## 4 Discussion
### Relation between GPP and RE  
The RE was positively correlated with GPP in both rice (0.917, \(p\) < 0.01) and wheat (0.698, \(p\) < 0.01) (Table 2). The regression relation between daily GPP and RE in rice and wheat over the crop season was positive, i.e., the higher the GPP (uptake of CO\({}_{2}\)) the higher the RE (CO\({}_{2}\) release) (Figure 6). The strength of this relationship in both the crops was highly significant (\(p\) < 0.01), though a higher R\({}^{2}\) of 0.94 was obtained in rice than the R\({}^{2}\) of 0.74 in wheat. Thus, plants respire more with increased photosynthesis in order to achieve higher growth and maintenance, thereby increasing the GPP. The maximum GPP and RE rates of 14.42 and 5.23 g CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\) in rice and 15.66 g C m\({}^{-2}\) d\({}^{-1}\) and 7.3 g CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\), respectively, in wheat were observed at the heading stage in our study. These were much less as compared to the maximum GPP rates of 70.8, 59.8, 41.7, 41.8 g CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\) and the maximum RE rates of 27.5, 32.3, 19.4, 28.7 g CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\) observed in the maize cropland, orchard, wetland and vegetable field ecosystems, respectively, by [PERSON] et al. [38]. The higher GPP in maize was due to the greater carbon assimilation capacities at the different growth stages of the maize. With a greater leaf area, maize had a greater ability to absorb PAR and this increased its CO\({}_{2}\) uptake capacity. The differences in RE were mainly due to the variations in air temperature, soil moisture and canopy radiative temperature. The cumulative RE at our site was 386.81 g C m\({}^{-2}\) in rice, whereas it was much lower at 303.89 g C m\({}^{-2}\) in wheat (Table 4), even though wheat has a longer duration of growth. Higher cumulative RE in rice of 521 and 743 was reported in the Philippines [34] and Japan [24] (Table 4). The cumulative RE in wheat was reported to be 676 and 529 g C m\({}^{-2}\) in winter wheat in Germany for two consecutive years by [PERSON] et al. [43] (Table 4). The much higher RE at these locations was probably due to higher soil organic carbon and longer rice and winter wheat crop durations compared to at our study site. Figure 6: Scatter plot of daily ecosystem respiration (RE) and gross primary productivity (GPP) in (**A**) rice and (**B**) wheat over the crop season. The annual GPP and RE in our study in a subtropical semi-arid rice-wheat crop rotation system was \(-1725\) and \(832\) g C m\({}^{-2}\), whereas [PERSON] et al. [19] recorded an annual GPP and RE of \(-1340\) and \(+883\) g C m\({}^{-2}\), respectively, in a tropical humid rice-rice system. A lower annual GPP and RE of \(-1174\), and\(810\) g C m\({}^{2}\), respectively, was reported in wheat and \(-1008\), and \(872\) g C m\({}^{2}\) in maize in the North China Plain by [PERSON] et al. [55]. The RE/GPP ratio at our study site was \(0.51\) in rice and \(0.34\) in spring wheat (Table 4). The ratio obtained for rice, in the range of \(0.51\)-\(0.67\), is comparable in magnitude to those reported by other researchers (Table 4). The GPP/RE ratio was higher than one, which implied that the most important factor affecting the GPP was PAR and the seasonal changes in PAR explained \(69\%\) and \(81\%\) of the variability in NEE in rice and wheat, respectively. The ratio obtained for spring wheat in this study was much lower than those reported by other researchers studying winter wheat. [PERSON] [56] reported an RE/GPP ratio of \(0.35\) for spring wheat grown in Australia, which is closer to the value obtained by us. Carbon budgets have become a powerful tool in guiding climate policy and for identifying the mitigation measures required to keep the global temperatures below \(2~{}^{\circ}\)C, the global average temperature increase limits set out in the United Nations Paris Agreement [57].