CO
2
enrichment and soil nitrogen effects on wheat evapotranspiration
and water use efficiency
D.J. Hunsaker
a,∗, B.A. Kimball
a, P.J. Pinter Jr.
a, G.W. Wall
a, R.L. LaMorte
a, F.J. Adamsen
a,
S.W. Leavitt
b, T.L. Thompson
c, A.D. Matthias
c, T.J. Brooks
daUS Water Conservation Laboratory, Agricultural Research Service, USDA, Phoenix, AZ 85040, USA bLaboratory of Tree Ring Research, University of Arizona, Tucson, AZ 85721, USA cDepartment of Soil, Water, and Environmental Science, University of Arizona, Tucson, AZ 85721, USA
dMaricopa Agricultural Center, University of Arizona, Maricopa, AZ 85239, USA
Received 7 December 1999; received in revised form 21 April 2000; accepted 28 April 2000
Abstract
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 370mmol mol−1during daytime) and enriched (200mmol mol−1above
ambient) CO2concentrations during 1995–1996 and 1996–1997 in Free-Air CO2Enrichment (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 CO2treatments during the seasons were
gen-erally 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.
Keywords: Global change; Soil water use; Crop coefficient; Subsurface drip
1. Introduction
Historical and modern records show that the atmo-spheric carbon dioxide (CO2) concentration increased from approximately 280mmol mol−1in pre-industrial times to about 315mmol mol−1by 1958, and to more
∗Corresponding author. Tel.: +1-602-379-4356/ext. 266; fax:
+1-602-379-4355.
E-mail address: dhunsaker@uswcl.ars.ag.gov (D.J. Hunsaker)
than 350mmol mol−1 by 1988 (Boden et al., 1994). The accelerated trend in the global CO2 growth rate during the first 30 years of modern records has led to various scenarios for the future CO2concentrations of the atmosphere. The report of the Intergovernmen-tal Panel on Climate Change (1996) projects CO2 in-creasing to about 500mmol mol−1before the end of the 21st century. Hoffman and Wells (1987) and Tren-berth (1991) estimate a near-doubling of atmospheric CO2 over the pre-industrial level by the end of the 21st century.
1983; Goudriaan and Unsworth, 1990).
Previous experimental studies to assess the effects of CO2 enrichment on crop water use were almost exclusively conducted in environments significantly altered from normal agricultural field conditions by the use of enclosures to confine the CO2around plants (Kimball et al., 1994a). Some of the earlier inves-tigations utilizing growth chambers or greenhouses indicated changes in water use (either ET or transpi-ration) owing to increased CO2 levels, although in general the effects have been small and inconsistent. Kimball et al. (1983), using lysimeters in open-top chamber studies in Arizona, indicated that a doubling of CO2 concentration reduced seasonal ET for cot-ton by 5–10%. However, in a second study (Kimball et al., 1984), they reported inconsistent effects on cot-ton ET for a doubling of CO2. Controlled, chamber experiments with soybeans (Jones et al., 1985a) indi-cated that differences in whole canopy transpiration between CO2levels of 330 and 800mmol mol−1were not significant, whereas in a similar experiment (Jones et al., 1985b), soybean transpiration was reduced to 10% at a CO2 level of 660mmol mol−1. Chaudhuri et al. (1990) reported little effect of CO2 on the ET of winter wheat grown in greenhouses during two out of the 3 years of study.
Recognizing the need to eliminate the effects im-posed by walled chambers in CO2enrichment exper-iments, a Free-Air CO2 Enrichment (FACE) system was developed to evaluate the effects of increased CO2 on crop response in a typical agricultural environment (Hendrey and Kimball, 1994). Experiments were con-ducted in 1989, 1990, and 1991 using the FACE sys-tem in a large, irrigated cotton field in central Arizona.
wheat crops grown at the FACE site under two levels of soil nitrogen. A parallel investigation by Kimball et al. (1999), using an energy balance approach, also sought to determine the impact of CO2on the water use for wheat in the same FACE experiments.
2. Methods and materials
A hard red spring wheat (Triticum aestivum L., cv. Yecora Rojo) was planted in rows spaced 0.25 m apart in an unbedded soil surface on 14–15 December, 1995 and on 15 December, 1996 on a 9 ha field site located at The University of Arizona, Maricopa Agricultural Center (MAC), in central Arizona. The seeding rates were 109 and 111 kg ha−1 and the plant densities at emergence were 189 and 194 plants m−2 during the first and second seasons, respectively. The soil at the site is classified as Trix clay loam (fine-loamy, mixed (calcareous), hyperthermic Typic Torrifluvents). The soil water retention characteristics for the site were estimated from data obtained by Post et al. (1988) and F.D. Whisler (personal communication, 1992). At soil matric potentials of −0.033 and−1.5 MPa, vol-umetric water contents are approximately 0.30 and 0.20 m3m−3, respectively, for the 0–0.7 m soil depth, whereas they are approximately 0.22 and 0.12 m3m−3, respectively, from 0.7 to 2.0 m below the surface. Al-though the field site was equipped with a subsurface drip irrigation system, the germination of the wheat was accomplished by applying 30 mm of water to all plots with a portable sprinkler system, shortly after both plantings.
