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The fluctuations of photosynthetically active radiation within different corn (Zea mays L.) varieties were studied in the frequency domain using spectral analysis. This approach which uses a single traversing sensor, provides a simple method for separating the temporal and spatial light fluctuations in a crop. It was found that the light spectra are influenced by the height of the light sensors within crop structure and by wind conditions. The most rapid fluctuations observed were about 10 cycles/second, which tends to discount their importance in the dark reaction of photosynthesis. Finally, these results give a better perspective on fluctuating light experiments.

A modified infrared CO2 gas analyzer, a small thermocouple assembly, a heated-thermocouple anemometer for horizontal wind, and a propeller-type vertical wind sensor were used to measure the eddy fluxes of heat and CO2 above a corn crop. Experimental results of these fluxes are discussed. The main sources of errors of the eddy fluxes using these instruments were estimated

Shawcroft, R. W., Lemon, E. R., Allen Jr., L. H., Stewart, D. W. and Jensen, S. E., 1974. The soil–plant–atmosphere model and some of its predictions. Agric. Meteorol., 14: 287–307. A general description of the soil–plant–atmosphere model (SPAM) is given. Emphasis is made as to the logical sequence of the operation of the model by use of various submodels depicting the soil, plant, and climatic interactions. Examples of the testing of the model are discussed. Some simulation studies are given to show how the model can be used in setting priorities on those variables that have the greatest influence on plant responses.

The penetration and distribution of light in leaf canopies are essential data to understanding the photosynthetic productivity of a crop. Light sensors which measure photosynthetically active radiation were constructed and traversed through the leaf canopies of six types of corn (Zea mays L.) to observe radiation penetration. Under clear sky conditions two major irradiance levels were observed in the corn canopies. One irradiance level was slightly less than the total radiation above the crop and represented the occurrence of sunflecks penetrating into the canopy. The second irradiance level was much less than the first and was comprised of sky radiation and radiation scattered by leaves. However, a decrease in the amount of direct solar radiation by either an overcast sky or a low sun angle resulted in a single, broad-band irradiance level. The data on sunfleck penetration also provided a basis for testing two physical models of light penetration in leaf canopies. Data from corn planted in a uniform, hexagonal pattern was in good agreement with the Poisson model which assumes a random leaf distribution. However, the Markov chain model which introduces a frequency gap parameter, λ, to account for nonuniform leaf distribution was required to obtain good agreement with data recorded in the canopy of corn planted in rows 76-cm apart. This was especially true for the top part of the row-planted corn where small, short leaves resulted in large gaps for radiation penetration. The Markov model would be necessary, for instance, to determine the penetration of photosynthetically active radiation to the leaves in the middle of the canopy. Nevertheless, the Poisson model gave good agreement with the measured values for radiation intercepted by the entire canopy of the row-planted crop.

The Bowen-ratio profile method for calculating total carbon dioxide, latent heat, and sensible heat flux density above a corn crop was used by measuring temperature, water vapor, and CO2 concentrations at several heights in the aerodynamic boundary layer of the crop. The ratio (α) of sensible heat flux density to carbon dioxide flux density as well as the Bowen ratio (β) were used in the computations. The two ratios, α and β were determined graphically from the slopes of linear plots of temperature vs CO2 concentration and vs water vapor. Each of the energy flux densities was computed from the two ratios and net radiation minus soil heat flux density. An analysis of probable error was performed on the Bowen-ratio profile method to evaluate the accuracy of the flux density estimates. Less than 10% error was found for latent heat flux density and less than 15% for carbon dioxide flux density under normal midday conditions for the instrumentation used. However, the carbon dioxide flux density error increased to over 40% when the sensible heat flux was small.

