## 3 Results and Discussion
### Variation Patterns of GPP, PAR and ELUE on Different Time Scales
#### 3.1.2 Seasonal Variations  
The dynamics of daily GPP, PAR and ELUE in different fields during the growing seasons were illustrated in Figure 3. PAR in the rice, soybean, and maize fields did not show apparent seasonal changes, while the variations of GPP showed apparent seasonal cycles and had a peak in active growth stages. Daily ELUE of paddy rice, soybean and summer maize increased firstly, peaked during the active growing period and decreased as the crops matured (Figure 3a-c). Winter wheat seed germination occurred typically in the autumn; GPP had a small peak and then fell to near zero after the wheat entered the wintertime (due to lack of PAR and heat during the wintertime) [49]. Thus, ELUE presented quite different seasonal variations that increased firstly after germination and then fell as photosynthetic productivity (namely GPP) was limited due to an unfavorable growing environment (Figure 3d). ELUE exhibited the highest value in summer maize with 0.92 \(\pm\) 0.06 g C M\({}^{-1}\), followed by soybean, paddy rice and winter wheat, with values of 0.80 \(\pm\) 0.16, 0.77 \(\pm\) 0.24 and 0.72 \(\pm\) 0.06 g C M\({}^{-1}\), respectively (Table 2). Summer maize possessed the highest GPP of 9.76 g C m\({}^{-2}\) d\({}^{-1}\), followed by soybean and paddy rice, with the values of 7.80 \(\pm\) 1.80 and 7.72 \(\pm\) 2.10 g C m\({}^{-2}\) d\({}^{-1}\), respectively. Winter wheat had the lowest GPP, 4.74 \(\pm\) 0.48 g C m\({}^{-2}\) d\({}^{-1}\), due to undergoing the wintertime during which physiological activity was weak under the lower temperature and PAR conditions [49] (approximately half the PAR of other ecosystems, Table 2). These findings suggested that the C\({}_{4}\) crop (summer maize) generally showed higher photosynthetic efficiency than C\({}_{3}\) crops (paddy rice, soybean and winter wheat), which was in line with previous reports [50; 51]. Under similar climatic conditions, C\({}_{4}\) plants possessed a stronger ability of environmental adaptation, absorbing more carbon using radiation energy that resulted in their robust photosynthetic capacity and leaf-cell anatomy. [PERSON] et al. (2018) found that the increase in both biomass production and CO\({}_{2}\) fixation with light intensity and CO\({}_{2}\) concentration in C\({}_{4}\) was faster than that in C\({}_{3}\)[24]. Therefore, the characteristics of C\({}_{4}\) plants determine a more efficient use of light and CO\({}_{2}\) than that of C\({}_{3}\) plants [50; 52; 53].  
Figure 2: Diurnal patterns (**a**–**d**) of gross primary production (GPP), photosynthetic active radiation (PAR) and ecosystem light use efficiency (ELUE) during daytime over the growing season at the four typical sites.  
Figure 3: Seasonal variations of gross primary production (GPP), photosynthetic active radiation (PAR) and ecosystem light use efficiency (ELUE) in the representative site of **(a)** paddy rice (IT-Cas), **(b)** soy-bean (US-Ne2), **(c)** summer maize (FR-Gri), **(d)** winter wheat (CH-Oe2). The grey area is marked as the growing season of the crops in the four typical sites. DOY is the day of the year. The vertical dotted line indicates the start of the year.  
\begin{table}
\begin{tabular}{l c c} \hline \hline
**Crop Type** & **GPP (g Cm\({}^{-2}\) d\({}^{-1}\))** & **ELUE (g C MJ\({}^{-1}\))** \\ \hline Paddy rice & 7.72 \(\pm\) 2.10 & 0.77 \(\pm\) 0.24 \\ Soybean & 7.80 \(\pm\) 1.80 & 0.80 \(\pm\) 0.16 \\ Summer maize & 9.76 \(\pm\) 0.80 & 0.92 \(\pm\) 0.06 \\ Winter wheat & 4.74 \(\pm\) 0.48 & 0.72 \(\pm\) 0.06 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Growing season mean + standard deviation of gross primarily productivity (GPP) and ecosystem light use efficiency (ELUE) across the typical croplands.