# Upscaling Gross Primary Production in Corn-Soybean Rotation Systems in the Midwest
## 3 Results  
### Eddy-Flux Station Data  
Eight EC stations measured carbon fluxes and meteorological data in fields with corn-soybean rotation during the growing season 2016. Mean daily IPAR decreased from 8.98 MJ m\({}^{-2}\) on DOY 159 to 1.02 MJ m\({}^{-2}\) on DOY 253 (Figure 2A). Mean Ta ranged from 16.4 and 28.8 \({}^{\circ}\)C (Figure 2B) and mean (\(\pm\) SE) seasonal precipitation was \(515\pm 51\) mm, which is warmer and with near normal rainfall compared to the 30-year seasonal average (see Section 2.1). The average daytime VPD ranged between 0.13-2.40 kPa (Figure 2C). Maximum average corn GPP (\(\pm\) SE) was reached at DOY 196 with 26.0 \(\pm\) 3.2 g C m\({}^{-2}\) d\({}^{-1}\), and maximum average soybean GPP was reached at DOY 218 with 13.7 \(\pm\) 2.3 g C m\({}^{-2}\) d\({}^{-1}\) (Figure 2D). Seasonal corn and soybean GPP were 1483 \(\pm\) 100 and 811 \(\pm\) 53 g C m\({}^{-2}\), respectively (Table 2). Mean corn NEP ranged from \(-\)3.6 to 17.1 g C m\({}^{-2}\) d\({}^{-1}\) (Figure 2E), with a site-average (\(\pm\) SE) seasonal total of 678 \(\pm\) 63 g C m\({}^{-2}\) (Table 2). The 95% CI are presented in Table 2. Cumulative corn NEP was greater than zero (i.e., carbon uptake \(>\) respiration) starting from DOY 169. Mean soybean NEP ranged from -3.7 to 7.8 g C m\({}^{-2}\) d\({}^{-1}\) (Figure 2E), with a seasonal NEP of 263 \(\pm\) 40 g C m\({}^{-2}\) (Table 2). Cumulative seasonal soybean NEP surpassed zero from DOY 181. Minimum corn RE (i.e., highest respiration) was reached at DOY 204 with -11.6 g C m\({}^{-2}\) d\({}^{-1}\), and maximum corn RE at DOY 237 with -1.6 g C m\({}^{-2}\) d\({}^{-1}\) (Figure 2F). Minimum soybean RE was at DOY 160 with -7.1 g C m\({}^{-2}\) d\({}^{-1}\), and maximum soybean RE at DOY 217 with -1.2 g C m\({}^{-2}\) d\({}^{-1}\). Seasonal corn and soybean RE were \(-\)805 \(\pm\) 40 and -548 \(\pm\) 14 g C m\({}^{-2}\), respectively (Table 2). NEP, RE, and GPP followed the seasonal and daily pattern of Ta, IPAR, and VPD (Figure 2). The site-specific daily NEP was weakly correlated to IPAR (r = 0.20-0.54), daytime VPD (r = 0.02-0.41), and Tmin (r = -0.01-0.21). Daily GPP was weakly correlated to IPAR (r = 0.11-0.45), daytime VPD (r = 0.02-0.47), and Tmin (r = 0.07-0.31) as well. Also, daily RE was weakly correlated to IPAR (r = -0.18-0.20), daytime VPD (r = 0.02-0.48), and Tmin (r = 0.22-0.42). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Crop**} & \multirow{2}{*}{**Variable**} & **NEP** & **RE** & **GPP** & **Yield\({}_{\text{GPP}}\)** & **Yield\({}_{\text{GPP}}\)** \\  & & **(g C m\({}^{-2}\))** & **(g C m\({}^{-2}\))** & **(g C m\({}^{-2}\))** & **(Mg h\({}^{-1}\))** & **(Mg h\({}^{-1}\))** \\ \hline Corn & Mean \(\pm\) SE & 678 \(\pm\) 63 & \(-805\)\(\pm\) 40 & \(1483\)\(\pm\) 100 & \(12.82\)\(\pm\) 0.65 & \(13.09\)\(\pm\) 0.09 \\ \hline  & 95\% CI & [547,769] & [897,–779] & [1334,1655] & — & — \\ \hline Soybean & Mean \(\pm\) SE & 263 \(\pm\) 40 & \(-548\)\(\pm\) 14 & \(811\)\(\pm\) 53 & \(6.73\)\(\pm\) 0.27 & \(4.03\)\(\pm\) 0.04 \\ \hline  & 95\% CI & [170,318] & [–602,–531] & [728,913] & — & — \\ \hline \hline \end{tabular} NEP = net ecosystem production, GPP = gross primary production, RE = ecosystem respiration, Yield\({}_{\text{GPP}}\) = yield as calculated with Equation (6).  
\end{table} 
Table 2: Seasonal (DOY 145–269) NEP, RE, and GPP as well as Yield [1] and calculated Yield\({}_{\text{GPP}}\), averaged (\(\pm\) SE) over four Iowa counties for corn and soybean (n = 4) in 2016. Figure 2: Meteorological data and carbon fluxes for the 2016 growing season in four Iowa counties: **(A)** Daily incoming photosynthetic active radiation (IPAR, MJ m\({}^{-2}\) d\({}^{-1}\)), **(B)** daily ambient air temperature (Ta, \({}^{\circ}\)C), and **(C)** daytime vapor pressure deficit (VPD, kPa) averaged (\(\pm\) SE) over eight eddy-flux stations.