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Fluid Journal : Winter 2011
crop model. It assumes optimal water supply without biotic and abiotic stresses to the maize crop. The likely yield outcome, or attainable yield (Ya) for the current season, is assumed to a fraction (fgap, default = 0.85) of Yp. In case the user specifies the average yield of past years, the simulated Ya will be replaced by the user-specified yield. Yield response. The response of maize yield to N uptake is founded on a large database of nutrient uptake by irrigated and favorable rain-fed maize grown in well-managed on-farm and on-station experiments in sub-tropical and tropical climates in southeast Asia (North Vietnam and Indonesia) as well as temperate climates (multiple sites in Nebraska, Figure 2). It covers a wide yield range from about 1 to nearly 20 Mg ha-1. The QUEFTS model employs two linear boundaries that describe the range from maximum accumulation ("excess") to maximum dilution ("deficiency") of N, P, and K in maize. These boundaries envelop an optimized yield response curved to N uptake, which follows a spherical response curve that plateaus at the maximum upper limit for yield potential. Note that the upper limit is greater in the temperate systems of Nebraska compared to tropical and subtropical systems in southeast Asia. The relationship in Figure 2 is thus used to calculate the necessary N uptake requirement from Hybrid-Maize- determined Yp or Ya, or user-defined yield input. Indigenous N. Soil Indigenous N-- including N from last season carryover, mineralization of SOM, crop residues (roots and stalks), any applied manure, and irrigation water--constitutes an important part of overall N supply to the maize crop. Maize-N simulates daily mineralization from SOM, crop residues, and manure based on temperature and soil moisture. The starting time of C and N mineralization differs for SOM, crop roots, stalks, and manures. For SOM, mineralization starts two weeks before the last crop matures, and ends two weeks before the current maize crop matures. For stalks of the last crop, mineralization starts during tillage (plow- or reduced-till). For no- till, mineralization starts on the day of planting because the planting operation likely leads to good contact of part of crop residues with the soil. For manures, 05 The Fluid Journal Winter 2011 Figure 1. Generalized inputs, outputs, and process models incorporated in the Maize N model. Figure 2. Maize yield response to N uptake in Nebraska and southeast Asian sites (North Vietnam and Indonesia). "D" = boundary of maximum N dilution and "A" boundary of maximum N accumulation. Figure 3. Relationship between observed unfertilized yield (Y0) and Indigenous N uptake for a large database (left) and simulated Indigenous N supply from DK-C&N model against observed Indigenous N uptake Y0 in Nebraska (right).
Early Spring 2011