Fig. 1.
Fig. 1.

Inputs, processes, and outputs of the Maize-N model to determine the economically optimum N rate (EONR).

 


Fig. 2.
Fig. 2.

Maize yield and fertilizer N rate as observed (symbols) and simulated with the spherical function used in the Maize-N model (solid line). The observed data are from the calibration data set from Clay Center, NE, in 2002. Yield potential (Yp) for the site was 15.6 Mg ha−1, attainable yield (Ya) was 14 Mg ha−1, and yield without applied fertilizer N (Y0) was 7.0 Mg ha−1. The relationship between net return to N and the fertilizer N rate is shown in the inset with an economically optimum N rate (EONR) of 153 kg ha−1, which corresponds to the maximum net return to N of US$547 ha−1.

 


Fig. 3.
Fig. 3.

Relationships between yield and N uptake in the global maize database used for the development of Maize-N. The relationship between observed indigenous N uptake (IN) and simulated indigenous N supply (INs) for the Nebraska irrigated (USA NE IR) data set is shown in the inset with a slope of 0.85, which represents recovery efficiency of indigenous N and was used as a default value in the Maize-N model (MW, Midwest; IR, irrigated; RF, rainfed; RFW, rainfed wet season; RFD, rainfed dry season; Y+N, yield with applied fertilizer N; Y0, yield without applied N fertilizer; YND, boundary line of maximum N dilution (kg grain kg−1 N); YNA, boundary line of maximum N accumulation (kg grain kg−1 N); YUN and Y0IN, yield vs. N uptake for Y+N and Y0, respectively, based on the QUEFTS model (Janssen et al., 1990).

 


Fig. 4.
Fig. 4.

Relationship between agronomic efficiency (AE) and maximum yield response (YaY0, where Ya is attainable yield and Y0 is yield without N fertilizer) to applied N at each site in the global maize database. The overall regression equation (for both irrigated, IR, and rainfed, RF, environments) is used in the Maize-N model to determine a baseline AE for a given environment. This baseline AE is then modified by climate, soil properties, and crop and soil management practices that influence the recovery efficiency of applied fertilizer N and the physiological efficiency of converting acquired N to yield.

 


Fig. 5.
Fig. 5.

Attainable maize grain yield (Ya) (left panel) and yield without applied fertilizer N (Y0) (right panel) as observed and simulated with the Hybrid-Maize model. Observed data (18 site–year observations) included locations in central Nebraska (Roberts, 2009), eastern South Dakota (Kim et al., 2008), and western Nebraska (Blumenthal et al., 2003). Simulated Ya and Y0 values are associated with simulation of the economically optimum N rate in Fig. 1.

 


Fig. 6.
Fig. 6.

Economically optimum N rate (EONR) as observed and simulated with (a) the Maize-N model (this study) and various yield-goal-based algorithms: (b) University of Nebraska-Lincoln (Shapiro et al., 2008); (c) Kansas State University (Leikam et al., 2003); (d) South Dakota State University (Gerwing and Gelderman, 2005); and (e) University of Missouri (Brown et al., 2004). The observed data (18 site–year observations) were independent of Maize-N model calibration and included locations in central Nebraska (Roberts, 2009), eastern South Dakota (Kim et al., 2008), and western Nebraska (Blumenthal et al., 2003).

 


Fig. 7.
Fig. 7.

Sensitivity of the economically optimum N rate (EONR) to (a and b) attainable yield, Ya, and (c and d) soil organic C, SOC, as simulated by Maize-N. Two baseline cases are shown: (a and c) an eastern South Dakota site (Brookings, SD) with Ya of 10.8 Mg ha−1, 2.42% SOC, a previous crop of soybean (soy), and no-till management (nt), and (b and d) a central Nebraska site (Clay Center, NE) with Ya of 11.6 Mg ha−1, 1.75% SOC, a previous crop of maize, and conventional tillage (ct). Sensitivity tests included counterpart options for soil management (no-till vs. conventional tillage) and the previous crop (maize vs. soybean) at each site, as shown by the different symbols. Vertical and horizontal dotted lines indicate baseline values for SOC and simulated EONR, respectively. For reference purpose, the values of Ya and SOC associated with the various percentages of Ya and SOC are shown near the horizontal axis at the top and bottom of each panel. Similarly, the percentage increase or decrease in EONR above or below the baseline is shown on the right side of each panel.