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Exercise objective:
· Sensitivity to and interaction among N management parameters.
In these short exercises, we will examine the sensitivity of ORYZA2000 to the N-management characteristics in the experimental data file, and their interacting effects.
Exercises:
Ex-III.9. Verify in CONTROL.DAT that the experimental data file is IR72DS2.T92 (remember: there is 225 kg ha-1 fertilizer-N application in this set!). Create a rerun file with different values for soil N supply SOILSP:
SOILSP = 0.4
SOILSP = 1.2
Run ORYZA2000 and fill out columns Sim1 (default run), Sim2 (rerun 1) and Sim3 (rerun 2) in Table III.2. Next, change in CONTROL.DAT the experimental data file to IR72DS0.T92 (remember: there is 0 fertilizer-N application in this set!), run ORYZA2000, and fill out columns Sim4 (default run), Sim5 (rerun 1) and Sim6 (rerun 2) in Table III.2. Comment on the results; in which of the two rerun sets does the change in SOILSP values have most effect, and why?
Table III.2.
View ANSWERS from the Tutorial_answer_sheet.pdf file.
In the first and second reruns using IR72DS2.T92, a total of 225 kg N ha-1 is applied as fertilizer. A 50% decrease or increase in soil N supply has a relatively modest effect, since the total amount of available N is high already: only 6-8% change in yield, 4-6% in total biomass, and 15-17% in total crop N uptake compared with the default run (Sim1). In the second set reruns using IR72DS0.T92, no fertilizer N is applied at all and crop performance is strongly constrained by limited N availability. A 50% change in soil N supply now has large effects: 28-42% change in yield, 21-34% in total biomass, and 45-48% in total crop N uptake compared with the default run (Sim1).
Ex-III.10. Restore in CONTROL.DAT the experimental data file to IR72DS2.T92. Create a rerun file with different fertilizer N applications FERTIL:
* rerun 1
FERTIL =
0., 0.,
1., 0.,
11., 0.,
12., 40.,
13., 0.,
29., 0.,
30., 40.,
31., 0.,
66., 0.,
67., 40.,
68., 0.,
94., 0.,
95., 40.,
96., 0.,
366., 0.
* rerun 2
FERTIL =
0., 0.,
1., 0.,
11., 0.,
12., 160.,
13., 0.,
366., 0.
In both reruns, the total amount of N applied is 160 kg ha-1, 65 kg ha-1 less than in treatment 2 of the experiment (see Box III.1, 225 kg ha-1). Run ORYZA2000 and fill out columns Sim1 (default run), Sim2 (rerun 1) and Sim3 (rerun 2) in Table III.3. Comment on the results. Q. Why are Sim2 and Sim3 different, despite the fact that total fertilizer N application is the same?
Table III.3.
View ANSWERS from the Tutorial_answer_sheet.pdf file.
Since the total N application rate is lower in the reruns than in the default run (Sim1; treatment 2 of the experiment), lower yields and lower biomasses are produced in the reruns than in the default run. In rerun 2, N uptake 22% is lower than in rerun 1 as a consequence of the timing of the fertilizer application. In rerun 1, the 160 kg N is divided in 4 splits. In table RECNIT (see the experimental data file), we see that fertilizer applications at later stages of crop development have higher recovery fractions (i.e., less fertilizer N is lost). In rerun 2, all 160 kg N is given at transplanting, and the recovery fraction for fertilizer applied at that time is only 30%. Therefore, less of the 160 kg N is available for uptake in rerun 2 than in rerun 1. Therefore, the yield in rerun 2 is 10% lower than in rerun 1. Total biomass production, however is only a negligible 1.5% less. Again this difference in reduction in yield and in total biomass is the consequence of the timing of the fertilizer N application. To understand this, we have to study N uptake and biomass formation in time.
Ex-III.11. Plot total N in the crop and in the panicles versus time for both rerun 1 and 2 in the same graph. Do the same for total above-ground biomass and panicle biomass. Explain on the basis of these graphs why total biomass production is almost the same, but yield 10% lower in rerun 2 than in rerun 1.
In rerun 1, the first N application at transplanting is only 40 kg ha-1, but enough for optimal crop N supply until the second application (Figure III.4a). However, this second dose is exhausted relatively early in the growing season (at around DVS=0.4, i.e., the tillering phase). Crop N uptake is then restricted until the third application, after panicle initiation. During that same period, the crop in rerun 2 still has ample supply of N and the 160 kg applied at transplanting is exhausted some days later than in rerun 1. As a result, (slightly) more vegetative biomass is produced in that period in rerun 2 than in rerun 1 (Figure III.4b). After panicle initiation, all the 160 kg N in rerun 2 is exhausted and crop growth is restricted by N shortage until harvest. The crop in rerun 1 benefits from two additional doses of fertilizer N, bringing its total crop and panicle N uptake considerably above that of rerun 2. However, the late N applications are less effective for total biomass production, since after panicle initiation there is considerable translocation from the stems and leaves to the panicle. Therefore, total biomass in rerun 1 only slowly approaches that in rerun 2 and attains about the same level (the extra increase in panicle weight after flowering in rerun 1 just matches the extra production of vegetative matter before flowering in rerun 2). The biomass of the panicles, however, benefits relatively strongly from the extra N applications, and the relative difference in panicle biomass is large (10% at maturity). This exercise illustrates the complexity of biomass accumulation and yield formation as function of N dynamics. Similar principles apply to other crop growth ‘drivers’, such as radiation, temperature and water. It often requires detailed dynamic analyses of a number of variables to understand what is happening.
Figure III.4a. Simulated N (kg ha-1) in the above-ground crop (ANCR) and panicles (ANSO) of cv. IR72 with 160 kg N ha-1 in 4 splits (Run 1) and with 160 kg N ha-1 all at transplanting (Run 2), IRRI, Los Baños, 1992.
Figure III.4b. Simulated above-ground biomass (WAGT) and panicle biomass (WSO) (kg ha-1) of cv. IR72 with 160 kg N ha-1 in 4 splits (Run 1) and with 160 kg N ha-1 all at transplanting (Run 2), IRRI, Los Baños, 1992.
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