Experimental Directory UMM :Data Elmu:jurnal:A:Agriculture, Ecosystems and Environment:Vol82.Issue1-3.Dec2000:

P.D. Jamieson et al. Agriculture, Ecosystems and Environment 82 2000 27–37 29 case simulation still uses daily weather data as input. The rate of photosynthesis depends on the light level at each layer, calculated from Beer’s law using an extinction coefficient of 0.6 [CO 2 ] and stomatal resis- tance calculated as a function of the vapour pressure deficit VPD. Respiration losses are also calculated so that the biomass increment is the difference be- tween what is gained and what is lost. This means that late in crop life total biomass may fall. The calculation of GAI is also done in more de- tail than in Sirius. The current sizes of leaves on the mainstem and tillers is calculated each day, and GAI is calculated by summing these per unit ground area. Leaves have finite lifetimes, and expand and contract at constant rates in thermal time. Hence, changes in GAI reflect the balance of births and deaths of both leaves and tillers. Drought reduces leaf extension rates and lifetimes. Accounting for N is done differently from Sirius. N demand is set assuming that there is a maximum [N] on a whole plant basis, and this decreases from 4.5 at emergence to 1.5 at the beginning of grain filling as a function of development stage. Minimum required [N] is set at 1.5 reducing to 0.3 in the same fashion, and optimum [N] is assumed to be mid- way between the maximum and minimum levels. N in excess of the optimum concentration is labile N in storage. If [N] is above optimum levels, then growth processes are unlimited by N. Below this level, a set of stress factors based on the ratio of current to opti- mum [N] is used to reduce tiller production rates, leaf expansion rates and lifetimes, and to reduce photosyn- thesis. Thus, AFRCWHEAT2 treats N-stress in very much the same way as it treats water stress. Both biomass and N demand per grain are constant in thermal time, so that grain growth and N uptake de- pend on grain population and temperature. The sources of biomass for grain growth are current assimilate, and a pool of carbohydrate that is initially calculated as 20 of the shoot biomass at anthesis, and increased by a fraction of the biomass accumulated between anthe- sis and the beginning of grain filling. Grain growth is unrestricted by assimilate provided this latter exceeds demand, in which case excess assimilate is added to the carbohydrate pool. The existence of the pool means grain growth can continue after the canopy dies. FASSET is a whole farm simulation model Jacobsen et al., 1998, which includes a soil–plant– atmosphere model that simulates N turn-over and crop production as affected by availability of water and N Olesen et al., 1996. The soil model has a one-dimen- sional vertical structure and simulates the daily movement and plant availability of N. The down- ward movement of water and N in the soil profile is simulated using the concepts of the SLIM model Addiscott and Whitmore, 1991 that is also incor- porated into AFRCWHEAT2. Soil temperature is simulated using an extended heat flow equation. The turn-over of soil organic matter in the soil is simu- lated using the structure of the DAISY model Hansen et al., 1991. A simplified version of the DAISY model is used to simulate crop water and N uptake. The crop model has three biomass pools: shoot biomass, below ground biomass and biomass in grains. The daily increase in biomass is proportional to the intercepted radiation. The LUE is reduced by drought, by low temperatures and by low N status in the crop according to the function by Sinclair and Horie 1989. The N status is the current N uptake scaled between a minimum and a maximum N uptake, which depend on both crop biomass and GAI. The crop GAI is assumed to increase up to the date of anthesis as a function of vegetative biomass and the nitrogen status. This func- tion was estimated using data from growth analysis experiments in winter wheat in Denmark. The senes- cence of the GAI follows crop development, but is hastened by drought stress. The extinction coefficient for light interception is 0.44. Crop phenology follows the Sirius approach with modifications as described by Olesen et al. 1999. Partitioning to grain is similar to Sirius.

3. Experimental

The experiment was conducted over 2 years at the University of Arizona, Maricopa Agricultural Center, AZ latitude 33.1 ◦ N, and is described in detail by Kimball et al. 1999. Wheat cv. “Yecoro Rojo” was sown in 1995 and 1996 on 15 December in 25 cm rows at a population of approximately 200 plants per m 2 . Irrigation water was applied from a subsurface irrigation system. There were four FACE rings kept at 200 mmolmol CO 2 above the concentration of four Blower rings, which had similar air movement but no added CO 2 . The Blower rings were the primary 30 P.D. Jamieson et al. Agriculture, Ecosystems and Environment 82 2000 27–37 experimental controls. All rings or main plots were split into high and low N treatment subplots. A profile of mineral N in the soil was determined at planting time in each treatment, and was used to set the initial conditions for all the models. The N-content of irri- gation water was sufficiently high that it needed to be accounted for in N-balance calculations 30 kg ha −1 , 19951996; 33 kg ha −1 , 19961997. In 19951996, an additional 70 kg N ha −1 was added to the low N treatment, and this treatment had 15 kg N ha −1 added in the second experiment. The high N treatments re- ceived additional N of 350 kg ha −1 each year, in four applications, plus 33 and 38 kg N ha −1 in 19951996 and 19961997, respectively, from the irrigation wa- ter. Measurements of biomass, leaf area, stem area, tiller number and, once grain filling was underway, grain biomass were made on each plot at intervals of about 10 days throughout growth. Final harvest results were recorded from large samples from an area adjacent to the one from which the sequential biomass measurements were made. Measurements of Haun stage Haun, 1973 on the mainstem were made at each harvest, and the anthesis dates were determined. In the 19951996 experiment, two extra rings had no extra CO 2 or air movement. These plots were added because the fans associated with the FACE treatment tended to increase foliage temperatures at night. Mea- surements comparing the extra plots with the FACE and Blower plots showed that the air mixing caused a rise in foliage temperature equivalent to an increase in minimum air temperature in the FACE and Blower plots of 1.2 ◦ C. Biomass results from these plots are not reported here, but phenological observations from them were used to calibrate the models for pheno- logy. These extra plots flowered 4 days later than the Blower plots.

4. Model calibration