Data from the Arizona FACE ( Free-Air CO 2 Enrichment ) Experiments on Wheat at Ample and Limiting Levels of Water and Nitrogen

Four free-air CO2 enrichment (FACE) experiments were conducted on wheat (Triticum aestivum L. cv. Yecora Rojo) at Maricopa, Arizona, U.S.A. from December, 1992 through May, 1997. The first two were conducted at ample and limited (50% of ample) supplies of water, and second two at ample (350 kg N ha) and limited (70 and 15 kg N ha) supplies of fertilizer nitrogen. More than 50 scientists participated, and they collected a large and varied set of data on plant, soil, and microclimatic responses to the elevated CO2 and its interactions with the water and N treatments. The dataset has been popular with wheat growth modelers who have utilized the growth, yield, and other data to validate their models, which get used to predict likely future wheat productivity with projected global change. The dataset assembled herein contains many of these data, including management, soils, weather, physiology, phenology, biomass growth, leaf area, yield, quality, canopy temperatures, energy balance, soil moisture, nitrogen assimilation, and other data.

flow, soil and plant elemental analyses, tissue nitrogen biochemistry, soil water content, photosynthesis, respiration, stomatal conductance, leaf water potential, carbohydrates, photosynthetic proteins, antioxidants, phenolics, stomatal density and anatomy, digestibility, decomposition, grain quality, bread baking quality, soil CO 2 fluxes, and changes in soil C storage from soil and plant C isotopes.Details about many of these measurements can be found in papers listed in the REFERENCES.
Besides the direct determination of the responses of wheat to elevated CO 2 and its interactions with water supply and nitrogen, the data collected in the FACE Wheat experiments have also proved useful for validating wheat growth models.To date, at least 13 papers have resulted from such model tests, as listed in the REFERENCES.Initially, the management, weather, growth, and yield data were formatted in the IBSNAT format, which is now obsolete, and then they were re-formatted to ICASA Version 1.0.More recently, they have been converted to ICASA Version 2.0 (White et al., 2013), which in turn is undergoing some modifications under the umbrella of the AgMIP (Agricultural Model Inter-comparison and Improvement Project; http://www.agmip.org/).These data are included in this dataset in file Biomass Area Phenology Management Weather Soil Moisture.ods.They currently are being used in another AgMIP model inter-comparison study involving wheat models which utilize canopy temperature information (as opposed to just growing the crop at air temperature).

METHODS:
Two experiments were conducted during the 1992-3 and 1993-4 growing seasons to determine the interactive effects of elevated CO 2 and limited soil water on spring wheat (Triticum aestivum L. cv.Yecora Rojo) at the University of Arizona Maricopa Agricultural Center (MAC), Maricopa, Arizona, USA (33.06°N latitude, -111.98W longitude, 361 m elevation).Two additional experiments were similarly conducted to determine the interactive effects of elevated CO 2 and limited soil nitrogen during the 1995-6 and 1996-7 growing seasons.A field plot plan for the 1992-3 and 1993-4 experiments is presented by Wall and Kimball (1993), while that for the 1995-6 and 1996-7 experiments is Figure 1 of Kimball et al. (1999).The soil is classified as a reclaimed, Trix clay loam [fine-loamy, mixed (calcareous), hyperthermic Typic Torrifluvents]; additional details about the soil properties are given by Post et al. (1988) and Kimball et al. (1992).

