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Elevated CO2 Improves the Net Photosynthesis and Water Use Efficiency of Soybean

Paper Reviewed
Wang, Y., Yan, D., Wang, J., Sing, Y. and Song, X. 2017. Effects of elevated CO2 and drought on plant physiology, soil carbon and soil enzyme activities. Pedosphere 27: 846-855.

Noting that severe drought is an oft-hypothesized claim of future climate change, Wang et al. (2017) sought to examine the interactive effects of elevated CO2 and drought on soybean (Glycine max, cv. Yu 19). This objective was accomplished by growing plants from seed for 40 days in controlled-environment greenhouses under ambient and twice ambient CO2 concentrations and three water regimes, including well-watered (80% water holding capacity of the soil), moderate drought (60% water holding capacity) and severe drought (40% water holding capacity).

So what did their study reveal?

As shown in the figure below, drought negatively impacted the net photosynthesis of the soybean plants, where it declined by 52% and 23% in comparing the well-watered to the severe drought treatment under ambient and elevated CO2 conditions, respectively. However, the positive influence of elevated CO2 was so great that even under severe drought conditions, the net photosynthetic rate was 73% greater than that observed under well-watered conditions at ambient CO2.

Water use efficiency was also enhanced by elevated CO2 (see figure 1 below, right panel), where it was "almost 2.5 times larger than that under ambient CO2." Furthermore, it increased (i.e., experienced the highest value) under severe drought conditions.

Wang et al. also report that elevated CO2 increased soil enzyme activities by stimulating plant root exudation and that it "resulted in a longer retention time of dissolved organic carbon (DOC) in [the] soil, probably by improving the soil water effectiveness for organic decomposition and mineralization." Consequently, in light of their many findings, the five Chinese scientists conclude that "drought stress had significant negative impacts on plant physiology, soil carbon, and soil enzyme activities, whereas elevated CO2 and plant physiological feedbacks indirectly ameliorated these impacts." And thus we have yet another example of the many benefits that rising atmospheric CO2 will bring to Earth's flora in the years and decades to come.

Figure 1. Net photosynthesis (Np; left panel) and water use efficiency (WUE; right panel) of soybean plants grown for 40 days under various treatments of drought (WW = well-watered; MD = moderate drought; SD = severe drought) and atmospheric CO2 (ambient and elevated, elevated = twice ambient).

Posted 9 April 2018