2.1. CO2treatments
A FACE system, similar to one used in prior exper-iments (Pinter et al., 1996a), was installed on the site just before planting the 1995–1996 and 1996–1997 spring wheat crops. Complete descriptions of the design, construction, and algorithms of the FACE ex-posure and monitoring system are provided by Lewin et al. (1994) and Nagy et al. (1994). The FACE sys-tem was used to enrich the CO2concentration of the air in four, open-field circular plots, 25 m in diameter, by 200mmol mol−1 above ambient (which averages about 370mmol mol−1 during the daytime). The CO2-enriched plots comprised the four replicates of
the FACE treatment. Four, circular non-CO2-enriched matching plots (Control treatment), equipped with blowers to provide air flow across plots similarly to that in the FACE plots, were also established in the field. The FACE and Control plot replicates were spaced approximately 90 m apart. In both seasons, CO2 enrichment commenced when 50% emergence of seedlings was observed, which occurred on 01 Jan-uary, 1996 and on 03 JanJan-uary, 1997, and enrichment continued 24 h a day through 15 May, 1996 and 12 May, 1997 for the first and second seasons, respec-tively. Final harvests of grain occurred about 2 weeks after enrichment was terminated in both seasons.
2.2. Soil nitrogen treatments
Two soil nitrogen (N) treatments were imposed within the two CO2treatments. Each of the main cir-cular CO2plots was split into two semicircular halves, with each half (subplot) either receiving an ample (High N) or limited (Low N) supply of nitrogen fertil-izer. During both wheat seasons, the High N treatment was given a total of 350 kg N ha−1. This total was ap-plied in four applications given at mid-tillering, stem extension, anthesis, and early grain-fill, as shown in Table 1. The Low N treatment received three fertilizer applications given at mid-tillering, stem extension, and anthesis (Table 1). The seasonal totals to the Low N treatment were 70 and 15 kg N ha−1 during the 1995–1996 and 1996–1997 seasons, respectively. During the 1996–1997 season, a fertilizer application error resulted when the amount intended for the High N treatments on 05 March, 1997 was inadvertently applied to one Low N subplot with the Control treat-ment. Due to the additional fertilizer, all analyses presented for the 1996–1997 season will exclude the data from that particular subplot.
wheat studies with differential irrigation (Hunsaker et al., 1996). Further details about the soil N treat-ments and a plot plan were provided by Kimball et al. (1999).
2.3. Crop development and yield
Development of the aboveground portion of the crop was monitored by plant sampling during the two seasons. Plants were sampled from all subplots at approximately weekly intervals for determinations of plant biomass, height, leaf area index, plant area index, phenology, and a number of other growth measures. Additional information on the plant sampling protocol was provided by Pinter et al. (1996b, 1997). Final grain yields were determined by machine harvesting an undisturbed, 18 m2area within each subplot on 29–30 May, 1996, and on 28–29 May, 1997. After harvest, the grain was oven-dried at 70◦C for a total of 14 days, and the yields were expressed on a dry weight basis.
2.4. Canopy reflectance and absorption parameters
On 55 and 56 days during the 1995–1996 and 1996–1997 seasons, respectively, measurements of canopy reflectance factors were made in all treat-ment subplots with a handheld radiometer. The Red and near-infrared (NIR) data were used to compute a normalized difference vegetation index (NDVI), as [(NIR-Red)/(NIR+Red)] (Pinter et al., 1997). Values of NDVI for days when reflectance measurements were not made were obtained by linear interpolation of the computed NDVI values.
NDVI data obtained in 1996–1997. Kimball et al. (1999) used the date at which fAPAR∗ had declined to 25% of the plot maximum as the crop senescence date. According to Kimball et al. (1999), the average crop senescence dates for both the Control–High N (C–H) and FACE–High N (F–H) treatments occurred on 10 May in the first and second seasons. However, the average crop senescence dates for Control–Low N (C–L) and FACE–Low N (F–L) treatments occurred on 29 April in both seasons, or 12 days earlier than for High N treatments.
2.5. Irrigation management
An irrigation scheduling program called AZSCHED (Fox et al., 1993) was used to schedule the times and amounts of subsurface drip irrigation during the two growing seasons. The general scheduling procedure was to apply water to all treatment plots after about 35% of the available soil water had been depleted from the wheat root zone. The depth of water applied for each irrigation was the amount calculated to replenish the wheat root zone water content to field capacity (0% depletion), as determined by AZSCHED. The program increased the root zone from a minimum of 0.15 m at planting to a maximum of 1.3 m at mid-season as a function of growing degree days. The program esti-mated the daily crop water use by multiplying a wheat crop coefficient by a grass reference ET (ETo) and then calculated the amount of soil water depletion by a daily water budgeting procedure. Daily EToand me-teorological data were provided by The University of Arizona, AZMET weather station (Brown, 1987), lo-cated on a well-irrigated grass field at the Maricopa Agricultural Center.