Leaf water potentials did not limit stomatal opening of Vitis labruscana Bailey cv. Concord during the summers of 1972 and 1973 in a New York vineyard. Midday leaf-water potentials ranged from −8 to −16 bars and were closely related to individual leaf irradiance. The diurnal variation of leaf, stem, and fruit cluster water potentials on a typical clear day were about 5, 11, and 6 bars, respectively. Water potential gradients at midday across the root, shoot, and petiole-leaf systems averaged about 10, 1 and 3 bars, respectively. The gradient across the root consistently increased throughout the day relative to plant transpiration rate. Minimum stomatal resistance on days after cold nights (less than 10°C) was 2.7 ± 1.1 s cm-1, while the mean resistance on all other days was 1.0 ± 0.5 s cm-1. Cool night temperatures inhibited stomatal opening and closing independently of leaf water potential.

The mean and short-term fluctuations of carbon dioxide concentration, air temperature, and horizontal wind speed were measured simultaneously within and above a maize crop. Although fluctuations were large at each of the 10 measurement heights, the largest were at the 180-cm level, the densest part of the crop. These short-term fluctuations indicated the broad range of environmental conditions to which plants are exposed (and which plants create or modify) in the field. The statistics of the distributions of these meteorological elements are discussed and the deviations from steady-state conditions are examined by determining linear trends. Finally, variance spectra showed the periodicity of the variations of these meteorological elements.

The vertical distribution of the photosynthetic fixation of carbon dioxide within a crop of corn was calculated from carbon dioxide profile data and transfer coefficients obtained by analysis of windspeed measurements. Infrared analyzers were used to measure the carbon dioxide concentration at several heights within and above the crop. The calculated total fixation for the day was approximately 60 g CO 2 in (equivalent to 470 pounds of sugar per acre per day). The results demonstrated the importance of the upper leaves in the fixation of carbon dioxide and showed the increased fixation by the lower leaves during periods of high light penetration. There was some indication that a coupling existed between the level of windspeed and fixation under conditions of high light and relatively low winds peed. With refinement in technique the method could be used to obtain more quantitative estimates of the distribution of photosynthesis in other crops.

Evapotranspiration is a function of soil, plant, and meteorological factors. Attempts to predict evapotranspiration without considering all pertinent factors can meet with only qualified success. It is found that evapotranspiration is controlled by: soil moisture tension, physiological factors, the relation of soil moisture of an irrigated area to that of its surroundings as well as purely meteorological factors of radiation, wind, air temperature, and humidity. Examples are given of the effects of these factors on measured evapotranspiration. View complete article To view this complete article, insert Disc 2 then click button8

Turbulent transfer within a crop of corn was characterized by an aerodynamic approach. The distributions of windspeed within an immature and a mature crop were measured using both cup and heated thermocouple anemometers. A combination of the statistical and mixing-length theories was employed to analyze the wind data for transfer coefficients. Eulerian time scales of turbulence were calculated from the windspeed fluctuations of selected 30-second periods of semi-steady wind. Momentum transfer coefficients were determined from the scale of turbulence by equating it to the mixing length and assuming isotropy. The resulting values were as large as 8,000 cm2 sec-1. The shearing stress profiles calculated with these values had unrealistically sharp maximums just below the top of the crop. This anomaly was considered an artifact of the method resulting from extreme anisotropy in the turbulent shear flow. The K values obtained by the statistical mixing length method were approximately ten times larger than those determined by logarithmic profile analysis. The latter values were considered more nearly correct and were used as a basis for correcting the other values. The results showed that the magnitude of turbulent transfer is several orders of magnitude greater than molecular diffusion even at levels deep within the crop. The transfer coefficient showed a marked attenuation with depth below the top of the corn but remained a function of windspeed at all heights.

Spectra of air pressure and wind were obtained that cover a frequency range from 10−4 to 102 Hz during both day and night with and without a corn crop. The pressure spectra were very similar to those previous workers obtained by different methods under different conditions. The spectral density for pressure decreased with approximately the −6/3 power of frequency for all conditions from about 108 to 10−3 μbar2/Hz over the entire frequency range. The corn crop had no obvious effects on the spectra, but the magnitude of the curves was strongly correlated with surface wind speed, at least for frequencies higher than 10−2.5 Hz.

