated Anet in the C3 species, while A. gerardii Anet increased only under dry conditions. However, Anet under elevated CO2 in the C3 species declined with drought but not in the C4 grass. Under wet conditions, gs reduced in elevated CO2 for all species. During dry periods, gs at elevated CO2 was sometimes higher than in ambient CO2. Our results support claims that elevated CO2 will stimulate tallgrass prairie productivity during dry periods and possibly reduce temporal and spatial variability in productivity in these grasslands.