During the 1995–1996 and 1996–1997 seasons, the High N treatments received a total of 653 and 621 mm of water from irrigations, and the Low N treatments received 592 and 548 mm, respectively. The Low N treatments were given the same irrigation amounts as the High N treatments through April in both seasons. However, due to the accelerated ma-turity of the nitrogen-stressed plants, irrigations to the Low N treatments were curtailed in May in both seasons. The cumulative precipitation received at the field site (germination through harvest) was 39 and 35 mm during the 1995–1996 and 1996–1997 seasons, respectively.
2.6. Soil water content measurements
Volumetric soil water contents were measured in each subplot using Time-Domain-Reflectometry (TDR) and neutron scattering equipment installed at the start of the each year’s experiment. The approach was to install the measurement equipment at the same location relative to drip emitters and plant rows in every subplot. Thus, the measurements of water con-tent were made uniformly in subplots with respect to field position, although the position selected may not have represented the average soil moisture condition of the subplot. The equipment was installed near the center of each semicircular subplot, about 7 m from the outside perimeter of the main plot.
Water content measurements by TDR were taken at one site within each subplot. The TDR system used was a Trase1 model 6050x1 cable tester (Soil-moisture Equipment Corp., Santa Barbara, CA). In each subplot, a non-coated, two-rod waveguide probe, 0.3 m in length was installed within a plant row, ver-tically into the soil to a depth of 0.3 m. The probe was installed such that it was equidistant between two drip emitters, and 0.125 m from the nearest drip tape (Fig. 1). Once installed, the TDR probe remained at the same location throughout the season. Measure-ments at the probe locations were made manually with the cable tester. The output of the cable tester provided a measurement of the integrated volumet-ric water content from 0 to 0.3 m below the soil surface.
Fig. 1. Location of TDR and neutron probes relative to drip tape, emitters, and plant rows.
Soil water content measurements by TDR and neu-tron scattering were taken in all subplots during early morning hours for 37 days during the 1995–1996 sea-son (from 18 December, 1995 to 18 May, 1996), and for 36 days during the 1996–1997 season (from 19 December, 1996 to 16 May, 1997).
2.7. Wheat evapotranspiration determinations
For all treatment subplots, wheat ET was deter-mined for periods between two successive soil water measurement dates in which precipitation was small (less than 4 mm) and irrigation water was not applied
(i.e. ‘soil water depletion’ periods). Assuming deep percolation was negligible during soil water depletion periods, the ET was calculated using a simplified soil water balance as
ET= (1S+P )
t (1)
where1S is the change in the root zone soil water
wetting’ periods), soil water balance determinations of ET were considered unreliable, since actual depths of applied water and drainage water losses were not measured at the TDR and neutron access tube loca-tions. For soil wetting periods, daily ET was estimated using procedures that will be described shortly.
For each treatment subplot, average daily ET values were calculated with Eq. (1) for soil water depletion periods that occurred between the commencement of CO2enrichment (early-January) and the soil water de-pletion period nearest to the average crop senescence date for the particular treatment. Thus for each C–H and F–H plot, an average daily ET was determined for 15 depletion periods between 04 January and 09 May in the 1995–1996 season and for 19 depletion periods between 02 January and 11 May in the 1996–1997 season. For the C–L and F–L plots, there were 13 pe-riods (04 January to 30 April) in 1995–1996 and 17 periods (02 January to 02 May) in 1996–1997. Soil water measurements made after the average treatment crop senescence dates indicated small amounts of soil water depletion occurred, but the depletion rates were highly variable among treatment replicates. Therefore, the ET after the periods above was not considered in treatment comparison. The length of the individual de-pletion periods varied from 2–8 days during 1996 and from 2–7 days during 1997.
The depth of the effective root zone used to compute the soil water depletion with Eq. (1) was estimated for each depletion period from temporal changes in the root biomass extension within the soil profile. The root biomass was determined from root core samples taken in plant rows in 0.15 and 0.2 m increments from 0 to1.0 m during the two wheat seasons by Wechsung et al. (1998). Analysis of the root core data indicated that the temporal changes in root mass extension in the soil profile were similar in all treatments during the growing seasons, although the root biomass was increased on average by about 12% in the FACE treat-ment and root mortality was slightly increased in the Low N treatments during the late season in both years (Wechsung et al., 1998). In 1996, all treatments ob-tained root biomass to a depth of 0.4 m on 10 Febru-ary, 0.7 m on 24 FebruFebru-ary, and 1.0 m on 23 March. In 1997, treatments obtained root biomass to a depth of 0.5 m on 12 February, 0.7 m on 26 February, and 1.0 m on 26 March. A maximum root depth of 1.3 m was applied uniformly to all treatments after anthesis
(late-March). Although the extension of the root depth beyond 1.0 m could not be determined from root core samples, the soil water content measurements indi-cated soil water depletion occurred to at least a 1.3 m depth for all treatments after anthesis. Erie et al. (1982) showed that about 10% of the total consumptive water use of wheat is extracted from soil water between the 0.9 and 1.2 m profile. From root core work during two previous FACE wheat experiments, Wechsung et al. (1995) reported that CO2enrichment did not induce a deeper rooting depth at any growth stage where sig-nificant differences between FACE and Control root biomass were found.