In a previous communication a meteorological method of assessing the vertical distribution of photosynthesis and respiration activity in a corn crop was described. The method, however, is tedious. Here the authors describe a much simpler method. Results indicate that all the leaves of a corn crop appear to follow the same near-linear light response curve, at least above the compensation point. Below the compensation point, all leaves evidently respire very little.

An attempt has been made to bridge the gap between the knowledge of the oxygen requirements of roots which has been gained through laboratory studies and the empiricism which now exists in field studies of soil aeration. This objective has been sought through a theoretical approach which combines field measurements of the oxygen supply in the gaseous and liquid phases of the soil and known quantitative demands by plant roots for oxygen into a single expression derived from Fick's Law for radial diffusion. The concentration of oxygen at the root surface is the soil aeration parameter determined. The application of the theory to the quantitative evaluation of soil aeration conditions in two contrasting soil types, Amarillo fine sandy loam and Miller clay, has led to the following conclusions: 1.At field capacity, the concentration of oxygen at certain root surfaces is suboptimal for normal root respiration in Miller clay, but optimum in Amarillo fine sandy loam. 2.The concentration of oxygen at the root surfaces increases linearly with the logarithm of soil moisture tension. 3.The apparent diffusion path length in the liquid phase about plant roots is more often a limiting factor in normal root respiration than the gaseous composition, per se, in the soil pores. The above conclusions, based upon the application of the theory, were tested against plant response. The results obtained indicate that the method shows promise as a tool to estimate the supply of oxygen to roots under field conditions.

An intensive study was conducted to evaluate the movement of dieldrin and heptachlor vapor from a treated watershed. The successful application of micrometeorological estimates of evapotranspiration using average hourly data indicates that reasonable confidence can be placed on the computation of vertical pesticide flux rates using the equation: $$P = K_Z \left( {\frac{{\partial p}}{{\partial Z}}} \right)$$ where vertical diffusivity coefficient, K i, for pesticide vapor is assumed equal to K z for water vapor as measured by micrometeorological methods. Total daily pesticide flux can easily be calculated by summing the total estimated flux for each of the 2 or 4 hr observations. Interpretations of within-canopy pesticide flux profiles and evapotranspiration flux rates indicate that the pesticide flux rate during nighttime hours is zero. No major sources or sinks for pesticide within the crop could be found, and it was concluded that the pesticide vapor flux was directly from the soil itself. Thus, vertical pesticide flux measurements made above the canopy are representative of total vertical flux rates. It is possible to evaluate the mass horizontal flow and the downwind pattern of adsorption of pesticide vapor on untreated soil as well as dissipation into the atmosphere.

Net radiation measurements were made with a traversing system in a 250 cm tall (LAI = 4.0) corn crop with east-west oriented rows. Frequency distributions of flux density of net radiation were plotted using 0.075 cal cm-2 min-1 class intervals. These distributions showed a shift from predominantly sunlit to predominantly shaded conditions with depth into the plant canopy. The frequency distributions showed that net radiation in sunflecks in a canopy can exceed that above the canopy, and that negative values of net radiation can exist in shadows. A Soil-Plant-Atmosphere Model (SPAM) also predicted that net radiation in sunflecks in the canopy would exceed net radiation above the canopy. The effects of spots with high radiation load within the plant canopy on leaf temperature and convective heat exchange were investigated theoretically.

The effect of soil moisture level on the uptake of Mo, Cu, and Co by alsike clover was investigated in a greenhouse experiment with two Brown Podzolic soils from New Hampshire and a Weisenboden and a Brown soil from Nevada. A wet soil condition increased the Mo in the soil solution and plant Mo content on all four soils. The Co concentration in the soil solution was higher in pots maintained at the high moisture level than at the low moisture level. This increase in Co concentration with soil wetness was reflected in alsike clover, over four cuttings, on the two Brown Podzolic soils, but not in plants from the two Nevada soils. Copper contents in the plants and in the soil solution did not differ consistently because of soil wetness.

