Studies of the vertical distributions of CO? fluctuation
in a cornfield were made in the 4-to 0.25-cycle/min frequency
range. Amplitude of fluctuations decreased with
height above the ground. Frequency in this range appeared
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

Drake, George M.

Lemon, E.R.

Wright, J.L.

Northwest Irrigation & Soils Research Laboratory Publications

Photosynthesis Under Field Conditions. XB. Origins of Short-Time CO2 Fluctuations in a Cornfield

Studies of the vertical distributions of CO₂ fluctuation
in a cornfield were made in the 4-to 0.25-cycle/min 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 to the fluctuations;
however, eddy structure originating inside and/or outside
the cornfield plays an important role too

USDA - ARS - NWISRL

171fiSTER COPY' 7.°Reprinted from AGRONOMY JOURNALVol. 61, May-June 1969, p. 411-413Photosynthesis Under Field Conditions. XB. Origins ofShort-Time CO, Fluctuations in a Cornfield'E. R. Lemon, J. L. Wright, and George M. Drake2ABSTRACTStudies of the vertical distributions of CO2 fluctuationin a cornfield were made in the 4-to 0.25-cycle/min fre-quency range. Amplitude of fluctuations decreased withheight above the ground. Frequency in this range ap-peared rather constant, however. Sources and sinks forCO, within the cornfield contribute to the fluctuations;however, eddy structure originating inside and/or outsidethe cornfield plays an important role too.Additional key words: Atmospheric CO 2 fluctuation,4-to 0.25-cycle/min frequency, Cornfield, CO 2 sources andsinks, Eddy structure.IT has long been known that the CO 2 content of theair near the earth's surface undergoes temporal andspatial fluctuations arising from inhomogeneous dis-tribution of sources and sinks of carbon dioxide aswell as the degree of air mixing by meteorologicalprocesses (Huber, 1952). The fluctuations can pre-sent serious problems when sampling to obtain repre-sentative values of the carbon dioxide content of theair. We have found this especially true in our studiesof 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 samplesfor 10- to 20-min sampling periods. However, previ-ously no attempts were made to study the short-timefluctuations themselves. This paper will discuss sucha 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 locationare pertinent to this study. The corn was relativelyisolated as a frost-sensitive crop in an area of mixedvegetation of frost-hardy species. Also, the wind fetchover the corn to the sampling site was approximately100 m. The significance of these two points will bediscussed later.SAMPLING PROCEDURE FOR CO2Two procedures for sampling CO2 were followed: (1) Duringall but one of the six sampling periods, time- and space-inte-grated samples were collected and analyzed to provide mean'Contribution from the Northeast Branch of the Soil andWater Conservation Research Division, ARS, USDA, in coopera-tion with the New York State Agricultural Experiment Stationat Cornell University, and the Atmospheric Sciences ResearchDivision, U. S. Army Electronics Command, Fort Huachuca,Ariz. Department of Agronomy Series Paper No. 821. ReceivedSept. 23, 1968.'Research Soil Scientist (Physics) and Professor of Agronomy,Ithaca, N. Y. 14850; formerly Soil Scientist, USDA, Ithaca, N. Y.(presently Research Soil Scientist, Snake River Conservation Re-search Center, SWC, ARS, USDA, Kimberly, Idaho); and Elec-trical Development Technician, USDA, Ithaca, N. Y.vertical profiles of CO, concentration above and within thecrop. The procedure has been reported in detail in an accom-panying paper (Lemon and Wright, 1969). In the present study,mean profile samples were taken at 380, 225, 135, 95, and 35cm above the ground. The profiles, which are identified bytime (EST), are presented in the left side of Fig. 1. (2) Con-current with the collection of samples for a given mean profile,a continuous record of the fluctuating carbon dioxide contentof the air at a single point above the ground was obtained.There were six "runs" of about 10-min duration each, with thecontinuous "fluctuation" samples taken in turn at the followinglevels above the ground: 520, 424, 305, 216, 140, and 51 cm.The corn was 285 cm tall with the uppermost large leaves atabout 225 cm. Thus three fluctuation samples were taken aboveand three within the canopy. The results are presented on theright side of Fig. 1 as fluctuations from the mean for a givenlevel of a specified mean profile. Each fluctuation sample isidentified by height above the ground and time (EST) in as-sociation with a particular mean vertical profile. Graphedpoints of the fluctuation samples are instantaneous values fromcontinuous chart recordings.Further details of the continuously recorded samplings arepresented. A single 50-m length of 0.5 cm ID tygon tubing wasattached to the suction side of an aquarium pump to providesample air direct to an infrared CO, gas analyzer. The inletof the tubing was supported on a mast in the corn upwindfrom the instrument trailer, and was sequentially positioned onthe mast at the