In order to determine the cumulative ET for the treatments, it was necessary to estimate ET for all days during the soil wetting periods. Hunsaker (1999) pre-sented a methodology for estimating daily crop ET for an entire season using soil water depletion measure-ments combined with the dual crop coefficient (Kc) approach and related procedures presented in the Food and Agriculture Organization Paper No. 56 (FAO-56) (Allen et al., 1998). The dual crop coefficient approach consists of separating Kc into a basal crop coefficient (Kcb) and a soil water evaporation coefficient (Ke) in which non-stressed and stressed soil water conditions, re-spectively.
Briefly, the methodology used by Hunsaker (1999) required three primary steps. First, Kcb Ks values are derived from the measured ET and corresponding ETo for all soil water depletion periods. Next, a continuous
Fig. 2. Measured Kcb values for soil water depletion periods and estimated daily Kcband Kccurves for one replicate of the Face-High N
(F–H) treatment in 1996–1997 (a) and the irrigation and precipitation dates and amounts applied to the treatment during the season (b).
ET and the remaining amount of measured ET is used to calculate a new KcbKs for the period (i.e. the first step is repeated using only the basal portion of the measured ET).
For the well-watered conditions of the present study, effects of water stress on ET were assumed negligi-ble and thus Ks was set equal to 1.0 for all periods. Fig. 2a illustrates the results of the procedures applied to one of the F–H replicates of 1996–1997, where the actual irrigation and precipitation events are shown in Fig. 2b. Since the measured Kcb were derived over soil depletion periods of two or more days, they repre-sented an average Kcbvalue for the period and, there-fore, are shown as horizontal lines along the Kcbcurve in Fig. 2a. Daily values of Kcb for periods between the horizontal Kcbsegments (i.e. for soil wetting peri-ods) were estimated by linear interpolation based on the Kcbvalues measured immediately before and after the period in question. The Kc on a given day is the summation of Kcbplus the soil water evaporation co-efficient (Ke) and, therefore, the value of Kc is equal to Kcbonly for conditions when the soil surface layer
is dry (i.e. when Ke=0). However, as shown in the figure, a portion of the measured ET during some soil depletion periods was contributed by soil evaporation (i.e. Ke>0). Thus, the measured Kcbin Fig. 2a repre-sent the basal portions of the measured ET divided by the corresponding ETo. The ET on any day can then be obtained by multiplying the estimated Kcby the ETo value on that specific day. The total cumulative ET for each treatment subplot was calculated as the summa-tion of the daily ET over all days from early-January through 09 May and 11 May for the High N treat-ments and from early-January through 30 April and 02 May for the Low N treatments in 1995–1996 and 1996–1997, respectively.
apply the FAO-56 calculations to the conditions of the present study. These included daily values for the crop height, the fraction of the soil surface wetted by irri-gation or precipitation (fw), and the fraction of the soil surface exposed to sunlight and air (denoted as 1−fc), as well as two drying parameters for the specific soil. For both the 1995–1996 and 1996–1997 experiments, daily values for crop height were estimated by lin-ear interpolation of the weekly plant height measure-ments. Also, for both years, a value of 1.0 was used for fwfollowing precipitation events (i.e. the entire soil surface was assumed to have been wetted). Follow-ing subsurface drip irrigations, the values used forfw were calculated using the FAO-56 recommendations for subsurface drip systems as follows:
fw=0.40(1−0.67fc) (4) where fc represents the fraction of the soil surface covered by canopy.
Values for the fraction of the exposed soil surface, 1−fc, were approximated from the fraction of PAR transmitted through the canopy, as determined from light bar measurements. For the 1995–1996 treat-ments, daily values of fTPAR(≈1−fc) were estimated by linear interpolation between the days that the fTPARwere measured. For the 1996–1997 treatments, estimates of daily fTPARwere obtained from the daily NDVI data for that season using a calibration model developed from the 1995–1996 fTPARand NDVI data. The soil parameters needed in the procedures are the readily evaporable water (REW), which is the maxi-mum depth of evaporation from the soil surface that can occur during the energy limiting stage, and the to-tal evaporable water (TEW), which is the maximum depth of water that can be completely evaporated from an initially wetted soil surface. For the clay loam soil at the FACE site, a value of 10 mm was assumed for REW, an average value suggested by FAO-56 for clay soils. Assuming a 0.10 m soil evaporation layer, TEW was calculated as 20 mm using the equation provided for estimating TEW in FAO-56, which is based on the field capacity and wilting point values for the surface layer.
