## 4 Discussion  
### Complexity of the Drivers of Spatio-Temporal Variation in GPP  
In this work, three rice-wheat-rotation cropland EC sites were quantified and used to explore the complexity of the drivers of spatio-temporal variation in GPP. In order to eliminate the errors induced by instrument measurements, Table 2 only summarized GPP values in EC-based literatures. The cumulative GPP for the wheat fields ranged from 609 to 848 g C m\({}^{-2}\) in eastern China, which was equivalent to that reported in India (621 g C m\({}^{-2}\), ref. [54]) but lower than that reported in Germany (1241 g C m\({}^{-2}\), ref. [55]) and northern China (1174 g C m\({}^{-2}\), ref. [56]). The total seasonal GPP during the rice growing season in eastern China (889-1170 g C m\({}^{-2}\)) was higher than that in India (811 g C m\({}^{-2}\), ref. [57]) and the Philippines (778 g C m\({}^{-2}\), ref. [51]). The discrepancy in the cumulative seasonal GPP among the previous studies is probably because of the difference in local meteorological conditions (e.g., precipitation, air temperature, and photosynthetically active radiation) and phenology (e.g., growth duration, NDVI and LAI) [55; 56; 57]. For example, the summer rice growing season was warmer (23-24 \({}^{\circ}\)C in eastern China) and received more precipitation (735-1028 mm in eastern China, Table 2) as compared with the winter wheat growing season. The mean seasonal GPP for rice paddas (889-1170 g C m\({}^{-2}\)) was obviously higher than those for wheat fields (609-848 g C m\({}^{-2}\)) in eastern China, mostly because of the different crop growth conditions. Furthermore, values of 609-848 g C m\({}^{-2}\) for winter wheat fields in eastern China were lower than the value of 1174 g C m\({}^{-2}\) in northern China, mainly because the growth duration of winter wheat in northern China is generally longer [56].  
Figure 13: Probability distribution functions of the error ranges of daily GPP at the grid scale (\(\Delta\)GPP) in the NYRD during the period of 2014–2018.  
In addition to the meteorological factors and phenology, crop management (e.g., fertilizer) and edaphoclimatic conditions (e.g., soil temperature, soil water content and microbial populations) also have influences on the GPP dynamics. The fertilization time for wheat and rice is basically fixed (Figure 2) during the sowing period (early June for rice and middle November for wheat) and the tillering period (early July for rice and late February for wheat). [PERSON] et al. [15] reported that the basic fertilizer applied during the rice/wheat sowing period had the most remarkable effect, with an increase in GPP reaching 2-3 g C m\({}^{-2}\)d\({}^{-1}\) over 8 days. Spraying leaf fertilization during the crop tillering stages had a minor effect on GPP, with an increase in GPP up to 1-2 g C m\({}^{-2}\)d\({}^{-1}\) over 8 days. Edaphoclimatic conditions (such as soil temperature and soil moisture) varied seasonally, modulating the GPP dynamics [18]. However, detailed information about the in situ fertilization and the edaphoclimatic conditions was not available for us to input into our RF models. Considering the importance of these factors, nevertheless, we cannot ignore the potential influence of the crop management or edaphoclimatic conditions.