Methods and materials Directory UMM :Data Elmu:jurnal:A:Agricultural & Forest Meterology:Vol104Issue2Agus2000:

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

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 m 3 m − 3 , respectively, for the 0–0.7 m soil depth, whereas they are approximately 0.22 and 0.12 m 3 m − 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. CO 2 treatments 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 CO 2 concentration of the air in four, open-field circular plots, 25 m in diameter, by 200 mmol mol − 1 above ambient which averages about 370 mmol mol − 1 during the daytime. The CO 2 -enriched plots comprised the four replicates of the FACE treatment. Four, circular non-CO 2 -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, CO 2 enrichment commenced when 50 emergence of seedlings was observed, which occurred on 01 Jan- uary, 1996 and on 03 January, 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 CO 2 treatments. Each of the main cir- cular CO 2 plots 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. The fertilizer, in the form of NH 4 NO 3 , was injected into the irrigation water of the subsurface drip system installed prior to the first experiment. Drip tape lines, spaced at 0.5 m parallel to plant rows with emitters every 0.30 m, were buried 0.23 m below the soil sur- face. The tape lines of the irrigation system extended across an entire main plot replicate. Thus, the High N sides of both the FACE and Control plot replicate shared the same irrigation tape and likewise for the Low N sides. Therefore, the experimental design was a strip-split-plot, such as that used in previous FACE 88 D.J. Hunsaker et al. Agricultural and Forest Meteorology 104 2000 85–105 Table 1 Fertilizer application occurrence and amounts for the High N and Low N treatments during the 1995–1996 and 1996–1997 FACE wheat experiments Growth period Season 1995–1996 1996–1997 Date Treatment kg N ha − 1 Date Treatment kg N ha − 1 High N Low N High N Low N Mid-tillering 30 January, 1996 50 15 30 January, 1997 50 5 Stem extension 22 February, 1996 125 30 05 March, 1997 125 5 Anthesis 30 March, 1996 125 25 27 March, 1997 125 5 Early grain-ill 18 April, 1996 50 22 April, 1997 50 Total 350 70 350 15 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 m 2 area 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-RedNIR+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. Every 2–3 weeks during the 1995–1996 season, on days when canopy reflectance measurements were made, the fraction of the photosynthetically active radiation PAR absorbed by the canopy fA PAR was calculated from measurements of the incident, trans- mitted, and reflected components of the radiation balance using a 0.80 m long light bar. The light bar measurements were made perpendicular to plant rows, both above and below the plant canopy in all treat- ment subplots. Similar measurements were made at a separate bare soil plot in the same field. To compute fA PAR , the fraction of PAR transmitted through the canopy fT PAR , the fraction of PAR reflected from the canopy, and the fraction of the PAR reflected from the soil, obtained from the light bar measurements, were used in the light balance equation described by Pinter et al. 1994. The biologically effective fraction of ab- sorbed PAR fA PAR∗ , which more truly describes the canopy photosynthetic competence than fA PAR , was used to determine the time at which crop senescence occurred. The fA PAR∗ was computed by multiplying fA PAR by the ratio of the green plant area index mea- sured for the plot to the maximum green plant area index measured for the plot up to that point in the sea- son. The resulting values of fA PAR∗ determined for all treatment and bare soil plots during 1995–1996 were then fitted to the corresponding NDVI values on the same days using a third-order polynomial regression model. The regression model fit pre- and post-anthesis wheat data equally well. Because similar light bar measurements were not made in the 1996–1997 sea- son, values of fA PAR∗ for treatments in that season were estimated from the regression model using the D.J. Hunsaker et al. Agricultural and Forest Meteorology 104 2000 85–105 89 NDVI data obtained in 1996–1997. Kimball et al. 1999 used the date at which fA PAR∗ 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 ET o and then calculated the amount of soil water depletion by a daily water budgeting procedure. Daily ET o and 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. One, 2 m long neutron gauge access tube was verti- cally installed in the plant row of each subplot, 0.9 m away from the TDR probe, but with the same orien- tation as the TDR probe, relative to the drip emit- ters Fig. 1. Water content measurements by neutron scattering were taken in 0.2 m intervals from 0.4 to 2.0 m using a Campbell Pacific Nuclear Martinez, CA model 503 neutron moisture gauge. The gauge was calibrated at the field site over a wide range of soil water contents, prior to the first experiment in 1995. The field calibration for the neutron gauge had a coefficient of determination of 0.96 and an