given sampling levels beginning at the top.The tubing from the mast to the trailer was strung on cornleaves through the crop at about 150 cm above the ground.This minimized possible diffusion of CO, into the tubing fromthe higher concentrations of CO, near the ground. Also higherdaytime temperatures near the ground induce evolution of or-ganic gases out of the tubing, causing additive errors in measure-ment by infrared methods. Both if these errors were minimizedby a high air sampling rate. Since emphasis here is on short-time fluctuations, such errors are likely to be of no significance.The disappearance of the fluctuations after the corn was killedby a frost indicated that the fluctuations were indeed causedby conditions external to the sampling system.The aquarium pump delivered air at a constant 2.5 liters/min. The exhaust side of the pump led directly into a specialcarbon dioxide infrared analyzer having a range of 12.5 ppmwith a sensitivity of ± 0.2 ppm. Other details of the instrumentare reported elsewhere (Wright and Lemon, 1966).Prior to each run the sampling airstream was split into twostreams before entering the analyzer, one going through the"sample cell" and the other through the "reference cell." Bothwere exhausted to ambient atmospheric pressure. In this way"zero" differential was established, with air of like compositionin each cell (i.e. both in CO, and water vapor). At the startof a run the "reference cell" was closed and the sample aircontinuously put through the "sample cell" only. Thus, thecontinuous fluctuations were relative to the air composition atthe beginning of a run. The analyzer had sufficient discrimina-tion between water vapor and carbon dioxide so that whenused in this way it was not necessary to remove the water vapor.Before the study was begun it was important to determinethe system response time. The problem of sample "smearing"during flow through the long sampling tubing could be a seriouscomplication. This problem was investigated by passing "square-wave" differentials of CO 2 concentration through the samplingtubes. Fig. 2 presents the results of two such studies on thetubing used in these studies (Lemon and Wright, 1969). The411412	 AGRONOMY JOURNAL, VOL. 61, MAY-JUNE 1969zcm . t,(in min)0 1 2 3 4 5 6 7 8 9 100-50-5105,-5cornSEPT. 18,1963Fig. 1. Mean CO2 concentration (C) profiles in a cornfield for the indicated time (EST)and the variation (ac) from the mean during the 10-min sampling period for each specifictime and level as indicated by arrow. Ellis Hollow, N. Y.503510500 510C, (in gm/cm3 x 1(19 )test procedure was this. Two tanks of standard gases differingin CO2 concentration by 5.0 0.2 ppm were connected to thedifferential CO 2 analyzer. Flow regulators and pressure regu-lators were used to assure equality of input to both "reference"and "sample" cells of the analyzer. Output was exhausted toambient air. In all cases the input air was passed throughmagnesium perchlorate absorbers immediately before enteringthe analyzer cells. First, equal and short lengths of copper tub-ing to both cells were used to establish that differential carbondioxide concentration was 5.0 ± 0.2 ppm without the longtest tubing. The differentials were reversed by switching inputstandard gases at the tank outlets. This served to check zeroand span characteristics of the analyzer. Following this thetest tubing was inserted in series to one cell of the analyzer,air flow and pressure regulated and equilibrium established.Not all the flow of air through the PVC garden hose was passedthrough the analyzer, however. A small subsample stream wasbled into the analyzer at the rate of 1 liter/min. This subsamplewas passed through the drier tube before entering the analyzer.Once the analyzer recorder indicated equilibrium ofthe system, the concentration differentials were re-versed at the tank outlets and the system was allowedto come to equilibrium once again. A continuouschart recording of each cycle permitted several cyclesto be clearly defined. Fig. 2 shows the elapsed timeof representative cycles for the two tubings tested. Itshould be noted first that there appears to be no CO2contamination with the rapid airflow rates we use inour experiments. The differentials were 5.0 ± 0.2 ppmwith and without the sample tubing. Apparently, too,the lag characteristics of the two types of tubing testedwere similar with the different airflow rates used.The shapes of the response curves need explanation.At time zero when concentration differentials have justbeen reversed there will be a period when the airentering both cells is alike in concentration due tothe fact that there is a large "ballast" of air in thelong sample tubing. Purging of the drier tubes andanalyzer cells takes about 10 to 20 sec at a flow rateof 2.2 liters/min or 20 to 30 sec at a flow rate of 1liter/min. At the end of this period there is a momen-tary or transient equilibrium of "no differential" airin the analyzer, producing the "step" in the tygon02.50	 20	 40	 60	 86 .0	 20	 43	 60	 80ELAPSED TIME, secFig. 