Studies of the vertical distributions of CO2 fluctuation in a cornfield were made in the 4-to 0.25-cycle/min fre- quency range. Amplitude of fluctuations decreased with height above the ground. Frequency in this range ap- peared rather constant, however. Sources and sinks for CO, within the cornfield contribute to the fluctuations; however, eddy structure originating inside and/or outside the cornfield plays an important role too. I T has long been known that the CO 2 content of the air near the earth's surface undergoes temporal and spatial fluctuations arising from inhomogeneous dis- tribution of sources and sinks of carbon dioxide as well as the degree of air mixing by meteorological processes (Huber, 1952). The fluctuations can pre- sent serious problems when sampling to obtain repre- sentative values of the carbon dioxide content of the air. We have found this especially true in our studies of CO2 exchange above active plant communities. We have been well aware of the shorter time fluc- tuations of IA to 2 cycles per minute, and as a conse- quence have attempted to obtain integrated samples for 10- to 20-min sampling periods. However, previ- ously no attempts were made to study the short-time fluctuations themselves. This paper will discuss such a study in a field of full-grown corn (Zea mays L.). Although the site has been described previously (Lemon et al., 1963), certain features of the location are pertinent to this study. The corn was relatively isolated as a frost-sensitive crop in an area of mixed vegetation of frost-hardy species. Also, the wind fetch over the corn to the sampling site was approximately 100 m. The significance of these two points will be discussed later.

Studies of the vertical distributions of CO2 fluctuation in a cornfield were made in the 4 to 0.25 cycle/minute frequency range. Amplitude of fluctuations decreased with height above the ground. Frequency in this range appeared rather constant, however. Sources and sinks for CO2 within the cornfield contribute in part to the fluctuations. However, eddy structure originating inside and/or outside the cornfield plays an important role too.

An instrument to measure the rate of gas movement from beneath the surface of soil or other porous media was constructed. Subsequent measured rates of gas movement from beneath surface coverings of coarse and fine gravel, very coarse and medium sand, Chenango silt loam and straw were correlated against concurrent measurements of wind and air pressure fluctuations. At a depth of 2 cm gas movement in silt loam was not significantly affected by air turbulence but in coarse gravel was increased 10-fold by wind velocities of 400 cm/sec. Intermediate effects were observed for media of intermediated pore sizes. Mathematical equations for calculation of soil air mass flow are derived. They are based upon the spectrum of air pressure fluctuations at the ground surface. Air pressure in the field was measured during both day and night with and without a corn crop. The spectra of air pressure were calculated and all were roughly straight lines with a slope of about -6/3 on a log-log plot. Spectral density decreased from 10 to the 8th power to 0.10 sq microbar/cycle/sec over a frequency range from 0.0001 to 100 cycle/sec. Soil air mass flow was calculated from the spectra for several soils. Using previous work, an attempt was made to evaluate the increase of soil gas movement beyond diffusion from the soil air mass flow.

Vegetative stands of alfalfa and orchardgrass, grown in flats in a glasshouse, were investigated in a small wind tunnel. The windspeed parameters of the crop surfaces were determined at low radiation intensity at each of two windspeeds; and the rates of net photosynthesis and evapotranspiration were measured at the two windspeeds at each of three radiation intensities. The roughness length (zo) increased with windspeed and was greater in orchardgrass. Rate of net photosynthesis, but not evapotranspiration, also increased with windspeed. In consequence, water-use was most efficient under the highest wind regime. The rate of net photosynthesis increased to the intermediate radiation intensity, whereas evapotranspiration increased and water-use efficiency decreased to the highest radiation regime. The work indicated that a small wind tunnel of the type described would be useful for further studies of the photosynthesis and transpiration of forage stands.

A crop of corn was treated as a random array of leaf elements. A mathematical model was developed which expressed the net photosynthesis rate of an individual leaf as a function of the light flux density and CO2 concentration at the leaf surface and the leaf temperature.