CO 2 TREATMENTS:
The free-air CO 2 enrichment (FACE) technique was used to enrich the air in circular plots within a wheat field similar to prior experiments (Hendrey, 1993;Wall and Kimball, 1993;Dugas and Pinter, 1994;Kimball et al., 1995Kimball et al., , 1999)).Briefly, four replicate 25-m-dia.toroidal plenum rings constructed from 0.305-m-dia.pipe were placed in the field shortly after planting.The rings had 2.5-m-high vertical stand pipes with individual valves spaced about every 2.4 m around the periphery.Air enriched with CO 2 was blown into the rings, and it exited near the canopy top through tridirectional jets in the vertical pipes.Wind direction, wind speed, and CO 2 concentration were measured at the center of each ring.A computer-control system used the wind speed and CO 2 concentration information to adjust the CO 2 flow rates to maintain the desired CO 2 concentrations at the centers of the FACE rings.The system used the wind direction information to turn on only those stand pipes upwind of the plots, so that CO 2 -enriched air flowed across the plots no matter which way the wind blew.When wind speeds were low (< 0.4 m/s), and it was difficult to detect direction, the CO 2 -enriched air was released from every other vertical pipe around the rings.Starting with the 1995-6 season, air blowers were installed in the non-CO 2 -enriched ambient control plots, hereafter called Blower plots, to provide air movement similar to that of the FACE plots (Pinter et al., 2000).The Blower plots had toroidal plenums like the FACE plots, but their vertical pipes were spaced about every 5 m around the periphery, and there were no valves on the vertical pipes.Thus, the air flowed all the time, and it was not changed in response to changing wind speed or direction.This strategy was justified because we believe the air flow in the Blower plots was important only under calm conditions (wind < 0.4 m/s) when the FACE plots were operated in the mode of releasing CO 2 -enriched air from every other vertical pipe.Also starting in 1995-6, unlike the prior experiments which had a constant set-point of 550 μmol/mol CO 2 concentration, the FACE plots were enriched by 200 μmol/mol above ambient (~ 360 μmol/mol).A separate sequential sampling system was used to measure the concentration in all of the FACE and Blower plots, as well as from two additional ambient sampling points north of Rep 1. Twenty seconds were required to measure the concentration in each plot, and 3 min for all of them.The minimum value from among the most recent observations of the four Blower plots and the two ambient points was selected to provide THE ambient value for the next 20 sec against which to reference the 200 μmol/mol enrichment in the FACE plots.By selecting the minimum value, we generally were choosing the values from the most upwind plots, thereby avoiding contamination of the ambient value by CO 2 from the FACE plots.The FACE treatment was applied continuously from emergence to harvest.For 1995-6, the average daytime CO 2 concentrations in the FACE and Blower plots were 548 and 363 μmol/mol, respectively, while the nighttime values were 598 and 412 μmol/mol.Thus, the daytime elevation of CO 2 concentration in the FACE plots above the Blower plots was 185 μmol/mol, and these data also indicate the average contamination of the Blower plots with CO 2 from the FACE plots was 15 μmol/mol above the upwind ambient concentration.

IRRIGATION TREATMENTS:
Irrigations were accomplished using a subsurface drip system with a tube depth of about 0.23 m, a tube spacing of 0.50 m, and an emitter spacing of 0.30 m.The irrigation scheduling methodology and soil water content data are presented in detail for the 1992-3 and 1993-4 seasons by Hunsaker et al. (1996).Irrigation water requirements for the ample (Wet) treatment were based on a computer-based irrigation scheduling program (AZSCHED; Fox et al., 1992).After about 30% of the available water in the rooted zone was depleted, they were irrigated by an amount calculated to replace 100% of the potential evapotranspiration since the last irrigation (adjusted for rainfall).The low-water (Dry) treatment plots received 50% of the amounts of the Wet treatment for each irrigation in 1993, whereas in 1994, they were irrigated only every other time the Wet plots were irrigated.The Wet and Dry sides of both the FACE and Control plots shared the same tubes, which extended across whole replicates.Therefore, the experimental design was strip-split-plot (Hunsaker et al., 1996).Cumulative irrigation totals between crop emergence and harvest were 600 mm and 620 mm for the Wet treatments in 1993 and 1994, respectively, and 275 and 257 mm for the Dry plots.Corresponding rainfall amounts were 76 mm and 61 mm, respectively.In 1995-6 and 1996-7 all plots were irrigated like the Wet plots in 1992-3 and 1993-4.In 1995-6 cumulative irrigation amounts were 692 and 631 mm for the high-and low-N treatments, respectively, and in 1996-7 they were 621 and 548 mm.(The amounts for the high-and low-N treatments would have been nearly identical except the last irrigations were curtailed due to earlier maturity of the low-N plants.Seasonal rainfall amounted to 39 and 29 mm, respectively for the two seasons.