estimated accuracy of ±2.4. Standard counts were taken be- fore and after each set of measurements with the neu- tron gauge sitting on a free-standing pipe, one-meter above a bare, dry soil. Counts during the calibration and measurements were taken for 15 s. 90 D.J. Hunsaker et al. Agricultural and Forest Meteorology 104 2000 85–105 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 where 1S is the change in the root zone soil water storage during the period mm, P is the precipitation during the period mm, t is the number of days in the period, and ET is the daily average rate during the pe- riod mm per day. However, for the periods between two successive soil water measurement dates when irrigation or heavier precipitation occurred i.e. ‘soil D.J. Hunsaker et al. Agricultural and Forest Meteorology 104 2000 85–105 91 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 CO 2 enrichment 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 February, 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 CO 2 enrichment 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 K c 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 K c into a basal crop coefficient K cb and a soil water evaporation coefficient K e in which K c = ET ET o 2 and K c = K cb K s + K e 3 where ET and ET o are in mm per day, and K s is a soil water stress coefficient. K s = 1 and K s 1 for non-stressed and stressed soil water conditions, re- spectively. Briefly, the methodology used by Hunsaker 1999 required three primary steps. First, K cb K s values are derived from the measured ET and corresponding ET o for all soil water depletion periods. Next, a continuous K cb K s curve is fit over the entire season based on the K cb K s values derived for the depletion periods. Third, the FAO-56 dual crop coefficient procedures are applied for the entire season to estimate the daily soil evaporation E s and, hence, K e following each irrigation or precipitation event. In the third step, if soil evaporation is determined to have contributed to the measured ET during a soil depletion period, that amount of E s is then subtracted from the measured 92 D.J. Hunsaker et al. Agricultural and Forest Meteorology 104 2000 85–105 Fig. 2. Measured K cb values for soil water depletion periods and estimated daily K cb and K c curves 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 K cb K s 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 K s 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 K cb were derived over soil depletion periods of two or more days, they repre- sented an average K cb value for the period and, there- fore, are shown as horizontal lines along the K cb curve in Fig. 2a. Daily values of K cb for periods between the horizontal K cb segments i.e. for soil wetting peri- ods were estimated by linear interpolation based on the K cb values measured immediately before and after the period in question. The K c on a given day is the summation of K cb plus the soil water evaporation co- efficient K e and, therefore, the value of K c is equal to K cb only for conditions when the soil surface layer is dry i.e. when K e = 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. K e 0. Thus, the measured K cb in Fig. 2a repre- sent the basal portions of the measured ET divided by the corresponding ET o . The ET on any day can then be obtained by multiplying the estimated K c by the ET o 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. Procedures for the calculation of K e are fully de- scribed in the FAO-56 publication. Briefly, the daily values for K e are determined using a daily water bal- ance computation of the surface soil evaporation layer ≈0.10–0.15 m to estimate daily cumulative evapora- tion of water from the wetted soil condition. In addi- tion to daily weather data and daily values for K cb and ET o , several crop and soil parameters were needed to D.J. Hunsaker et al. Agricultural and Forest Meteorology 104 2000 85–105 93 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 f w , and the fraction of the soil surface exposed to sunlight and air denoted as 1−f c , 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 f w following precipitation events i.e. the entire soil surface was assumed to have been wetted. Follow- ing subsurface drip irrigations, the values used for f w were calculated using the FAO-56 recommendations for subsurface drip systems as follows: f w = 0.401 − 0.67f c 4 where f c represents the fraction of the soil surface covered by canopy. Values for the fraction of the exposed soil surface, 1−f c , 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 fT PAR ≈1−f c were estimated by linear interpolation between the days that the fT PAR were measured. For the 1996–1997 treatments, estimates of daily fT PAR were obtained from the daily NDVI data for that season using a calibration model developed from the 1995–1996 fT PAR and 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 kgm 3 . 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 CO 2 main effect α; the error term used for evaluating the effect of CO 2 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