<b>Cooley</b>, S. R., D. J. P. <b>Moore</b>, S. R. Alin, D. Butman, D. W. Clow, N. H. F. French, R. A. Feely, Z. I. Johnson, G. Keppel-Aleks, S. E. Lohrenz, I. B. Ocko, E. H. Shadwick, A. J. Sutton, C. S. Potter, Y. Takatsuka, A. P. Walker, and R. M. S. Yu, 2018: Chapter 17: Biogeochemical effects of rising atmospheric carbon dioxide. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 690-727, https://doi.org/10.7930/SOCCR2.2018.Ch17.
Biogeochemical Effects of Rising Atmospheric Carbon Dioxide
The CO2 fertilization effect is defined in SOCCR1 as the “phenomenon in which plant growth increases (and agricultural crop yields increase) due to the increased rates of photosynthesis of plant species in response to elevated concentrations of CO2 in the atmosphere.” SOCCR1 concluded that the CO2 fertilization effect was widespread, but whether enhanced photosynthesis would translate into a persistent land carbon sink was unclear (CCSP 2007). The global land carbon sink, calculated as the difference between human emissions and carbon accumulating in the atmosphere and ocean, has grown from 0.2 ± 0.5 petagrams of carbon (Pg C) per year in the 1960s to 3.0 ± 0.7 Pg C per year in 2014 (Le Quéré et al., 2015). This change consists of the effects of land-use change and the residual land sink (Le Quéré et al., 2016). The residual carbon sink is carbon that is stored on land but is calculated as the remainder of other observed carbon sinks rather than observed itself. Growth in the residual sink is attributed to global changes in CO2, nitrogen deposition, and climate in both observational studies and modeling efforts (Ballantyne et al., 2012; Le Quéré et al., 2016; Schimel et al., 2015). However, predicting how the land carbon sink will respond to changing atmospheric CO2 is challenging because the land sink is inferred by accounting rather than experimental testing. The research community has evaluated the CO2 fertilization effect through experimental manipulations such as Free-Air CO2 Enrichment (FACE) projects (see Figure 17.1), tree rings, observational networks, and modeling experiments.
Plants take up carbon through the process of photosynthesis and synthesize biomass (e.g., leaves, wood, and roots) from simple, carbon-rich sugars derived from CO2. As CO2 increases in the atmosphere, plants can photosynthesize more quickly. