Global climate change associated with rise in concentration of atmospheric greenhouse gases (GHGs) has emerged as an important environmental challenge with the considerable impact on agriculture sector. According to the 5th Assessment Report (AR5) of the Inter-Governmental Panel on Climate Change (IPCC), atmospheric carbon dioxide (CO2) concentration is increasing at the rate of nearly 2 μmol mol−1 annually [1]. Although increased CO2 concentration has a fertilization effect on crops in terms of their yield increase, but to achieve the benefits of enriched CO2, supply of adequate amount of nutrients is essential. Inadequate supply of nitrogen could limit the development of new sinks and alter the source–sink balance in plants grown under high CO2 concentration [2]. Several workers have earlier reported decrease in N concentration in plants under elevated CO2 level [3, 4]. Abebe et al. [5] reported that N and crude protein content in maize decreased under elevated CO2 concentration. Legumes, however, have the ability to fix atmospheric nitrogen (N) and may have advantage over non-legumes when grown under increased CO2 concentration [6].

Wheat (Triticum aestivum L.) and chickpea (Cicer arietinum L.) are the two important crops which contributed significantly in India’s food security. Considering the imminent impacts of climate change on crops, a study was conducted to quantify the impacts of elevated atmospheric CO2 on wheat (C3 cereal) and chickpea (C3 legume) crops. The study was aimed for comparing the effect of elevated CO2 on tissue N concentration and crop N uptake in wheat and chickpea.

The experiment was carried out for two consecutive years (2011–2012 and 2012–2013) at ICAR—Indian Agricultural Research Institute (IARI) farm, New Delhi (28°35′N and 77°12′E), India. Wheat (variety PBW 343) and desi chickpea (variety BGD 72) crops were grown both inside and outside the Free Air Carbon dioxide Enrichment (FACE) facility at IARI, New Delhi. Design of the experiment was randomized block design (RBD). The FACE ring was made up of eight horizontal pipes, which release CO2-enriched air at the crop canopy level. Pipes were perforated with holes of 3 mm diameter facing the inner side of the ring to disperse the air inside the ring [7]. Concentration of CO2 inside the ring was maintained at around 550 ppm, while crops grown outside the FACE ring were subjected to ambient CO2 concentration (390 ppm). Nitrogen was applied through urea in both the crops. In wheat crop, nitrogen (N) was applied at the rate of 120 kg N ha−1 in 3 splits (half as basal; remaining half in 2 equal splits at vegetative and flowering stage) while in chickpea a starter dose of 25 kg N ha−1 was applied.

Both grain and biomass yields were recorded by harvesting plants from the four quadrants of the FACE ring. Harvest Index of both the crops was calculated using formula:

$$ {\text{Harvest}}\,{\text{Index}}\,\left( {\text{HI}} \right) = \left( {{\text{Grain}}\,{\text{or}}\,{\text{seed}}\,{\text{ yield/total}}\,{\text{dry}}\,{\text{matter}}} \right) \times 100 $$
(1)

Plant samples collected at harvesting stage were dried in oven at 65 ± 2 °C for 72 h and ground in a Wiley mill. Nitrogen content in plant samples was analyzed following the micro-Kjeldahl method as described by Jackson [8]. Available nitrogen content of soil was estimated at flowering stage of the crops in both the years using Subbiah and Asija [9] method. Statistical analysis of the observations was performed using ANOVA (analysis of variance) technique recommended for the design [10] to test whether the differences between means were statistically significant or not. Unless indicated otherwise, differences were considered significant at p ≤ 0.05.

Wheat yield significantly increased under elevated CO2 level. Grain yield increased by 15.1% and 16.7% (p ≤ 0.05) in the first and second years of study “respectively” (Fig. 1). Earlier workers reported an increase in biomass yield by 11.8% and grain yield by 10.4% in wheat under elevated CO2 condition [11]. A review of fifty experiments studying the impact of elevated CO2 on wheat crop showed that on average doubling of CO2 from 350 to 700 ppm increased wheat yield by 31% [12]. Among different yield parameters, maximum increase was observed in number of grains per spike in high CO2 treatment. Higher C assimilation by the wheat crop under increased CO2 level resulted in higher biomass partitioning causing more number of spikes as well as number of grains. Chakraborty et al. [13] also found that the increase in net photosynthesis caused more carbohydrates accumulation leading to higher productivity of Brassica cultivars. Ruhil et al. [14] also found that increased photosynthesis rate coupled with a higher leaf area increased biomass and yield under elevated CO2 condition.

Fig. 1
figure 1

Impact of elevated carbon dioxide (CO2) level on grain yield in wheat crop (*indicate significant difference between ambient and elevated CO2 treatments in a year at p ≤ 0.05 level) (error bars in figure indicate standard deviation)

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Chickpea crop showed positive response under elevated CO2 condition. Seed yield of chickpea increased by 21.1% and 21.9% (p ≤ 0.05) in high CO2 treatment in the first and second years, respectively (Fig. 2). Harvest index (HI) of the crop significantly increased (p ≤ 0.05) in the first year. This shows that allocation of assimilates to storage organs increased with increased CO2 level. Earlier workers also reported increase in seed as well as biomass yield under high CO2 condition in leguminous crops like mungbean [15] and pigeonpea [16]. Singh et al. [17] reported that among different crops studied, CO2 fertilization effect was maximum in chickpea crop. Earlier researchers also found significant increase in chickpea productivity under elevated CO2 condition; however, decline in nutritional quality was reported [18]. Nitrogen content in wheat grains decreased significantly under elevated CO2 condition. Grain N content was 1.89% and 1.93% in high CO2 treatment during the first and second years of our study, while under ambient condition grain N content, it was 1.95% and 2.04%, respectively (Table 1). Reduction in wheat grain N content may be the result of dilution effect of more carbohydrate accumulation under increased CO2 condition [19]. Several researchers have reported similar decrease in overall N concentration in plants when grown in elevated CO2 condition [3, 4, 20]. Contrufo et al. [20] reported an average of 14% reduction in N concentration in plants grown under elevated CO2 condition. However, this effect is different for different plants.

Fig. 2
figure 2

Impact of elevated carbon dioxide (CO2) level on seed yield in chickpea crop (*indicate significant difference between ambient and elevated CO2 treatments in a year at p ≤ 0.05 level) (error bars in figure indicate standard deviation)

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Table 1 Effect of elevated carbon dioxide (CO2) on N content in wheat and chickpea crop
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In case of chickpea crop, no significant difference was found in seed N content between ambient and elevated CO2 treatments. Seed N content was 3.01% in both ambient and elevated CO2 treatments in the first year. During the second year, seed N was 2.84% and 2.85% in ambient and elevated CO2 treatments (Table 1). This might be attributed to the fact that chickpea being a leguminous crop got the advantage of N fixation under elevated CO2 condition. Earlier results showed that in forage crops leaf N content and C/N ratio remained unaltered in legumes while it decreased in non-legumes [21]. Contrasting results are also being reported in chickpea as high atmospheric CO2 concentration could adversely affect the chickpea grain quality [22].

Decreased plant N content with increase in total N uptake by wheat crop was attributed to increased grain and biomass yield under elevated CO2 treatment. Total N uptake was 10.7 and 12.9 g m−2 in the first and second year, respectively, under high CO2 concentration (Fig. 3a). Similarly higher seed as well as biomass yield of chickpea crop in high CO2 treatment led to higher N uptake. Total N uptake by chickpea plant ranged from 11.6 to 12.1 g m−2 in high CO2 treatment (Fig. 3b). Higher C assimilation and higher N uptake under elevated CO2 condition resulted in higher biomass accumulation by the crop. Earlier workers also reported that in cereal crops like rice, total nitrogen (N) uptake for the whole plant increased, while leaf N concentration decreased at elevated CO2 level [23, 24].

Fig. 3
figure 3

Nitrogen uptake under ambient and elevated carbon dioxide (CO2) conditions in a wheat and b chickpea crop (*indicate significant difference between ambient and elevated CO2 treatments in a year at p ≤ 0.05 level) (error bars in figures indicate standard deviation)

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Partitioning coefficient of nitrogen to grain and straw was calculated by dividing grain and straw N uptake by total N uptake of the crop. In wheat crop, nitrogen partitioned to grains was higher (0.74) in elevated CO2 treatment than ambient (0.7) during the first year. But in the second year N partitioning was same (0.77) as that of ambient CO2 condition. On the other hand, N partitioned to wheat straw got reduced in the first year and remained same in the second year. In chickpea N partitioning to seeds increased under high CO2 condition during both years of study (0.76 in ambient CO2 and 0.79 in elevated CO2 treatment during the first year and 0.76 in ambient CO2 and 0.78 in elevated CO2 treatment in the second year). Under high CO2 condition, increase in seed yield (21.1% and 21.9% in the first and second years) in chickpea was more as compared to increase in straw yield (12.4% and 12.7% in the first and second years) which led to partitioning of more amount of N to seeds. Available N of soil also significantly decreased under elevated CO2 concentration in wheat crop. Under ambient condition soil available N was 203.2 and 198.6 kg ha−1, respectively, in the first and second years of study which decreased to 185.3 and 174.5 kg ha−1 under elevated CO2 concentration (Table 2). On the other hand, soil available N was not affected by increased CO2 concentration in chickpea crop. Chickpea being a leguminous crop can fix atmospheric N which might have benefitted the crop in terms of maintaining seed N content and soil available N as compared to wheat. The results indicate that increased atmospheric CO2 level will increase yield of C3 crops like wheat and chickpea, but grain N concentration and soil available N status might decrease in non-leguminous crop like wheat, while it may not be affected for leguminous crop like chickpea.

Table 2 Effect of elevated carbon dioxide (CO2) on soil available N in wheat and chickpea crop
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