Temporal changes in soil water content
During 1998, volumetric soil water content measurements averaged from 0.0 to 1.8 m of depth (Fig. 1a) and showed two distinct dry-down periods for the dry plots; the first DOY 228–254 and the second DOY 275–330. We observed three dry-down periods in the dry treatment during 1999: DOY 198–218, DOY 228–258, and DOY 268–290. During 1998, periods of plant stress in dry treatments, as inferred from a 30% drop in soil moisture below field capacity, occurred during DOY 247–253 and DOY 290–325 whereas they occurred from DOY 202–218, DOY 244–258, and after DOY 280 until maturity during 1999 (Fig. 1b).
Figure 1. Mean volumetric soil water contents (0.0–1.8 m depth) in the control-dry (CD; open circles), FACE-dry (FD; closed circles), control-wet (CW, open squares), and FACE-wet (FW, closed squares) plots for the 1998 (a) and 1999 (b) sorghum growing seasons. Bars denote standard errors. Each mean is the average of four replicates. A, when only wet plots were irrigated; B,when both wet and dry plots were irrigated. NS, *, **, *** = not significant at P > 0.25 and significant at P < 0.25, 0.15, 0.05, respectively.
Download figure to PowerPoint
During the first year (1998), differences in cumulative ET between wet and dry treatments were apparent by DOY 273 and were significant over the season (Tables 1, 2). Aside from a divergence during DOY 260–290, when plants were undergoing reproductive growth, FD and CD treatments showed similar patterns in cumulative ET with essentially no difference. However, clear trend differences between CO2 treatments were evident in wet plots beginning on DOY 260. FW plots evapotranspired 60 (± 117) mm or 11% (P = 0.40, df = 11.7) less than CW over the growing season (Fig. 2a). During 1999, differences in cumulative ET between wet and dry treatments began at DOY 220, became more pronounced after DOY 240, and were significant over the season (Tables 1, 2). Seasonal ET of FD tended to be 25 (± 14) mm or 6% (P = 0.27, df = 12) less than CD in 1999; whereas under the wet treatment there was a clearly significant CO2 effect, where FW evapotranspired 58 (± 34) mm or 9% (P = 0.02, df = 12) less water than CW plants (Fig. 2b).
Table 1. Sorghum grain yield (Ottman et al., 2001), evapotranspiration (ET), water-use efficiency based on grain yield (WUE-G), the percent difference in WUE-G due to FACE enrichment and related ANOVA test P-values, ** signifies P = 0.10
| ||Yield (g m−2)||ET (mm)||WUE-G (g m−2 mm−1)||WUE-G % diff.||Yield (g m−2)||ET (mm)||WUE-G (g m−2 mm−1)||WUE-G % diff.|
|FACE-Dry (FD)||553 (± 30)||292 (± 21)||1.93 (± 0.24)||15% (± 24%)||142 (± 33)||404 (± 9)||0.36 (± 0.09)||45%** (± 26%)|
|Control-Dry (CD)||472 (± 58)||293 (± 22)||1.68 (± 0.32)|| ||106 (± 18)||429 (± 6)||0.25 (± 0.04)|| |
|FACE-Wet (FW)||677 (± 22)||498 (± 72)||1.43 (± 0.24)||15% (± 21%)||424 (± 21)||605 (± 15)||0.70 (± 0.05)||−3% (± 6%)|
|Control-Wet (CW)||670 (± 11)||559 (± 58)||1.24 (± 0.15)|| ||475 (± 13)||664 (± 25)||0.72 (± 0.05)|| |
|CO2|| ||0.57||0.41|| || ||0.02||0.44|| |
|H2O|| ||< 0.01||.03|| || ||< 0.01|| < 0.01|| |
|H2O × CO2|| ||0.55||0.87|| || ||0.30||0.03|| |
Table 2. Sorghum biomass (Ottman et al., 2001), evapotranspiration (ET), water-use efficiency based on biomass (WUE-B), the percent difference in WUE-B due to FACE enrichment and related ANOVA test P-values
| ||Yield (g m−2)||ET (mm)||WUE-B (g m−2 mm−1)||WUE-B % diff.||Yield (g m−2)||ET (mm)||WUE-B (g m−2 mm−1)||WUE-B % diff.|
