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Paul J. Pinter
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ABSTRACT: The diurnal behavior of canopy reflectance and emittance was characterized for six spring wheat cultivars using two ground-based radiometers that had spectral bandpass characteristics similar to the multispectral scanner and thematic mapper radiometers on Landsats 4 and 5. Nadir measurements of spectral reflectance and emittance were made over well-watered canopies with and without dew to determine its effect on each wavelength interval. The diurnal patterns of reflectances from canopies without dew were symmetric with respect to solar noon. However, when dew was present on canopies, morning reflectances in wavelengths shorter than 0.7 μm and longer than 1.15 μm were significantly different than those observed during the afternoon under similar solar zenith angles. Quantitative measurements of dew density in each cultivar established that moderately high dew levels increased reflectance in visible wavelengths by 40–60%, and decreased reflectance in wavelengths between 1.15 and 2.35 μm by 25–60%. No effect of dew was noted in the near-IR region of the spectrum between 0.7 and 1.1 μm or the thermal IR (10.4–12.5 μm).
Remote Sensing of Environment.
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Steven W. Leavitt,
Eldor A. Paul,
Bruce A. Kimball,
George R. Hendrey,
Jack R. Mauney,
Roy Rauschkolb,
Hugo Rogers,
Keith F. Lewin,
John Nagy, Paul J. Pinter,
Hyrum B. Johnson
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ABSTRACT: A role for soils as global carbon sink or source under increasing atmospheric CO2 concentrations has been speculative. Free-air carbon dioxide enrichment (FACE) experiments with cotton, conducted from 1989 to 1991 at the Maricopa Agricultural Center in Arizona, maintained circular plots at 550 μmol mol−1 CO2 with tank CO2 while adjacent ambient control plots averaged about 370 μmol mol−1 CO2. This provided an exceptional test for entry of carbon into soils because the petrochemically derived tank CO2 used to enrich the air above the FACE plots was depleted in both radiocarbon (14C content was 0% modern carbon (pmC)) and 13C () relative to background air, thus serving as a potent isotopic tracer. Flask air samples, and plant and soil samples were collected in conjunction with the 1991 experiment. Most of the isotopic analyses on the plants were performed on the holocellulose component. Soil organic carbon was obtained by first removing carbonate with HCl, floating off plant fragments with a NaCl solution, and picking out remaining plant fragments under magnification. The δ13C of the air above the FACE plots was approximately , i.e. much more 13C depleted than the background air of approximately −7.5‰. The δ13C values of plants and soils in the FACE plots were 10–12‰ and 2‰13C-depleted, respectively, compared with their control counterparts. The 14C content of the FACE cotton plants was approximately 40 pmC lower than tha tof the control cotton, but the 14C results from soils were conflicting and therefore not as revealing as the δ13C of soils. Soil stable-carbon isotope patterns were consistent, and mass balance calculations indicate that about 10% of the present organic carbon content in the FACE soil derived from the 3 year FACE experiment. At a minimum, this is an important quantitative measure of carbon turnover, but the presence of 13C-depleted carbon, even in the recalcitrant 6 N HCl resistant soil organic fraction (average age 2200 years before present (BP)), suggests that at least some portion of this 10% is an actual increase in carbon accumulation. Similar isotopic studies on FACE experiments in different ecosystems could permit more definitive assessment of carbon turnover rates and perhaps provide insight into the extent to which soil organic matter can accommodate the ‘missing’ carbon in the global carbon cycle.
Agricultural and Forest Meteorology.
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Elise Pendall,
Steven W. Leavitt,
Talbot Brooks,
Bruce A. Kimball, Paul J. Pinter,
Gerhard W. Wall,
Robert L. LaMorte,
Gabriele Wechsung,
Frank Wechsung,
Floyd Adamsen,
Allan D. Matthias,
Thomas L. Thompson
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ABSTRACT: SummaryUnderstanding the response of soil carbon (C) dynamics to higher atmospheric CO2 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 CO2 Enrichment (FACE) where spring wheat was grown in an open field at two CO2 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 CO2 was used to partition the soil CO2 flux into C from rhizosphere respiration and decomposition of pre-existing C. Decomposition contributed 100% of the soil CO2 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 Verständnis der Antwort der Kohlenstoff-Dynamik (C) im Boden auf höhere CO2-Konzentrationen in der Atmosphäre ist bedeutsam für die Bewertung des Potentials für die C-Sequestration in Zeiträumen von Jahrzehnten bis Jahrhunderten. Hier berichten wir über Veränderungen in der Bodenatmung unter Free-Air CO2 Enrichment (FACE), bei dem Sommerweizen in einem offenen Feld unter zwei CO2-Konzentrationen (Umgebung und Umgebung + 200 (mol mol−1) und unter natürlichen meteorologischen Bedingungen angebaut wurde. FACE erhöhte die Bodenatmungsraten um 40—70% während des Maximums des Weizenwachstums. Auf den FACE Plots wurde die Zusammensetzung an stabilen C Isotopen des Boden-CO2 genutzt, um den Boden CO2-Fluss zu C durch Rhizosphären-Atmung von der Zersetzung von zuvor existierendem C zu trennen. Die Zersetzung trug 100% des Boden-CO2-Flusses vor dem Beginn des Weizenwachstums bei, und nur 35—45% des Flusses während des Maximums des Wachstums. Die Zersetzungsraten waren nicht mit der Bodentemperatur korreliert, aber die Rhizosphären-Atmungsraten waren eng korreliert mit dem grünen Blattflächen-Index.
Basic and Applied Ecology.
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Paul J. Pinter
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ABSTRACT: Multispectral vegetation indices are often used to estimate the proportion of incident photosynthetically active radiation (PAR, 0.4–0.7 μm) that is absorbed by plants for potential use in photosynthesis. Field experiments were conducted near Phoenix, Arizona to establish such predictive capabilities for alfalfa and also to determine the effect of varying solar zenith angles (θs) on the relationships. The fraction of absorbed PAR () was measured using a 1-m long line quantum sensor. Canopy reflectance measurements (red, 0.61–0.68 μm; near-infrared, 0.79–0.89 μm) were obtained with a hand-held radiometer. Data were collected for θs from 27° to 72°. Statistically significant relationships were observed between and red reflectance factors (r2 = 0.97) and several commonly used vegetation indices (ratio, r2 = 0.96; normalized difference, r2 = 0.96; and soil adjusted, r2 = 0.93). Actual values of these parameters varied with time of day, but the relationships between and various indices derived from reflectance observations were independent of θs extending the potential usefulness of remote sensing approaches for inferring changes in at various times of the day and different seasons and latitudes.
Remote Sensing of Environment.
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ABSTRACT: Anticipated changes in global climate and atmospheric CO2 concentrations have very important, albeit poorly understood consequences for production agriculture. Effects of these changes on plants have usually been examined in controlled-environment enclosures, glass-houses, or open-top field chambers. Beginning in 1989, an innovative experimental free-air CO2 enrichment (FACE) facility was operated in central Arizona to evaluate crop response to increased CO2 levels within a large, open-field production environment. Cotton (Gossypium hirsutum L.) was grown for three consecutive seasons under well-watered conditions and exposed to either ambient (control, about 370 μmol mol−1) or elevated (FACE, 550 μmol mol−1) CO2 concentrations. Deficit irrigation regimes supplying 75% (beginning in July 1990) or 67% (beginning in mid-May 1991) of the crop's evapotranspiration requirement were included as additional treatment variables. Plant growth was monitored by periodic sampling. Canopy reflectances in visible (blue, 0.45-0.52 μm; green, 0.50-0.59 μm; red, 0.61-0.68 μm) and near-infrared (NIR; 0.79-0.89 μm) wavebands were measured frequently with an Exotech radiometer and related to absorbed photosynthetically active radiation (PAR; 0.4-0.7 μm) measured with a line quantum sensor. Dry biomass of plants in the FACE treatment was significantly (P < 0.05) greater than control values during each year of the study. The FACE plant canopy also absorbed significantly more PAR than controls during the early and middle portion of the 1990 and 1991 seasons. Light use efficiency (LUE, biomass produced per unit absorbed PAR) was significantly higher in FACE plots during each year. In the well-watered irrigation treatment, the 3 year mean LUE was 1.97 g MJ−1 for FACE and 1.56 g MJ−1 for controls. The deficit irrigation treatment in 1991 produced significantly smaller plants, which absorbed less PAR and had lower LUE than plants in the well-watered treatment (P < 0.05). No interaction was observed between CO2 and irrigation treatments. FACE research under realistic field conditions revealed positive consequences of increased CO2 on cotton plant biomass, PAR absorption, and LUE. It also demonstrated the effectiveness of this new technology for examining community-level plant responses to possible changes in global environment.
Agricultural and Forest Meteorology.