Values of the water use efficiency were calculated for each treatment plot using the total cumulative ET estimates and measured grain yields, where WUE equals final grain yield per unit of ET, expressed as kg/m3.
2.8. Data analysis
Statistical analyses to evaluate significant treat-ment effects on ET and WUE were made using a strip-split-plot model, as described by Little and Hills (1978). The analyses were performed using the Gen-eral Linear Models (GLM) procedure of SAS (SAS Institute Inc., 1988). The strip-split-plot model had three parts. Part (1) included the replication effect (ρ) and the CO2main effect (α); the error term used for evaluating the effect of CO2 was ρα. Part (2) included the soil nitrogen effect (β), which was eval-uated with the error term,ρβ. Part (3) included the interaction term (αβ), which was evaluated by the residual mean-square error.
3. Results
3.1. Basal crop coefficients, Kcb
Fig. 3 (1995–1996) and Fig. 4 (1996–1997) show the seasonal variation of the average Kcb values, de-rived from the soil water depletion measurements, for each of the four, CO2by soil nitrogen treatment com-binations. In both years, the Kcbfor the C–H treatment increased to a maximum value of about 1.20 during mid-April (early grain-fill), and then decreased to a fi-nal value between 0.45 and 0.55 in early-May. As seen in the figures, the Kcbfor the F–H treatment were gen-erally slightly lower than those for the C–H treatment during the two seasons. The Kcbfor Low N treatments were similar to the Kcbfor High N treatments through about early-March in both seasons. However, starting in mid-March, the Kcb values for Low N treatments were lower relative to the High N treatments, indicat-ing less water use among Low N plots durindicat-ing the latter half of the season.
3.2. Evapotranspiration
Fig. 3. Average basal crop coefficients measured for soil water depletion periods during the 1995–1996 season for the Control–High N (C–H), Control–Low N (C–L), FACE–High N (F–H), and FACE–Low N (F–H) treatments.
were generally increased for several days following ir-rigation and precipitation events due to wet soil evap-oration. In both years, the soil evaporation component of ET was most pronounced following irrigation and precipitation during January and February, prior to full canopy development. The amount of total soil evapo-ration calculated for the High N treatments averaged 25 and 22 mm in 1995–1996 and 32 and 30 mm in 1996–1997 for the C–H and F–H treatments,
respec-Fig. 4. Average basal crop coefficients measured for soil water depletion periods during the 1996–1997 season for the Control–High N (C–H), Control–Low N (C–L), FACE–High N (F–H), and FACE–Low N (F–H) treatments.
Fig. 5. Average daily ET with day of year during the 1995–1996 season for the Control–High N (C–H), Control–Low N (C–L), FACE–High N (F–H), and FACE–Low N (F–H) treatments (a) and the irrigation and precipitation dates and amounts applied during the season (b).
trogen treatment, since the periods occurred prior to the treatment fertilizer applications that were begun on 30 January.) The period in late-March, when the effects of the nitrogen treatment on ET were first sig-nificant, corresponded approximately to the beginning of anthesis. At that time the Low N treatment had re-ceived a total of 45 and 10 kg N ha−1for 1995–1996 and 1996–1997, respectively, compared to a total of 175 kg N ha−1for the High N treatment (Table 1). The limited N fertilizer applied to the Low N treatments resulted in a significant decline in the leaf area and the rate of aboveground biomass production relative to the High N treatments during the latter half of the two growing seasons. Thus the reductions in water use ob-served for the Low N treatments during the latter half of the seasons were consistent with decreasing leaf area and growth rate for the Low N treatments dur-ing that period. The total cumulative ET (shown at the bottom of Tables 2 and 3) for Low N treatments was 25 and 22% less than for High N treatments in Con-trol and 23 and 20% less in FACE in the1995–1996 and 1996–1997 seasons, respectively. In both years, the difference in total cumulative ET between soil N treatments was highly significant (p<0.01). Each treat-ment in 1996–1997 attained a cumulative ET that was within 6 to 16 mm of the treatment’s cumulative ET in 1995–1996 suggesting little variation in climatic con-ditions between the two seasons. Table 4 indicates that differences in average climatic data between the two seasons were small.
The GLM results also revealed that the effects of CO2enrichment on ET over 10-day periods were not often statistically significant. However, the GLM did indicate that ET for FACE was significantly lower than
that for C–L in the first and second years, respectively. The GLM analyses indicated that the cumulative ET differences between CO2treatments were not highly significant.