SOIL NITROGEN TREATMENTS:
During 1992-3 and 1993-4, the plots received ample amounts of nitrogen fertilizer via the irrigation system.A total of 271 kg N ha -1 was applied in 1992-3 and 261 kg N ha -1 in 1993-4.During 1995-6 and 1996-7, half of each of the main FACE and Blower plots received either an ample (High-N) or a limiting (Low-N) level of nitrogen fertilizer.The High-N plots received a total of 350 kg N/ha from ammonium nitrate in 4 applications during both seasons.The Low-N plots received 70 and 15 kg N/ha during 1995-6 and 1996-7, respectively.An additional 33 and 30 kg/ha of N were added to the High-and Low-N plots, respectively, from the irrigation water itself.An unfortunate oversight during the 1996-7 season resulted in the nitrogen applications being switched between the High-and Low-N sides of the Blower-Replicate 3 plots on 5 March 1997 (DOY 64).The mistake was discovered about a week later, and the High-N plot was salvaged by applying additional nitrogen to it.Thus, any analysis of these data needs to account of this complication.

CROP CULTURE:
In 1992 FACE and Control arrays were moved to new areas within the same field where the FACE cotton experiments had been conducted.Then in 1995 between the FACE x water stress and FACE x nitrogen stress experiments, the FACE and Blower arrays were moved again within the same field to virgin areas where extensive soil sampling had not been done.During the 1994-5 winter growing season (between the FACE experiments), domestic oats (Avena sp.) were grown as an N-removal crop and cut several times as green forage.Following the oat crop, the soils were deeply ripped in two directions and disked.Then new drip irrigation tubing was installed, as described above.A large pre-plant irrigation (about 150 mm) was applied with sprinklers between 30 Nov and 2 Dec 95 in order to leach as much initial soil nitrogen as possible and germinate remaining oat seeds which were suppressed with Roundup [Note: Trade names and company names are included for the benefit of the reader and do not imply any endorsement or preferential treatment of the product by the authors, the U.S. Department of Agriculture, or the University of Arizona] on 12 December 1995.Despite these efforts, a substantial number of volunteer oat plants appeared in the later wheat crops, and they had to be rogued by hand from experimental plots.Certified Yecora Rojo wheat seed was planted at mid-December in all seasons in east-west rows that were spaced 0.25 m apart (parallel to the drip irrigation tubing).In 1992-3 a large initial irrigation was applied with the drip irrigation system for germination, while in the subsequent seasons, germination was accomplished with smaller irrigation amounts applied using sprinklers.Fifty percent emergence of seedlings was observed about 1 January in all seasons, and FACE treatments commenced at that time.A combination of biological and chemical methods were used for control of oat bird cherry aphids and broadleaf weeds; losses caused by these pests were judged to be minimal.Air temperatures (2-m height) typically ranged from -5 to 42ΕC.Growing-degree-days amounted to about 2000, except for the Low-N treatments which matured earlier as evidenced by an accelerated decline in the fraction of absorbed photosynthetically active radiation.Final harvests of grain occurred at the end of May for each season.Approximately 24 wheat plants were sampled from all replicates of each treatment combination at 7-10 day intervals during each season (about 18 samplings).Plant phenology of the main stem was determined using both Zadoks and Haun development scales.Green leaf and stem areas were determined on a subsample of 3 median-sized plants using an optical planimeter.Numbers of stems and heads were counted.Crown, stem, green and non-green leaf, and head components of subsample and remaining plants were separated.Component biomass was measured after ovendrying at 65-70°C.Leaf area index was computed from subsample specific leaf weight and green leaf biomass of all plants.About one week after anthesis developing grains were separated from chaff by a combination of hand and machine threshing of heads that were pooled by subplot.Grain was ovendried for a total of 14 days at 65-70°C.In addition to the interval sampling of individual plants, final grain yield was determined by machine harvesting and threshing an approximately 20 m 2 area of each plot, which had been reserved for only non-destructive, non-contact, remote-sensing measurements during the season.(deceased) and his staff at the Maricopa Agricultural Center.The FACE apparatus was furnished by Brookhaven National Laboratory, and we are grateful to Keith Lewin, John Nagy, and George Hendry for assisting in its installation and consulting about its use.We also acknowledge the advice and participation of Floyd J. Adamsen in the soil nitrogen treatments and measurements, as well as Thomas R. Sinclair for his wheat tissue N determinations.