|FACE-Dry (FD)||1332 (± 80)||292 (± 21)||4.64 (± 0.61)||12% (± 17%)||970 (± 69)||404 (± 9)||2.41 (± 0.22)||26% (± 15%)|
|Control-Dry (CD)||1176 (± 73)||293 (± 22)||4.13 (± 0.55)|| ||822 (± 58)||429 (± 6)||1.91 (± 0.16)|| |
|FACE-Wet (FW)||1658 (± 43)||498 (± 72)||3.51 (± 0.57)||22% (± 23%)||1551 (± 54)||605 (± 15)||2.56 (± 0.15)||8% (± 3%)|
|Control-Wet (FW)||1554 (± 5)||559 (± 58)||2.87 (± 0.30)|| ||1566 (± 17)||664 (± 25)||2.37 (± 0.11)|| |
|CO2|| ||0.57||0.28|| || ||0.02||0.04|| |
|H2O|| ||< 0.01||0.02|| || ||< 0.01||0.07|| |
|H2O × CO2|| ||0.55||0.87|| || ||0.30||0.28|| |
Figure 2. Mean cumulative evapotranspiration (ET) for the control-dry (CD; open circles), FACE-dry (FD; closed circles), control-wet (CW; open squares), and FACE-wet (FW; closed squares) plots in 1998 (a) and 1999 (b) sorghum growing seasons. Dashed line, modeled ET. Bars denote standard errors. Each mean is the average of four replications. The dotted line represents modelled ET from the Arizona Meteorological Network AZMET (Brown, 1987) adjusted for sorghum. NS, *,**, *** = not significant at P > 0.25 and significant at P < 0.25, 0.15, 0.05, respectively.
Download figure to PowerPoint
Grain yield and total above-ground biomass were measured in 1998 and 1999 by Ottman et al. (2001). Water-use efficiency based on grain yield (WUE-G) was calculated as the ratio of grain yield m−2 mm−1 ET (Table 1). WUE based on biomass (WUE-B) was calculated as the ratio of shoot biomass m−2 mm−1 ET (Table 2). In 1998, WUE-G and WUE-B differences between dry and wet treatments were significant (Tables 1, 2). However, the ANOVA result for the two-way interaction effect was nonsignificant. In 1999, significant differences between wet and dry treatments were again evident for WUE-G and WUE-B (Tables 1, 2). Additionally, the interaction effects between CO2 and irrigation were significant in WUE-G (P = 0.03, df = 2.84). WUE-G for FD was 0.11 (± 0.08) g m−2 mm−1 or 45% significantly greater (P = 0.10, df = 5.3) than CD, while FW was not significantly different from CW. The interaction between CO2 and irrigation was not significant for WUE-B in 1999 (P = 0.28, df = 6). Nevertheless, the overall CO2 effect tended to be larger under dry (26%) than wet (8%) (Tables 1, 2). These data suggest an increasing WUE trend due to CO2 enrichment exists with increasing drought. Under extreme drought, increases in WUE may become significant, as was the case for WUE-G in 1999. Therefore a yield increase is likely without additional use of water resources at higher than present day ambient CO2 concentrations.
Increased drought resulting from increased water demand, coupled with a decreased total of applied water can explain the large increase in yield, biomass, and WUE in 1998 relative to 1999. Drought was partially due to an earlier planting date in 1999 (June 15th (DOY 166) in 1999, and July 16th (DOY 197) in 1998). Additionally, the average air temperature over the growing season was 22.5°C in 1998 and 28.8°C in 1999. Therefore 1999 sorghum plants were growing at a hotter time of the year and experienced a greater degree of heat stress. This is consistent with the corresponding lesser (100 mm) modelled ET adjusted for sorghum in 1998 compared with 1999 (Fig. 2a,b). Another factor that may have decreased grain yield and biomass in 1999 is a wind and hail storm, which shredded the sorghum leaves at the time of grain fill on DOY 262 and consequently disrupted carbon translocation to grain.