Table 2
Treatment means and GLM results for wheat evapotranspiration (ET) determined for 10-day periods during the 1995–1996 season
Period (1996) Treatments (mm)a Effects (P>F)b
Julian days Dates C–H C–L F–H F–L CO2 N CO2×N
004–011c 04–11 January 3 2 3 1 0.82 0.04 0.96
012–021 12–21 January 9 5 9 6 0.98 0.05 0.97
022–031 22–31 January 14 11 15 12 0.97 0.08 0.63
032–041 01–10 February 30 30 30 29 0.92 0.78 0.49
042–051 11–20 February 29 24 26 25 0.70 0.38 0.10
052–061 21 February–01 March 34 30 28 30 0.03 0.35 0.08
062–071 02–11 March 51 44 48 42 0.10 0.19 0.99
072–081 12–21 March 45 44 42 38 0.04 0.15 0.60
082–091 22–31 March 63 50 60 51 0.80 0.01 0.72
092–101 01–10 April 79 62 76 64 0.78 0.01 0.29
102–111 11–20 April 88 73 86 72 0.38 0.02 0.64
112–121 21–30 April 88 74 88 76 0.94 0.03 0.65
122–130d 01–09 May 68 nae 69 nae 0.79 nae nae
Cumulative total 601 449 580 446 0.30 0.01 0.06
aTreatments are Control–High nitrogen (C–H), Control–Low nitrogen (C–L), FACE–High nitrogen (F–H), and FACE–Low nitrogen
(F–L).
bThe probability of a larger F statistic (P>F) for the CO
2, nitrogen (N), and the CO2×N interaction effects from the GLM. cThe first period was 8 days.
dThe final period was 9 days. ena: Not applicable.
Table 3
Treatment means and GLM results for wheat evapotranspiration (ET) determined for 10-day periods during the 1996–1997 season
Period (1997) Treatments (mm)a Effects (P>F)b
Julian days Dates C–H C–L F–H F–L CO2 N CO2×N
002–012c 02–12 January 3 3 3 2 0.66 0.65 0.99
013–022 13–22 January 12 12 10 11 0.40 0.90 0.95
023–032 23 January–01 February 17 18 15 13 0.02 0.91 0.65
033–042 02–11 February 21 23 23 21 0.49 0.47 0.07
043–052 12–21 February 38 37 39 36 0.91 0.17 0.37
053–062 22 February–03 March 28 26 27 26 0.70 0.33 0.88
063–072 04–13 March 52 52 47 44 0.05 0.80 0.67
073–082 14–23 March 64 55 63 58 0.74 0.08 0.03
083–092 24 March–02 April 72 62 67 61 0.03 0.01 0.51
093–102 03–12 April 68 53 64 58 0.69 0.02 0.41
103–112 13–22 April 83 68 81 70 0.91 0.02 0.05
113–122 23 April–02 May 83 56 79 59 0.57 0.03 0.23
123–131d 03–11 May 54 nae 54 nae 0.84 nae nae
Cumulative total 595 465 572 459 0.20 0.01 0.25
aTreatments are Control–High nitrogen (C–H), Control–Low nitrogen (C–L), FACE–High nitrogen (F–H), and FACE–Low nitrogen
(F–L).
bThe probability of a larger F statistic (P>F) for the CO
2, nitrogen (N), and the CO2×N interaction effects from the GLM. cThe first period was 11 days.
Fig. 8. Average daily cumulative ET and average daily difference in cumulative ET between FACE (F–L) and Control (C–L) for the Low N treatment in the 1995–1996 season (a) and for the Low N treatment in the 1996–1997 season (b).
F–H was nearly equal to that for the C–H treatment. However, this trend was reversed during the remainder of the season and the cumulative ET for F–H gradually decreased relative to the C–H treatment, reaching a reduction of 4.7% on DOY 106 (≈early grain-fill). The total cumulative ET for F–H at the end of the 1996–1997 season was 4.0% less than the total for C–H.
For the Low N treatment in 1995–1996 (Fig. 8a), the cumulative ET for F–L was higher than that for C–L through about early-March. However, from
mid-March through late-March, there was a change in the ET response between the two Low N treatments, such that the cumulative ET for F–L fell behind that for C–L by as much as 3.8% on DOY 082 (22 March). This trend then reversed starting in late-March when F–L attained more ET than C–L. Consequently, the total cumulative ET for F–L for the 1995–1996 season was only 0.7% less than that of the C–L treatment.
the season. On DOY 090 (31 March), the reduction in cumulative ET for the F–L treatment had declined to 5%. By the end of the 1996–1997 season, the total cumulative ET for F–L was only 1.2% less than the total cumulative ET for C–L.
3.3. Water use efficiency
3.3.1. 1995–1996
Final grain yield of spring wheat in 1995–1996 in-creased an average of 15 and 12% under CO2 enrich-ment for the High and Low N treatenrich-ments, respectively. The Low N treatment reduced grain yield an average of 22 and 24% within Control and FACE treatments, respectively. Note that the reduction in grain yield for the Low N treatments was nearly proportional to the reduction in total cumulative ET for the treatments (i.e. 25 and 23% less ET occurred in the C–L and F–L treatments than the C–H and F–H treatments, respec-tively).
