## 4 Discussion  
### Diurnal and Seasonal Variation in NEE  
The average nighttime NEE was, on average, 0.11 and 0.12 mgm\({}^{-2}\)s\({}^{-1}\) during the rice and wheat growth periods, respectively. These values are close to those measured by [PERSON] et al. [36] in Japan (<0.2 mgm\({}^{-2}\)s\({}^{-1}\)) and [PERSON] et al. [37] in China (0.12 mgm\({}^{-2}\)s\({}^{-1}\)) in intermittently flooded rice paddise. The mean diurnal variation of NEE showed that both rice and wheat crops acted as net CO\({}_{2}\) sinks, as CO\({}_{2}\) uptake during the day was higher than emission after daylight hours. The highest values of half-hourly NEE were observed during the reproductive stage and the variations in NEE followed those in incident PAR. [PERSON] et al. [38] also observed relatively large fluctuations in diurnal NEE in maize cropland, orchard, wetland and vegetable field ecosystems with highest CO\({}_{2}\) uptake levels observed in maize crops due to the differences in the light saturation point (\(\alpha\)) and carbon absorption capacities of the crops. The highest leaf area index in rice (4.3) and wheat (4.6) was observed during the reproductive stages of heading to flowering. As PAR values increased beyond 2000 \(\mu\) mol m\({}^{-2}\) s\({}^{-1}\), there was an increase in carbon assimilation in rice and wheat. As the wheat crop advanced to the heading stage, the peak NEE increased to \(-1.48\) mg CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\) with a PAR of 2342 \(\mu\) mol m\({}^{-2}\) s\({}^{-1}\). Higher positive values of NEE at harvesting could be attributed to increased CO\({}_{2}\) emission from soil and plant respiration than uptake level by the plants. The magnitude of NEE in rice was lower during the vegetative stages and reached its maximum during reproductive stages of heading-flowering at \(-8.83\) g C m\({}^{-2}\) d\({}^{-1}\) in rice and \(-11.96\) g C m\({}^{-2}\) d\({}^{-1}\) in wheat. Similar values of daily NEE between vegetative and reproductive stages were reported for rice [34; 39] and wheat [40]. With the onset of  
Figure 5: Average diurnal pattern of relative humidity (RH), air temperature (AT), soil temperature (ST), soil heat flux (SHF), net radiation (NR)and PAR in rice (empty circle) and wheat (filled circle). senescence during ripening and maturity, the daily NEE became less negative in rice and turned positive at harvesting. The daily average NEE during wheat and rice in our study was \(-\)5.01 and \(-\)3.74 g C m\({}^{-2}\), respectively, showing that spring wheat was a stronger CO\({}_{2}\) sink than rice. [PERSON] et. al. [12], in a rice-winter wheat rotation system in China, observed a daily NEE of \(-\)2.35 and \(-\)3.96 g C m\({}^{-2}\) in wheat and rice, respectively, even though (Table 4) the winter wheat field took up more CO\({}_{2}\) on a seasonal basis as compared to the rice paddy fields because of the longer growing season. Much higher levels of CO\({}_{2}\) uptake have been reported in maize as compared to soybean and wheat in other eddy covariance studies. [PERSON] and [PERSON] [41] observed a maximum CO\({}_{2}\) uptake of 51.3 g CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\) in a maize crop in maize/soybean ecosystem in the USA which was similar to the maximum CO\({}_{2}\) uptake of 45.8 g CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\) in a maize ecosystem in the North China Plain [42]. The CO\({}_{2}\) uptake levels are different for crops such as corn, as compared to soybean and wheat, due to biochemical and physiological differences between C3 (soybean, wheat) and C4 crops ( maize). Water management, duration and weather conditions are factors which differ in crops and lead to differences in the NEE and uptake of CO\({}_{2}\). In our study, the soil and air temperatures were significantly different between the rice and wheat crop seasons impacting the NEE. The rice duration was 98 days and was irrigated every 2-3 days, whereas wheat irrigated for a duration of 115 days was irrigated five times.