Agricultural and Forest Meteorology 104 2000 85–105
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
d
a
US Water Conservation Laboratory, Agricultural Research Service, USDA, Phoenix, AZ 85040, USA
b
Laboratory of Tree Ring Research, University of Arizona, Tucson, AZ 85721, USA
c
Department of Soil, Water, and Environmental Science, University of Arizona, Tucson, AZ 85721, USA
d
Maricopa 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 370 mmol mol
− 1
during daytime and enriched 200 mmol mol
− 1
above ambient CO
2
concentrations during 1995–1996 and 1996–1997 in Free-Air CO
2
Enrichment FACE experiments in central Arizona. The enriched FACE and ambient Control CO
2
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 CO
2
treatments 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 CO
2
concentration increased from approximately 280 mmol mol
− 1
in pre-industrial times to about 315 mmol mol
− 1
by 1958, and to more
∗
Corresponding author. Tel.: +1-602-379-4356ext. 266; fax: +
1-602-379-4355. E-mail address: dhunsakeruswcl.ars.ag.gov D.J. Hunsaker
than 350 mmol mol
− 1
by 1988 Boden et al., 1994. The accelerated trend in the global CO
2
growth rate during the first 30 years of modern records has led
to various scenarios for the future CO
2
concentrations of the atmosphere. The report of the Intergovernmen-
tal Panel on Climate Change 1996 projects CO
2
in- creasing to about 500 mmol mol
− 1
before the end of the 21st century. Hoffman and Wells 1987 and Tren-
berth 1991 estimate a near-doubling of atmospheric CO
2
over the pre-industrial level by the end of the 21st century.
0168-192300 – see front matter Published by Elsevier Science B.V. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 1 5 7 - X
86 D.J. Hunsaker et al. Agricultural and Forest Meteorology 104 2000 85–105
If the atmospheric CO
2
concentration continues to rise in the future, the growth and yield of many impor-
tant agricultural crops is expected to be significantly increased Kimball, 1983. One natural concern for
future irrigated agriculture is how higher CO
2
concen- trations will affect crop evapotranspiration ET and,
hence, the management of irrigation water. Larger and more vigorous plants from a CO
2
-enriched environ- ment may be associated with increased crop ET. How-
ever, CO
2
enrichment may also slow plant transpi- ration by partially closing stomata Morrison, 1985;
Allen, 1990. Some investigations have reported a de- crease in leaf transpiration per unit leaf area at higher
atmospheric CO
2
concentrations Kimball and Idso, 1983; Goudriaan and Unsworth, 1990.
Previous experimental studies to assess the effects of CO
2
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 CO
2
around 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 CO
2
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 CO
2
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 CO
2
. Controlled, chamber experiments with soybeans Jones et al., 1985a indi-
cated that differences in whole canopy transpiration between CO
2
levels of 330 and 800 mmol mol
− 1
were not significant, whereas in a similar experiment Jones
et al., 1985b, soybean transpiration was reduced to 10 at a CO
2
level of 660 mmol mol
− 1
. Chaudhuri et al. 1990 reported little effect of CO
2
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 CO
2
enrichment exper- iments, a Free-Air CO
2
Enrichment FACE system was developed to evaluate the effects of increased CO
2
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.
During the 1990 and 1991 FACE experiments, cotton ET was measured using three independent approaches;
sap flow gauges Dugas et al., 1994, energy balance Kimball et al., 1994a, and soil water balance Hun-
saker et al., 1994. All three investigations were in gen- erally good agreement, and the overall conclusion was
that any effects of CO
2
enrichment to 550 mmol mol
− 1
on ET were too small to be detected. Without a signifi- cant change in ET, however, the water use efficiency of
the cotton was increased in proportion to the increase in lint yield due to CO
2
enrichment, which was more than 50 for both 1990 and 1991 Pinter et al., 1996a.
Experiments to investigate the responses of wheat to CO
2
enrichment and irrigation were conducted at the FACE system site during the 1992–1993 and
1993–1994 spring wheat seasons Pinter et al., 1996a. In those studies, wheat, exposed to ambient about
3770 mmol mol
− 1
and elevated 550 mmol mol
− 1
CO
2
concentrations, was grown under well-watered and water-stressed treatments and supplied with am-
ple soil nitrogen. Hunsaker et al. 1996, using a soil water balance to determine wheat ET, reported
that seasonal ET for the enriched CO
2
treatment was reduced by an average of 4.5 and 5.8 under the
well-watered treatment, but was increased by an aver- age of 4.8 and 0.9 under the water-stress treatment
for the 2 years, respectively. The relatively small changes in ET when coupled with the increases in
grain yields caused by elevated CO
2
represented an average increase in the water use efficiency WUE
of 13–15 and 18–24 for the well-watered and water-stress treatments, respectively. In the same
FACE wheat experiments, Kimball et al. 1994b, 1995, using an energy balance approach, and Senock
et al. 1996, using sap flow gauges, determined wheat ET and transpiration, respectively, for the
well-watered treatment. Kimball et al. 1994b, 1995 reported a greater reduction in the wheat ET due to
CO
2
enrichment 8 and 11 in the first and second experiments, respectively than that determined by
the soil water balance. In the first FACE wheat exper- iment, Senock et al. 1996 reported that the reduction
in transpiration due to elevated CO
2
ranged from 7 to 23, but the variability was high and the differences
were often not statistically significant. The objective of this paper was to evaluate the effects of CO
2
en- richment on ET and water use efficiency, using a soil
water balance approach, for two well-watered, spring
D.J. Hunsaker et al. Agricultural and Forest Meteorology 104 2000 85–105 87
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 CO
2
on the water use for wheat in the same FACE experiments.
2. Methods and materials