The mean values for WUE, expressed as final grain yield per unit of seasonal ET (kg/m3), are presented for the 1995–1996 treatments in Table 5. The FACE treatment resulted in a 19 and 12% increase in WUE
Table 6
Treatment means for water use efficiency (kg/m3) in 1996–1997a
Treatments High soil N Low soil N Ratio of Low N/High N
Control 1.004 a 1.085 a 1.08
FACE 1.232 b 1.163 b 0.94
Ratio of FACE/Control 1.23 1.07
aData are means of four replicates for all treatments except the Control, Low N treatment, which had three replicates; means followed
by different letters in a row or column are significantly different at the 0.01 probability.
for Control, the WUE for the Low N treatment was 5% higher than that for the High N treatment. The GLM also indicated that the effects on WUE due to nitrogen level (p=0.77) or CO2by nitrogen interaction (p=0.30) were not significant.
3.3.2. 1996–1997
Final grain yield of spring wheat in 1996–1997 in-creased an average of 17 and 5% under CO2 enrich-ment for the High and Low N treatenrich-ments, respectively. The Low N treatment reduced grain yield an average of 13 and 24% in the Control and FACE treatments, respectively. Thus, the reduction in grain yield for the Low N treatments was less than the reduction in to-tal cumulative ET under the Control treatment (22%), but under FACE, the yield reduction for the Low N treatment was slightly greater than the ET reduction of 20%.
higher (8%) than that for the Control High N treatment, whereas the WUE for the FACE Low N treatment was again lower (6%) than that for the FACE High N treat-ment. As in the first year, the effects on WUE due to nitrogen level (p=0.90) or due to CO2by nitrogen interaction (p=0.25) were not highly significant.
4. Discussion
Soil water depletion measurements combined with FAO-56 basal crop coefficient procedures were used to determine ET. It is important to note that the ET values derived for the various treatments in the study have a degree of uncertainty due to potential errors made in measurements of soil water or in parameter es-timates used in the crop coefficient procedures. Since soil water content measurement was the fundamental measurement used in determining ET, it represented the most significant potential source of error. The esti-mated accuracy of the water content measurement for the TDR system used in this study is±2%, accord-ing to the manufacturer of the device. The estimated accuracy of the water content measured by properly calibrated neutron moisture gauges has been often re-ported to be about±2% (Sinclair and Williams, 1979; Haverkamp et al., 1984). The estimated accuracy of the neutron gauge used in this study (±2.4%) was sim-ilar to the reported accuracy. However, water content measurement may have resulted in ET accuracy less than 2.4% because ET was primarily based on the dif-ference in the measured soil water between two suc-cessive dates (i.e. soil water depletion). Sinclair and Williams (1979) reported that the error associated with the change in water content measured by a neutron gauge at a depth of 0.30 m was typically about 5% for a properly calibrated gauge. In a more recent study, Evett et al. (1993) used a combination of TDR mea-surements at the soil surface (0–0.40 m) and neutron gauge measurements at deeper depths (0.4–2.0 m) to determine the change in water storage over a 16-day period for a winter wheat crop grown in precision weighing lysimeters. Their results indicated excellent agreement (<1 mm) between the change in water stor-age determined from soil water measurements and lysimeters. Since the wheat ET for the16-day period was about 95 mm, the error in ET associated with the soil water measurement was less than 1%. This error
was about one fifth of that realized when neutron scat-tering alone was used (Evett et al., 1993), indicating the great improvement in accuracy when the TDR is used to determine water content changes at the soil surface. Since the ET in our study was determined us-ing the TDR and neutron scatterus-ing combination, we estimate that the uncertainty in ET introduced during measurement of soil water depletion was no larger than 3%. However, additional errors in the soil depletion determination of ET may have been introduced under the assumption of zero water flux below the estimated crop rooting depth.
Any errors in ET determined during soil water de-pletion periods would also introduce errors in the sim-ulated ET for soil wetting periods. Furthermore, the uncertainty in ET during soil wetting periods would be expected to have been greater than 3%, since crop, soil, and climatic parameters used in the crop coeffi-cient evaluation could also introduce errors that might impact the resultant ET and soil evaporation values during those periods. Hunsaker (1999) using the ap-proach presented in this study determined crop ET for irrigated cotton in Arizona. In that 2-year study, comparison between predicted and measured ET dur-ing soil wettdur-ing periods indicated that the error in ET based on the crop coefficient approach averaged about 9%. Assuming a similar accuracy in ET during the soil wetting periods of the current study, we estimate that the uncertainty in total ET for the various treat-ments in the study was probably greater than 5% but less than 10%.
