Allan D. Matthias

Free-air CO2 enrichment and soil nitrogen effects on energy balance and evapotranspiration of wheat

In order to determine the likely effects of the increasing atmospheric CO2 concentration on future evapotranspiration, ET, plots of field-grown wheat were exposed to concentrations of 550 µmol/mol CO2 (or 200 µmol/mol above current ambient levels of about 360 µmol/mol) using a free-air CO2 enrichment (FACE) facility. Data were collected for four growing seasons at ample water and fertilizer (high N) and for two seasons when soil nitrogen was limited (low N). Measurements were made of net radiation, Rn; soil heat flux; air and soil temperatures; canopy temperature, Ts; and wind speed. Sensible heat flux was calculated from the wind and temperature measurements. ET, that is, latent heat flux, was determined as a residual in the energy balance. The FACE treatment increased daytime Ts about 0.6° and 1.1°C at high and low N, respectively. Daily total Rn was reduced by 1.3% at both levels of N. Daily ET was consistently lower in the FACE plots, by about 6.7% and 19.5% for high and low N, respectively.

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.

Leaf nitrogen concentration of wheat subjected to elevated [CO2] and either water or N deficits

Leaf N concentration is important because it is associated with the CO2 assimilatory capacity of crops, and in grasslands, it is an important determinant of forage nutritive value. Consequently, the productivity of both domestic and native animals in future global environments may be closely linked to possible changes in leaf N concentration of grasses. Since grasslands are frequently subjected to water-deficit or N-deficit conditions, it is important to investigate the interactive responses between elevated [CO2] and these stress conditions. Therefore, this 4-year research program was undertaken with wheat (Triticum aestivum L.) as a model system for forage grasses, to document the potential changes in leaf N concentration in response to global environment changes. Wheat crops grown under field conditions near Phoenix, AZ, USA, were subjected to elevated [CO2] and either water-deficit or N-deficit treatments using large Free Air Carbon dioxide Enrichment (FACE) arrays. Surprisingly, the elevated [CO2] treatment under optimum conditions resulted in little change in leaf N concentration. Therefore, no change in the nutritive value of forage from highly managed pastures would be expected. Further, water-deficit treatment had little influence on leaf N concentration. To some extent, the lack of response to the water-deficit treatment resulted because severe deficits did not develop until late in the growing seasons. Only on one date late in the season was the water-deficit treatment found to result in decreased leaf N concentration. The low N treatment in combination with elevated [CO2], however, had a large influence on leaf N concentration. Low levels of applied N resulted in decreased leaf N concentration under both [CO2] treatments, but the lowest levels of leaf N concentration were obtained under elevated [CO2] through much of the growing season. These results point to a potential problem with grasslands in that the nutritive value of the forage consumed by animals will be decreased under future global environment changes.

CO2 enrichment and soil nitrogen effects on wheat evapotranspiration and water use efficiency

Evapotranspiration (ET) and water use efficiency (WUE) were evaluated for two spring wheat crops, grown in a well-watered, subsurface drip-irrigated field under ambient (about 370 mmol mol 1 during daytime) and enriched (200mmol mol 1 above ambient) CO2 concentrations during 1995‐1996 and 1996‐1997 in Free-Air CO2 Enrichment (FACE) experiments in central Arizona. The enriched (FACE) and ambient (Control) CO2 treatments were replicated in four, circular plots, each 25 m in diameter. Two soil nitrogen (N) treatments, ample (High N) and limited (Low N), were imposed on one-half of each circular plot. Wheat ET, determined using soil water balance procedures, was significantly greater in High N than Low N treatments starting in late-March (anthesis) during both years. Differences in ET between CO2 treatments during the seasons were generally small and not statistically significant, however, there was a tendency for the ET to be lower for FACE than Control under the High N treatment. The reduction in the cumulative seasonal ET due to FACE averaged 3.7 and 4.0% under High N and 0.7 and 1.2% under Low N in the first and second years, respectively. However, WUE (grain yield per unit seasonal ET) was significantly increased for the FACE treatment under both soil N treatments. For the High N treatment, the WUE was 19 and 23% greater for FACE than Control and for the Low N treatment the WUE was 12 and 7% greater for FACE than Control in the 2 years, respectively. Published by Elsevier Science B.V.

