5.1 Introduction
In irrigated maize production in dry areas in Spain, the risk of nitrogen leaching is high. However, there are technical opportunities to match crop nitrogen demand with nitrogen availability. Water management and a fertilization scheme are the instruments to improve nitrogen use efficiency and hence reduce nitrogen losses by leaching. This is why the Universidad Politécnica de Madrid has started research and evaluation projects in irrigated maize cultivation in the Albacete and the Aranjuez region. A dataset from Albacete was used in this report.
5.2 Methods: collection and characteristics of the dataset
In a three-year experiment from 2003 to 2005, seven fertilizer treatments were compared with a zero treatment, resulting in eight treatments without replicates. The treatments are listed in Table 19. Maize was sown in the beginning of May and harvested in the beginning of October. Both seeds and crop residues were harvested, resulting in a very limited return of organic matter to the soil. The soil is a Calcixerolli – xerochrept with an Ap soil layer 0-25 cm and a Bk layer up to 40 cm. Root growth is supposed to be limited to 40 cm depth.Table 19 Treatments in the experiment
Name Description 0 No fertilizer
40 40 kg N/ha artificial fertilizer, four weeks after sowing 120 120 kg N/ha artificial fertilizer, four weeks after sowing 200 200 kg N/ha artificial fertilizer, four weeks after sowing 280 280 kg N/ha artificial fertilizer, four weeks after sowing 360 360 kg N/ha artificial fertilizer, four weeks after sowing
280_2 140 kg N/ha artificial fertilizer, four weeks after sowing, 140 kg three weeks later 360_2 180 kg N/ha artificial fertilizer, four weeks after sowing, 180 kg three weeks later
Soil sampling for soil mineral N analysis was done twice a year at the start and the end of crop growth, resulting in six measurements in each treatment.
The crop was irrigated every 1-3 days, in 2003 and 2005 with 18.3 mm each turn and in 2004 with 13.3 mm each turn. In 2005 there were more irrigation days with less water applied each time. Fresh yield was measured from product (kernels) and crop residue, and from the product the dry matter content was analyzed. From both, nitrogen content was measured.
For building the NDICEA scenarios, real product yield and nitrogen content were used. For the crop residue, due to the lack of dry matter analysis, the dry matter yield was estimated using results of the Aranjuez experiment as default values. In the Aranjuez experiment the dry matter content of the residue was measured.
For NDICEA a topsoil of 40 cm was used and the subsoil was supposed to reach up to 60 cm depth. In the subsoil no root growth was modeled. In NDICEA, evapotranspiration is usually calculated
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according to Makkink (1957). However, this approach lead to high calculated surpluses of water that were not accordance with practical experience from farmers. On the farms were the data for this study where obtained, the advanced Penman-Monteith-equation (Monteith, 1973) is used to calculate evapotranspiration. When incorporating this equation in NDICEA, calculated water surpluses were better in accordance with practical experience. Thus, evapotranspiration was calculated to Penman- Monteith, as is common practice in FAO-studies. The Penman-Monteith approach implied that environment-files were adapted to include additional parameters; the crop factor for transpiration was increased from 1.1 to 1.2 and the evapotranspiration from bare soil was increased from 0.25 to 0.3.For all scenarios, the calibration function of NDICEA was used. This can be done when there is a consistent difference between simulation and reality. Calibration will then result in a consistent (i.e. in all treatments the same direction) change of the parameters in question. This calibration was done in two steps. First the eight scenarios were calibrated, resulting in changed values of the ten model parameters included in the procedure. Second, the average of the obtained parameter values in ‘Cal’ was calculated and this average value for each parameter was used in all eight scenarios.
5.3 Results
5.3.1 Visual observations
In Figure 16 the course of mineral N in top- and subsoil is given. In 2004 and 2005 the level of available N reached zero, indicating a modelled N shortage, which should not occur. The shortage was around 10 kg in 2004 and 15 kg in 2005 (Figure 17). All other treatments did not show a calculated N shortage (graphs not shown).
Figure 16. Course of mineral N in treatment 0. Green line: simulated mineral N value topsoil. Blue line: simulated mineral N value subsoil. Green dots: measured mineral N values topsoil.
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Figure 17. Cumulative nitrogen uptake (red line) and nitrogen availability (green line) of treatment 0,in kg N ha-1.
5.3.2 RMSE
The RMSE of the simulations with the default soil model parameters was mostly >20, and only satisfying for the zero treatment (Table 20, column ‘Basic’). In all cases except for the zero treatment, simulated soil mineral N values were higher than measured values (data not shown).
Table 20. RMSE of three series of simulations. Basic = original default model parameters. Cal = calibrated individually. CalAv = Average model parameters after calibration. n=6
RMSE RMSE RMSE
name Basic Cal CalAv
0 12 7 8 40 21 5 6 120 37 16 16 200 23 8 9 280 39 11 14 360 54 23 25 280_2 32 8 9 360_2 53 23 25
Calibration considerably improved model performance in terms of RMSE (Table 20, columns Cal and CalAv). Only the two 360 treatments kept a RMSE > 20 kg N ha-1. Replacing the individual soil parameters by the average parameters slightly increased the RMSE , as expected, but the number of scenarios with an RMSE below 20 remained the same (6 out of 8, Table 20).
In Table 21 the default values and the average calibrated values of the ten soil parameters included in the calibration procedure are given. Most important differences between default and average values are in Nleach1 and IAgeFresh.
Table 21. Default and new average values of calibrated soil parameters.
Parameter* Default Average
Text 0,78 0,80
Nleach1 0,85 1,10
IAgeOld 24 22,80
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IAgeFresh 1,4 2,1 Nleach2 0,85 0,84 MaxWaterUptake1 0,75 0,78 C/N 8,3 6,79 A/D 0,45 0,35 Denitr 0,1 0,10* For information about the parameters, contact the authors
5.3.3 RSR
The RSR is presented in Table 22. The STDEV was close to ten in all treatments. Four out of eight treatments had an RSR below one. The two ‘360’ treatments with a high RMSE also had a high RSR.
Table 22. RSME, STDEV and RSR of the CalAv scenarios of the treatments. RMSE and STDEV in kg N ha-1, others without dimension