The physical composition of the soil can determine grassland plant responses to rising atmospheric carbon dioxide.
Increasing atmospheric carbon dioxide (CO2) concentrations affect the rate at which plants can take up CO2 during photosynthesis1. But what might seem to be a simple relationship — the more CO2 in the atmosphere, the more plants will photosynthesize and grow — is often not so straightforward. Writing in this issue of Nature Climate Change, Fay et al.2 demonstrate some of the potential complexities governing the relationship between vegetation and the atmosphere. In their CO2-enrichment study in a Texas prairie, they find that the response of grasses, in terms of their leaf production, to increased levels of atmospheric CO2 is determined by the type of soil in which the grasses are rooted, with coarser-textured soils (sands and silts) yielding greater plant growth than finer-textured soils (clays).
Carbon dioxide is of course a key reactant in photosynthesis and the ultimate source of carbon for carbohydrates and other plant-synthesized compounds; it enters the plant through openings in the leaves known as stomata. But terrestrial plants face a dilemma in that as CO2 enters through the stomata, water (the other key reactant in photosynthesis) exits through the same openings. Given this 'problem' with photosynthesis, some plant taxa have evolved physiological and morphological strategies for conserving water3,4. With regard to physiology, the most common photosynthetic pathway (but least water-use efficient) is termed C3, because there are three carbons in the first carbon compound generated in the process. Other main photosynthetic pathways that have evolved, at least in part to address water limitations, are C4 (first carbon compound generated has four carbons) and CAM (crassulacean acid metabolism, found largely in desert succulents). The C4 photosynthetic pathway is more efficient in water-use than C3, as it essentially uses the CO2 within its leaves more efficiently, so less water is lost in the process; whereas CAM plants use water more efficiently by opening their stomata at night (when the atmospheric demand for water is relatively low) and closing them during the day. From a morphological perspective, some plant species conserve water by reducing leaf area (and therefore stomatal area) and focusing their attention on roots for maximizing water and nutrient access.
Studies of CO2-enrichment conducted in grasslands with mixed communities of C3 and C4 grasses were expected to benefit the C3 grasses, as they use CO2 less efficiently than C4 grasses, and therefore have more room for improvement (it was assumed that C4 plants were not limited by CO2 and therefore CO2 additions would not yield increased photosynthesis rates). But this was not always the case; in practice it was often found that both C3 and C4 plants benefited from increased atmospheric CO2 (refs 5,
Now here's where the soils come in. Soils differ in their constituencies of particle types and sizes (that is, sand, silt, clay), which determine the soil texture. Soils with different textures differ in their porosities — the amount of open space in the soil — and in the distribution of pore sizes. Because of this, they have differing abilities to hold water7. Clay soils, with smaller pores and greater total particle surface area than coarser-textured soils, will tend to hold water more strongly against other forces, such as the 'suction' exerted by plant roots and the atmosphere, whereas sandier soils, with larger pores and less surface area, allow water to move more freely. In the study by Fay et al., increased atmospheric CO2 led to increased efficiency in the use of water and decreased losses of water through stomata (transpiration) for both of the C4 grasses examined. This in turn led to increases in soil water in all of the three soil types in the experiment (sandy loam, silty clay, clay). But it was only in the two coarser-textured soils (sandy loam, silty clay), with greater plant-accessible water, that the changing conditions translated to an increased dominance of the more productive tall grass at the expense of the more drought-tolerant mid-sized grass. Thus, the grassland productivity response to increased atmospheric CO2 was mediated by soil texture through interactions with soil water and changes in grass species composition (Fig. 1).
Fay et al. highlight the importance of indirect effects, which may not be readily apparent, on ecosystem responses to environmental changes. The variable that they cleverly manipulated in their study was atmospheric CO2, between 250 and 500 ppm, a very reasonable range given that we are currently close to 400 ppm and rising. But, as we well know, there are many other dynamic components of these systems than just CO2. The south-central grassland region of the United States has recently experienced some of its warmest and driest years on record; Texas, for example, in 2011 had its second warmest and second driest year since 1895, not to mention the fires8. So, if understanding CO2 effects on ecosystems is complicated, tackling the combined effects of dynamic CO2, temperature and precipitation is a great challenge, but one that can be addressed with more field experiments and simulation modelling. Interestingly, though, one property of this Texas grassland ecosystem that is unlikely to change over relatively short timeframes (decades) is the key factor in this study: the soil texture. While everything else is rapidly moving around it, the soil (in texture at least) remains the same.