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Fluid Journal : Winter 2011
Winter 2011 The Fluid Journal 06 mineralization starts on the day of application. The amount of roots and stalks is estimated based on the grain yield in the previous crop and generic coefficients for harvest index and shoot- root ratio. The amount of N mineralization from stalks also depends on tillage method. By default, 100 percent of stalks will enter the mineralized pool when plowed, 75 percent in reduced- till, and 50 percent in no-till. Note that these values can be modified by the user, depending on expert information about these proportions. Results of soil C and N mineralization are attained for each substrate for each day. If soil nitrate before or at planting is measured, the simulated N mineralization is corrected by subtracting from values of daily total soil nitrate the difference between measured soil nitrate and simulated value on the day of soil sampling. If soil nitrate is not measured, a default amount of N carryover from last season is used (=50 kg/ha, modifiable). Indigenous soil N simulated by the DK --C and N model is used to set Y0 using the cubic-solver for N uptake vs. yield. The magnitude of the yield response to applied N, called the delta yield (Ya -- Y0) is used to estimate AE calculated for estimation of a yield-potential-based N requirement. The efficiency of Indigenous N uptake is set at 0.85 (Figure 3). Empirical estimation. RE of N is defined as the ratio of N uptake by crop (U) to N supply from indigenous sources and fertilizers. Nitrogen from different sources likely has different RE. Nitrogen from mineralization of SOM and other organic sources becomes available to the crop in a gradual manner, which results in a high RE (Figure 3), while N carryover from the previous season would have a relatively lower RE because a large part of it may be present deep in the soil and is at risk of leaching. By default, an RE value of 0.85 is assigned to N from mineralization of SOM based upon the observed relationship between U0 and simulated Indigenous N supply, crop residues and manures, as well as N in irrigation water. RE of fertilizer N is estimated based on fertilizer type, number of doses, timing of application, and soil properties. For each type of fertilizer, there is a default RE value that occurs when the best fertilizer management practices are used (i.e., one preplant application and two in- season side dresses) in an optimal soil Figure 4. Observed relationship between delta yield (Ya-Y0) and agronomic efficiency (AE). Figure 4. Maize-N simulated vs. observed economic optimum N rate (EONR) for sites in USA, Asia, and South America. (i.e., loam texture with neutral or acid pH). When N fertilizer management and/or soil properties deviate from the optimal conditions, smaller RE values are used to account for less efficient uptake. As a default, RE can be calculated as a function of the regression in Figure 4 divided by the PE generated by QUEFTS. Carryover residual nitrate is assigned the same RE as N fertilizer. Optimal N rate. Economically optimal N rate (EONR) within Maize-N is estimated as the rate of N application at which marginal profit of the maize yield increase due to applied N equals the cost of N fertilizer to achieve the increase. The required amount of fertilizer (F) is estimated as: F = EONR/N content. Figure 5 shows the relationship between the simulated EONR and measured EONR for U.S., southeast Asian, and Brazilian sites where sufficient data were available on residue inputs, and N application rates and methods. Within the irrigated NE sites, there were approximately four sites where
Early Spring 2011