Elevated CO2 stimulates soil respiration in a FACE wheat field

Summary Understanding the response of soil carbon (C) dynamics to higher atmospheric CO 2 concentrations is critical for evaluating the potential for soil C sequestration on time scales of decades to centuries. Here, we report on changes in soil respiration under Free-Air CO 2 Enrichment (FACE) where spring wheat was grown in an open field at two CO 2 concentrations (ambient and ambient+200 μmol mol −1 ), under natural meteorological conditions. FACE increased soil respiration rates by 40—70% during the peak of wheat growth. On the FACE plots, stable C isotopic composition of soil CO 2 was used to partition the soil CO 2 flux into C from rhizosphere respiration and decomposition of pre-existing C. Decomposition contributed 100% of the soil CO 2 flux before crop growth commenced, and only 35—45% of the flux at the peak of the growing season. Decomposition rates were not correlated with soil temperature, but rhizosphere respiration rates were strongly correlated with green leaf area index. Ein Verstandnis der Antwort der Kohlenstoff-Dynamik (C) im Boden auf hohere CO 2 -Konzentrationen in der Atmosphare ist bedeutsam fur die Bewertung des Potentials fur die C-Sequestration in Zeitraumen von Jahrzehnten bis Jahrhunderten. Hier berichten wir uber Veranderungen in der Bodenatmung unter Free-Air CO 2 Enrichment (FACE), bei dem Sommerweizen in einem offenen Feld unter zwei CO 2 -Konzentrationen (Umgebung und Umgebung + 200 (mol mol −1 ) und unter naturlichen meteorologischen Bedingungen angebaut wurde. FACE erhohte die Bodenatmungsraten um 40—70% wahrend des Maximums des Weizenwachstums. Auf den FACE Plots wurde die Zusammensetzung an stabilen C Isotopen des Boden-CO 2 genutzt, um den Boden CO 2 -Fluss zu C durch Rhizospharen-Atmung von der Zersetzung von zuvor existierendem C zu trennen. Die Zersetzung trug 100% des Boden-CO 2 -Flusses vor dem Beginn des Weizenwachstums bei, und nur 35—45% des Flusses wahrend des Maximums des Wachstums. Die Zersetzungsraten waren nicht mit der Bodentemperatur korreliert, aber die Rhizospharen-Atmungsraten waren eng korreliert mit dem grunen Blattflachen-Index.

Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere with variable soil nitrogen regimes. 2. Net assimilation and stomatal conductance of leaves.

Atmospheric CO2 concentration continues to rise. It is important, therefore, to determine what acclimatory changes will occur within the photosynthetic apparatus of wheat (Triticum aestivum L. cv. Yecora Rojo) grown in a future high-CO2 world at ample and limited soil N contents. Wheat was grown in an open field exposed to the CO2 concentration of ambient air [370 μmol (CO2) mol−1; Control] and air enriched to ∼200 μmol (CO2) mol−1 above ambient using a Free-Air CO2 Enrichment (FACE) apparatus (main plot). A High (35 g m−2) or Low (7 and 1.5 g m−2 for 1996 and 1997, respectfully) level of N was applied to each half of the main CO2 treatment plots (split-plot). Under High-N, FACE reduced stomatal conductance (gs) by 30% at mid-morning (2 h prior to solar noon), 36% at midday (solar noon) and 27% at mid-afternoon (2.5 h after solar noon), whereas under Low-N, gs was reduced by as much as 31% at mid-morning, 44% at midday and 28% at mid-afternoon compared with Control. But, no significant CO2 × N interaction effects occurred. Across seasons and growth stages, daily accumulation of carbon (A′) was 27% greater in FACE than Control. High-N increased A′ by 18% compared with Low-N. In contrast to results for gs, however, significant CO2 × N interaction effects occurred because FACE increased A′ by 30% at High-N, but by only 23% at Low-N. FACE enhanced the seasonal accumulation of carbon (A′′) by 29% during 1996 (moderate N-stress), but by only 21% during 1997 (severe N-stress). These results support the premise that in a future high-CO2 world an acclimatory (down-regulation) response in the photosynthetic apparatus of field-grown wheat is anticipated. They also demonstrate, however, that the stimulatory effect of a rise in atmospheric CO2 on carbon gain in wheat can be maintained if nutrients such as nitrogen are in ample supply.

Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere with variable soil nitrogen regimes. 3. Canopy architecture and gas exchange.

