In order to test our hypothesis and to evaluate the soil water response differences occurring within dryland and non-dryland systems, a total of 45 studies from 8 countries (yielding 1705 measurements from 21 distinct sites), were included in the meta-analysis (Fig. 1A, Supplementary Table S1, Supplementary Appendix S1). The meta-analysis revealed that increasing atmospheric CO 2 to between 1.2 to 2.0 times the ambient CO 2 level has a positive effect on soil water content, indicated by the fact that the effect size was greater than zero in both drylands and non-drylands (Fig. 1B). When considering the entire data set, higher CO 2 levels resulted in an 11% increase in soil water content across all systems (Fig. 1B). Importantly, the analysis revealed that elevated CO 2 significantly enhanced soil water levels in drylands more so than it did in non-drylands (P < 0.05, Fig. 1C), with soil water content increasing by 9% in non-drylands compared to 17% in drylands (P < 0.05, Fig. 1C). According to our meta-analysis data set, the mean soil water content was 11.6% under the ambient CO 2 level in drylands, while it was 24.1% in non-drylands. Based on the meta-analysis results, the enhanced CO 2 level would result in a 1.9% absolute soil moisture change in drylands and 2.2% change in non-drylands. Although the absolute change of soil moisture in drylands is comparable to that in non-drylands, studies have shown that even small change of soil moisture in drylands could be significant enough to cause large changes in vegetation productivity18. The CO 2 induced soil water increase seems contrary to the conventional understanding that any additional soil water should be transpired or evaporated in drylands, as water is a limiting resource. However, similar responses have been observed across many individual studies (Supplementary Table S1) and are apparent in our global synthesis at both dryland and non-dryland sites, highlighting the strong role vegetation plays in the soil water balance17. Importantly, the observed response lends weight to the hypothesis that any additional soil water in the root zone is then available to facilitate vegetation growth and greening under enhanced atmospheric CO 2 . Determining the mechanisms of stronger soil water responses in drylands requires further investigation, since it is generally thought that elevated CO 2 has a smaller effect on stomatal response during dry periods or under extreme drought19.

Figure 1 Global climate map and a comparison of mean effect size and soil water response under elevated CO 2 . (A) Site locations of the CO 2 enrichment experiments together with globally distributed climate zones based on a standard aridity index formulation (precipitation/potential evapotranspiration); (B) Mean effect size of soil water content under elevated CO 2 for the entire data set, under dryland and non-dryland regimes. The effect size was calculated as the natural log of the magnitude of an experimental treatment mean (the soil water under elevated CO 2 ) relative to the control treatment mean (the soil water under ambient CO 2 ); The dashed line indicates the threshold of statistically significant CO 2 effect on soil moisture. The effect is positive when above the line and vice versa. (C) Enhancement of soil water content under elevated CO 2 for dryland versus non-dryland regimes. The number of cases is shown in brackets. Error bars are bootstrapped confidence intervals (CI). All the statistics are significant at P < 0.05. The map was generated using ArcGIS for Desktop 10.3.1 (http://www.arcgis.com). Full size image

The direct effects of elevated CO 2 on photosynthesis can act to increase plant productivity through the alleviation of any carbon limitation20. However, CO 2 is not a limiting factor in most drylands, where productivity is governed mainly by water and nutrient constraints21. Assuming that a direct CO 2 effect occurs through the alleviation of carbon limitation in both dryland and non-dryland ecosystems, as shown earlier, our analysis has demonstrated that the indirect soil water response to elevated CO 2 levels is 89% higher in drylands (P < 0.05, Fig. 1C), indicating that factors other than a direct CO 2 effect play a role in increasing plant productivity in dryland systems.

To explore this idea further, a SEM approach22 was used to test the relative importance of direct (increased CO 2 removing any carbon limitation) versus indirect (i.e., increased CO 2 increasing soil water content) links between CO 2 enrichment and vegetation productivity for both drylands and non-drylands. SEM results show that the CO 2 effect on productivity was stronger for both direct effects on growth (path coefficients = 0.86 for drylands and 0.2 for non-drylands) and indirect effects on soil water content (path coefficients = 0.74 for drylands and 0.13 for non-drylands) (Fig. 2), providing additional support that CO 2 induced soil moisture increases are important in drylands.

Figure 2 Structural equation modeling of direct and indirect effects of CO 2 enrichment on vegetation productivity for both drylands and non-drylands. The number of cases is shown in brackets. Arrow thickness is proportional to path coefficient. Full size image

There are other variables that could affect the interaction between soil water content and elevated CO 2 level, including soil texture, vegetation type and system type. However, with the protocols developed in this exercise, the meta-analysis shows no evidence for any significant effects of these on soil water under higher CO 2 levels (Fig. 3A–C). In addition to accounting for the potential influence of other factors on vegetation response, the use of different methodologies to quantify soil water content has the capacity to influence the interpretation of results. To test any introduced methodological bias, we compared the results of studies reporting volumetric water content (the predominant unit used in the studies included in our analysis) and results using techniques such as gravimetric water content. The meta-analysis results were consistent between the different approaches (Fig. 3D).

Figure 3 Enhancement of soil water content for elevated CO 2 levels (A) under different management systems; (B) under different vegetation types; and (C) under different soil texture; and (D) using results from different soil water content (SWC) measurement methods (volumetric method, gravimetric method, etc., Extended Table 1). The number of cases is shown in brackets. Error bars are bootstrapped confidence intervals (CI). All the statistics are not significant at P > 0.05. Full size image

To date, the global average concentration of CO 2 in the atmosphere has increased by nearly 27% (from 315 ppm to approximately 400 ppm) over the period 1960–201523, with the expectation of a continued rise into the 21st century. To establish the validity of using results from higher CO 2 enrichment experiments (1.2 to 2.0 times ambient atmospheric CO 2 ) to explain the soil water-vegetation responses observed under current CO 2 levels, we examined the sensitivity of soil water change to varying levels of CO 2 using a regression analyses. Using the global meta-analysis data, a significant positive change in soil water along the CO 2 enrichment gradient was determined (P < 0.05, Fig. 4), supporting the CO 2 enrichment effect on soil water. At the same time, the rate of change was low (slope = 0.138, Fig. 4), indicating that soil water changes in response to CO 2 are comparable between higher CO 2 enrichment levels (1.2–2.0) and currently observed CO 2 enrichment (~1.27). The stability of the rate of change justifies using higher CO 2 enrichment levels to interpret soil water responses to currently observed CO 2 enrichment.

Figure 4 Sensitivity of the soil water response to CO 2 enrichment for the entire data set. The response index was calculated as the soil water content under elevated CO 2 divided by the soil water content under ambient CO 2 . The closed circles are the observations, with the solid black line providing a linear regression. The red lines represent the 95% confidence intervals of the observations and the dashed grey lines represent the 95% confidence interval of the model. m is the slope of the regression line. Full size image