2. CO. sampling plus analytical system response charac-teristics to a square wave input of CO. differential of 5 ppmin air.curve. (Evidently the transition was so gradual inthe PVC garden hose that no step is apparent.) Weinterpret the response in the tygon tubing to begin at30 sec and to be 2/3 complete at 40 sec, with almostcomplete equilibrium in 60 sec. We also interpretfrom the same curve the analyzer response to be 2/3complete in 15 sec with almost complete response in20 sec. Thus as a conservative estimate, the completesystem as used in this fluctuation study was capableof detecting fluctuations with a frequency of up to 4cycles/min. Higher frequencies were filtered out byboth lead in hoses and instrument response time.Interestingly enough, however, the fluctuations ap-pearing in Fig. 1 are primarily of lower frequency, i.e.,1/4 to 1 cycle/min.As to the matter of the wind velocity in the cornfieldduring the study, we can only present mean velocitiesfor the six sampling periods (Table 1). The methodsof obtaining these results are discussed by Lemon andWright (1969).garden hose1.0.• 1.11 Cm.length • 61 m.flow •17.5 I./min.tygon tubeI.D.•0.5Cm.length.50m.flow• 2.2 I./min.   5PPM2.5413Table 1. Mean wind velocity at various levels above and withina canopy of corn. Ellis Hollow, N. Y. Sept. 18, 1963.Time(EST)Wind velocity	 (cm/sec) at various height above ground, cm520 424 305 216 140	 511358-1408 272 241 187 99 32	 141435-1445 227 202 156 82 27	 111510-1520 205 182 141 74 24	 101545-1555 218 193 150 79 26	 111615-1625 141 125 97 51 17	 71645-1655 198 122 95 50 16Height of corn was 285 cm. Underlined figures indicate mean velocities at time andheight of CO, fluctuation sampling,Table 1 indicates that general wind flow decreasedgradually as the afternoon progressed. However, thisdecrease was minor compared to the decrease in windas sampling height decreased. Thus in Fig. 1, the in-crease in amplitude of the CO 2 fluctuations as theground is approached can be associated with a decreasein wind velocity caused primarily by canopy resistance.Because of the scale of height of measurement andthe filtering out of frequencies more rapid than 4cycles/min, it is doubtful that any change in frequencyof CO2 fluctuation with height can or should be de-tected.We are now at a point where it may be appropriateto discuss the origin of the observed low frequencyfluctuations. When we first observed this phenomenonseveral years ago at the same site, we reasoned that thefluctuations were generated outside the field becausethe eddy scale was larger than the upwind fetch inthe field. For example, at a low mean wind flow of1 m/sec just above the canopy and a V2 cycle/minfluctuation, the eddy scale would be 120 m (1 meter/sec X 60 sec/min X 1/2). At a moderate wind of 2or 3 m/sec, the eddy scale would be 240 to 360 m,considerably greater than our 100-m fetch in the corn-field.However, two findings in this study have caused usto question our earlier reasoning: (1) Three daysfollowing the fluctuation study the corn was killedby a frost, causing a subsequent cessation of CO 2fluctuation at all levels (surrounding frost-hardy vege-tation appeared to be unharmed). (2) The frequencyof fluctuation before frost, appeared to be independentof height despite the wide range of windspeeds duringthe day.Therefore, we now propose that the CO 2 fluctua-tions are indeed generated in the cornfield by variablesources and sinks associated with photosynthesis andrespiration. Furthermore, the eddies contributing tothe fluctuations could be originating either inside oroutside the field or both. In any case, they have tobe moving across the field at a much slower velocitythan the mean windspeed. Nothing can be inferredabout their size or shape. We now need to make simul-taneous fluctuation measurements at several horizontaland vertical points to throw additional light on thisphenomenon. Further studies of this phenomenonalso can lead to useful application in measuring CO 2exchange in the field by a "direct" method (Inoue,1964).REFERENCESHuber, B. 1952. Der Einfluss der vegetation aus die Schwan-kungen des CO2-Gehaltes der Atmosphare. Arch. Met.Geophy. Und Klimat. "B" 4:154.Inoue, Eiichi. 1964. Turbulent fluctuations in atmospheric CO 2concentration over the vegetated fields. 'Studies on Oceano-graphy.' Div. of Meterol., Nat. Inst. Agr. Sci. Tokyo, Japan.p. 232-237.Lemon, Edgar R., Joseph H. Shinn, J. H. Stoller, and Conrad S.Yocum. 1963. The energy budget at the earth's surface.Part I. U. S. Dep. Agr. Prod. Rep. No. 71. 33 p.Lemon, E. R., and J. L. Wright. 1969. Photosynthesis underfield conditions. XA. Assessing sources and sinks of carbondioxide in a corn crop using a momentum balance approach.Agron. J. 61:405-411.Wright, J. L., and E. R. Lemon. 1966. Photosynthesis underfield conditions. IX. Vertical distribution of photosynthesiswithin a corn crop. Agron. J. 58:265-268.

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Photosynthesis Under Field Conditions. XB. Origins of Short-Time CO2 Fluctuations in a Cornfield

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