The response of whole-canopy net CO2 exchange rate (CER) and canopy architecture to CO2 enrichment and N stress during 1996 and 1997 for open-field-grown wheat ecosystem (Triticum aestivum L. cv. Yecora Rojo) are described. Every Control (C) and FACE (F) CO2 treatment (defined as ambient and ambient +200 μmol mol−1, respectively) contained a Low- and High-N treatment. Low-N treatments constituted initial soil content amended with supplemental nitrogen applied at a rate of 70 kg N ha−1 (1996) and 15 kg N ha−1 (1997), whereas High-N treatments were supplemented with 350 kg N ha−1 (1996 and 1997). Elevated CO2 enhanced season-long carbon accumulation by 8% and 16% under Low-N and High-N, respectively. N-stress reduced season-long carbon accumulation 14% under ambient CO2, but by as much as 22% under CO2 enrichment. Averaging both years, green plant area index (GPAI) peaked approximately 76 days after planting at 7.13 for FH, 6.00 for CH, 3.89 for FL, and 3.89 for CL treatments. Leaf tip angle distribution (LTA) indicated that Low-N canopies were more erectophile than those of High-N canopies: 48° for FH, 52° for CH, and 58° for both FL and CL treatments. Temporal trends in canopy greenness indicated a decrease in leaf chlorophyll content from the flag to flag-2 leaves of 25% for FH, 28% for CH, 17% for CL, and 33% for FL during 1997. These results indicate that significant modifications of canopy architecture occurs in response to both CO2 and N-stress. Optimization of canopy architecture may serve as a mechanism to diminish CO2 and N-stress effects on CER.

Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere with variable soil nitrogen regimes. 1. Leaf position and phenology determine acclimation response

We have examined the photosynthetic acclimation of wheat leaves grown at an elevated CO2 concentration, and ample and limiting N supplies, within a field experiment using free-air CO2 enrichment (FACE). To understand how leaf age and developmental stage affected any acclimation response, measurements were made on a vertical profile of leaves every week from tillering until maturity. The response of assimilation (A) to internal CO2 concentration (Ci) was used to estimate the in vivo carboxylation capacity (Vcmax) and maximum rate of ribulose-1,5-bisphosphate limited photosynthesis (Asat). The total activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), and leaf content of Rubisco and the Light Harvesting Chlorophyll a/b protein associated with Photosystem II (LHC II), were determined. Elevated CO2 did not alter Vcmax in the flag leaf at either low or high N. In the older shaded leaves lower in the canopy, acclimatory decline in Vcmax and Asat was observed, and was found to correlate with reduced Rubisco activity and content. The dependency of acclimation on N supply was different at each developmental stage. With adequate N supply, acclimation to elevated CO2 was also accompanied by an increased LHC II/Rubisco ratio. At low N supply, contents of Rubisco and LHC II were reduced in all leaves, although an increased LHC II/Rubisco ratio under elevated CO2 was still observed. These results underscore the importance of leaf position, leaf age and crop developmental stage in understanding the acclimation of photosynthesis to elevated CO2 and nutrient stress.

SOIL WATER AVAILABILITY AND TEMPERATURE DYNAMICS AFTER ONE-TIME HEAVY CATTLE TRAMPLING AND LAND IMPRINTING

Abstract Soil disturbance by livestock hooves and by mechanical cultivation has been suggested as a way to increase soil water intake on crusted rangeland soils. In conjunction with a seedling establishment study on a sandy loam soil in southern Arizona, soil water content and temperature were measured continuously for the 1988 and 1989 summer rainy seasons on seedbeds that were one‐time intensively trampled by cattle, furrowed with a land imprinter, or left undisturbed. Diurnal surface temperatures (1‐cm depth) fluctuated up to 31°C even when surface soils were wet. Maximum temperatures were lowest at the basin or bottom and highest on the south‐facing slope of the imprint furrows. Soils disturbed by trampling or imprinting wet up more (had a higher maximum water content) than undisturbed soils after summer rains in mid July to early August, but wet up less than undisturbed soils near the end of the rainy season, presumably due to reformation of the surface crust. Increased wetting of trampled or imprint...

Comparison of three resistance methods for estimating heat flux under stable conditions

Aerodynamic resistance to sensible heat transfer (rah) is an important parameter in bulk resistance models of energy exchange between the Earths surface and the atmosphere. To estimate sensible heat flux density (H) over irrigated fields, it is often necessary to evaluate rah under stable atmospheric conditions. This is because irrigated fields are often cooler than air temperature as a result of net radiative cooling at night and advective influences. Sensible heat fluxes are generally small at night, but may become a relatively large component of the surface energy budget during advective conditions. The purpose of this paper was to compare H modeled for stable conditions over an extensive area of irrigated bermudagrass (Cynodon dactylon) cv. Midiron) using three rah methods from the literature, and to determine the sensitivities of the methods to the various parameter inputs. In addition to comparing modeled H values with each other, comparisons were made with H values measured by eddy correlation over the bermudagrass. The three methods studied were expressed as functions of the bulk Richardson number (RiB). The methods differ mainly in the stability correction function, and the added resistance term (kB−1, where k is von Karmans constant and B−1 is a non-dimensional bulk parameter), which arises as a result of dissimilarities between heat and momentum transfer mechanisms near surface obstacles. Application of the methods to the bermudagrass data resulted in marked differences between the various modeled H values, and between modeled and measured H values. The modeled H values were more sensitive to surface emissivity and temperature inputs.