## 1 Introduction  
Carbon dioxide (CO2) is an important greenhouse gas (GHG) that is emitted from agricultural fields [1]. The concentration of CO2 in the atmosphere has substantially exceeded, by about 40%, that of pre-industrial levels [1, 2, 3, 4, 5]. Forests, agricultural crops, soil and water bodies contribute towards the reduction in CO2 and act as major sinks [6, 7, 8, 9]. Quantification of exchange of CO2 between agricultural soils and the atmosphere is important in order to assess the global carbon budgets as CO2 accounted for 76% of total anthropogenic GHG emissions in 2010 [1, 2, 3, 4, 5, 10]. Rice (_Oriyza sativa_) and wheat (_Triticum aestivum_) are the two important crops that are essential for global food security. The rice-wheat cropping system is the largest agricultural production system in the world, occupying around 13.5 million hectares (M ha) in South Asia [11] and around 13 M ha in China [12]. In China, the rice harvest is followed by winter wheat (7-8 month duration), whereas in South Asia, where the winters are milder, the wheat grown during the winter is called spring wheat and is of shorter duration (4 months). In South Asia, 85% of the rice-wheat area lies in the Indo-Gangetic Plains (IGP) which produce about 50% of the total food grains from this region [13]. In the rice-wheat system of the study region, rice is grown during the summer months from July to October when the climate is warm and sub-humid, whereas the successive wheat is grown from December to April in a relatively dry winter [14]. There is a high level of variation in the temperatures and the amount of rainfall received between the rice and wheat crop growing seasons. Rice and wheat are grown not only in contrasting climatic conditions but the agricultural practices of tillage, manure, fertilizer, seeding, transplanting and water management also differ [15]. Rice is transplanted and grown in submerged anaerobic soil, whereas wheat is grown in upland aerobic soil [16]. Rice is a tropical crop, while wheat is a winter crop, with temperature playing a key role in their growth due to its influence on photosynthetic efficiency [17]. The contrasting soil environments influence the carbon and nitrogen dynamics, affecting the degradation of soil organic matter and, thus, the net ecosystem exchange (NEE) of CO\({}_{2}\). Net ecosystem exchange (NEE) is a measure of the net exchange of C between an ecosystem and per unit ground area of the atmosphere and is a primary gauge of ecosystem C sink strength [18]. GPP represents the gross CO\({}_{2}\) uptake through photosynthesis by plants, whereas ecosystem respiration (RE) is the gross CO\({}_{2}\) release from the ecosystem through both autotrophic and heterotrophic respiration [19]. Rice and wheat ecosystems may be significant sinks/sources of CO\({}_{2}\), depending on the soil type, crop duration, management and the climate of the region. In Asian regions, measurements of net ecosystem CO\({}_{2}\) exchange have been conducted in rice and wheat [12; 20; 21]. The crop duration of both rice and spring wheat in the South Asian region is about 100 to 120 days, unlike the winter wheat which has a much longer duration. Winter wheat is mostly grown as a rain-fed crop, whereas spring wheat is grown under irrigated conditions in the IGP [22]. [PERSON] et al. [12] quantified the NEE of CO\({}_{2}\) in a rice-winter wheat cropping system in the North China Plain, however, no measurements have been collected of NEE in South Asian rice-spring wheat systems, where two contrasting crops are grown in one annual cycle. The factors controlling CO\({}_{2}\) gas exchange between the rice canopy and atmosphere are different from those in upland wheat fields, as rice is flooded during most of its cultivation period, thus, leading to a great deal of uncertainty in the net CO\({}_{2}\) budgets [23].