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The Interactive Effects of Elevated CO2 and Drought on Grassland Species

Paper Reviewed
Niboyet, A., Bardoux, G., Barot, S. and Bloor, J.M.G. 2017. Elevated CO2 mediates the short-term drought recovery of ecosystem function in low-diversity grassland systems. Plant and Soil 420: 289-302.

Most CO2 enrichment studies typically examine the individual impacts of rising atmospheric CO2 on plant growth. Few are the studies that examine the interactive effects of CO2 with other growth-impacting variables, such as temperature, moisture or light. In the present work, therefore, Niboyet et al. (2017) set out to examine the interactive effects of atmospheric CO2, drought and plant community composition.

Their study was conducted in controlled-environment mesocosms in a glasshouse at the University of Paris XI, where monoculture and mixed-species grass communities were exposed to ambient (463 ppm) or elevated (702 ppm) CO2 conditions and then subjected to an artificially-induced drought and subsequent rewetting period. The monoculture treatment consisted of Dactylis glomerata, while the mixed-species grass community contained a mixture of four perennial species (Dactylis glomerata, Lolium perenne, Festuca arundinacea and Trifolium repens). Initially, all experimental treatments were well-watered to field capacity. Then, approximately four months after initial CO2 treatment began, half of the plants in each treatment were subjected to drought, where no irrigation was supplied over a period of three weeks. Following the drought exposure period, the droughted mesocosms were rewatered, and then watered regularly until final harvest approximately 2 months later.

In describing their findings, the four French scientists report that, as expected, drought had a negative effect on biomass production, reducing it on average by 46%. In contrast, elevated CO2 had a stimulatory effect (approximately 29% increase relative to ambient in both droughted and control mesocosms, see Figure 1A). Other encouraging findings reported by Niboyet et al. include, (1) elevated CO2 had a positive impact on soil moisture content, boosting it an average of 21%, (2) plant carbon pools in both roots and shoots decreased an average 40% with drought, but increased under elevated CO2 by an average 27% (see Figure 1B and 1C), (3) "elevated CO2 reduced the magnitude of drought effects on both ecosystem respiration and N2O emissions measured after rewetting," (4) "elevated CO2 counteracts short-term drought-induced increases in soil N availability, reducing the magnitude of drought effects on plant C:N and thus promoting the stability of forage quality" and (5) plant community composition did not modify the effects of elevated CO2 on grassland drought recovery."

Given the several positive findings noted above, Niboyet et al. conclude with the obvious, i.e., that "elevated CO2 mitigates the effects of extreme drought on multiple grassland functions."

Figure 1. Effects of drought on aboveground plant biomass (Panel A), shoot carbon (Panel B) and root carbon (Panel C) under interactive CO2 and species composition treatments. Values for aboveground biomass are at the end of experimental drought (prior to soil rewetting). Values for shoot and root carbon (C) were obtained following 55 days of soil rewetting. Treatment codes are given by: amb, ambient CO2; +CO2, elevated CO2; Dac, Dactylis monoculture, Mix, mixed grassland community. Source: Niboyet et al. (2017).

Posted 11 May 2018