Large water applications to the dry treatments at the time of flag leaf emergence in 1998 may have prevented late season drought from developing. The only period of substantial drought for the dry treatment in 1998 was around DOY 250 (Fig. 1a). Thus, plant roots in the dry 1998 treatment were able to extend to depths more typical of well-watered plants (1.6 m) after the second irrigation event. These plants were able to utilize water from the second irrigation as well as residual soil water from the first irrigation still stored at lower depths. This use of deeper water, along with decreased late season climatic demand allowed dry plants to complete the season with only moderate drought. Wet plants received adequate water throughout the 1998 season. However, in 1999 a possibility existed that they did not, due to the extreme summer climate and possible imprecision in application of AZSCHED for irrigation timing. A possible water shortage in wet treatments may have resulted in some degree of drought occurring from DOY 200–220 and again from DOY 244–263 (Fig. 2b). Even a short period of drought in the 1999 wet plants during vegetative growth could have caused decreased yields. Dry plants were substantially droughted in 1999. The only period of recovery for dry plants occurred over DOY 222–240.
Soil substrate properties such as texture and water holding capacity can vary across location and over depth. This was the case at the FACE sorghum field site as evidenced by the consistent difference between the FACE and control soil water contents following re-wetting (Fig. 1a,b). Factors that directly influence root growth include soil strength, soil water, soil temperature and composition of soil atmosphere (McMichael et al., 1993). However, minimal plot substrate variability will not significantly influence the water extraction front velocity (root growth) or the consequent above ground biomass accumulation (Meinke et al., 1993). Therefore relative differences in soil moisture between each treatment should be representative of the treatment ET effect. Differences in ET within the magnitude of 5–90 mm, or 1–20% are difficult to discern with the neutron equipment and soil water balance technique. Consequently, significant statistical differences in ET for CO2 treatments were not evident over both years at the P < 0.25 level. This parallels the results of previous FACE experiments utilizing similar methodology (Hunsaker et al., 1996). Over both years, there was no significance in WUE-G or WUE-B (P > 0.25) as a result of CO2 enrichment. However, there was a significant (P = 0.09, df = 5.28) WUE-G increase in plants grown in the dry treatment, in 1999, under CO2 enrichment.
CO2 enrichment at 200 µmol mol−1 above today’s ambient mole fraction (c. 570 µmol mol−1) reduced ET by 13 (± 18) mm or 4% and 60 (± 151) mm or 10% averaged over both years, for droughted and well-watered sorghum, respectively. CO2 enrichment also increased WUE-G by 0.18 (± 0.21) g m−2 mm−1 or 19% and 0.08 (± 0.1) g m−2 mm−1 or 9% and WUE-B by 0.50 (± 0.49) g m−2 mm−1 or 17% and 0.42 (± 0.33) g m−2 mm−1 or 16% over both years, for droughted and well-watered sorghum, respectively. Elevated CO2 caused partial stomatal closure, reduced stomatal conductance, and decreased transpiration per unit of leaf area in both wet and dry plots (Wall et al., 2001). However, the C4 sorghum did not exhibit an increased leaf area in wet plots (Ottman et al., 2001) consistent with observations on C4 maize (Samarakoon & Gifford, 1995), so water was conserved in these plots (Fig. 2a,b). In the dry plots, CO2-enriched plants exhibited reduced stomatal conductance (Wall et al., 2001), which conserved water and enabled them to grow further into a drying cycle. Cumulative ET of FD and CD plants were similar (Fig. 2a,b), however, because FD plants were able to maintain growth longer into a drying cycle than CD plants (Ottman et al., 2001) they had a greater WUE trend (Tables 1, 2). Additionally, FD plants were expected to have greater root length and mass, which would enable them to better mine the soil profile for water and nutrients (Chaudhuri et al., 1986; Wechsung et al., 1999). Therefore, we accept the hypothesis that elevated CO2 will cause a C4 crop (sorghum) to decrease ET under well-watered conditions and to increase WUE under both well-watered and droughted conditions.
Averaging over both years and over irrigation treatments, CO2 enrichment caused a reduction in ET by 36 (± 84) mm or 7%. Both WUE-G and WUE-B, were increased 0.13 (± 0.16) g m−2 mm−1 or 14% and 0.46 (± 0.41) g m−2 mm−1 or 16%, respectively, as a result of to CO2 enrichment. Therefore, our data show that future water requirements for irrigated sorghum should decrease slightly, provided global warming is minimal. Under rain-fed conditions, where sorghum is more likely to experience drought, elevated CO2 will likely cause a productivity increase. Moreover, under ample-water or water-limited conditions, increases in CO2 are likely to cause WUE to increase.