Conceivably, real differences in the ET between CO2 treatments could have been masked amid the potential errors noted above and therefore possibly were not detected with the approach used in these studies. However, although differences in calculated ET between CO2 treatments were not detected, dif-ferences in ET between high and low soil nitrogen treatments were large and statistically significant. It should also be pointed out that the total ET values of 601 and 595 mm in 1995–1996 and 1996–1997, re-spectively, that were obtained under ambient CO2and well-fertilized conditions were similar to the seasonal wheat ET determined under those conditions in other studies in Arizona, e.g. 650 mm (Erie et al., 1982) and 580–610 mm (Hunsaker and Bucks, 1992).
1996), was 4.5 and 5.8% less in FACE than Control in 1992–1993 and 1993–1994, respectively. Thus, in all four studies with wheat, the total cumulative ET was decreased for FACE when water and nutrient in-puts were adequate. In three of the four studies, the reduction was 4% or more.
Parallel investigations using a residual energy bal-ance approach to measure ET (Kimball et al., 1995, 1999) in the same four FACE wheat studies were in general agreement with the soil water balance ET results under well-watered and well-fertilized con-ditions. The energy balance approach determined a 4-year average reduction of 6.7% in wheat ET for the FACE treatment, although the reduction was less than 6.0% in the 1995–1996 and 1996–1997 years. Considering the two independent investigations, it is apparent that a modest reduction in the ET of wheat occurs under elevated CO2, when water and nitrogen are not limited.
In the present studies, the total ET under limited soil N was determined to be only 0.7 and 1.2% less for FACE than for Control. However, seasonal trends re-vealed that the cumulative ET for F–L was 3–5% less than that for C–L through (late-March) in both sea-sons. Following the final fertilizer application to the Low N treatment in late-March of both years, the ET rate for F–L was increased relative to C–L through the remainder of the seasons. Because the CO2 enrich-ment effect produced larger and more vigorous plants than those in the Control treatment, the FACE wheat plants under Low N may have been able to more fully exploit the available nutrients during the late season, and thus consume more water than the Control dur-ing that period. Although the 25% fAPAR∗ data did
was used to determine the relative differences in ET between the treatments but not the absolute cumulative values. Also, as discussed by Kimball et al. (1999), there were differences in leaf area, canopy architec-ture, and leaf greenness between the CO2treatments, particularly under the Low N treatments. Thus, it is conceivable that the lower leaf area in the Control plots under Low N resulted in the infrared thermometers to have viewed warmer soil temperatures rather than leaf temperatures. Consequently, the sensible heat compo-nent of the energy balance may have been overesti-mated in the Control Low N plots, which would result in ET estimates that were too low. Similarly, if canopy coverage was overestimated for the Control Low N plots, the ET determined from the soil water balance procedures may have also been too low. In this case, the soil water evaporation component of ET for the Control Low N treatment would have been underes-timated following irrigation and large rains resulting in underestimated total ET during the soil wetting pe-riods. On the other hand, as indicated above, the soil water approach was subject to a possible accumula-tion of errors that may have been more exaggerated under Low N than High N conditions.
in WUE for FACE under the Low N treatment for the 1995–1996 and 1996–1997 seasons, respectively, were considerably lower than the 15 and 24% in-creases in WUE for FACE under limited water con-ditions in the 1992–1993 and 1993–1994 studies, respectively. These results suggest that the benefits of higher CO2 on the yield and water use efficiencies of wheat, which appear to be increased under water stress, were greatly diminished when limited nitrogen supplies were encountered.
5. Conclusions
The results from these studies support previous findings that suggested that elevated CO2 causes a small reduction in wheat ET under well-watered and high soil N conditions. The implication to irrigation water management is that the seasonal water require-ments for fully-irrigated wheat may be slightly lower in the future high-CO2 environment, provided that a concomitant change in climate warming does not occur. The net effect of a small decrease in water con-sumption combined with the expectant higher yields under increased CO2 is expected to result in an in-crease in wheat water use efficiencies of about 20%. Under limited irrigation and adequate nutrient supply, the stimulatory effects of CO2on wheat tend to coun-teract the effects of water stress on reducing yield. This is particularly important if water supplies for agricultural food production in the future high-CO2 world decline and/or become more expensive. How-ever, under a limited soil nitrogen, the beneficial effects of elevated CO2on wheat yield and water use efficiency appear to be diminished.
Acknowledgements
This research was supported by grant DE-FG03-95ER-62072 from the Office of Biological and Environmental Research, Environmental Sciences Division, Department of Energy Terrestrial Carbon Processes Research Program to the University of Ari-zona, Tucson, and Maricopa, Arizona (S. Leavitt, T. Thompson, A. Matthias, R. Rauschkolb, and H. Cho are principal investigators). We acknowledge the help-ful cooperation of Roy Rauschkolb (deceased) and his
staff at the Maricopa Agricultural Center. The FACE apparatus was furnished by Brookhaven National Laboratory, Upton, New York, and we are grateful to Keith Lewin, John Nagy, and George Hendrey for as-sisting in its installation and consulting about its use.
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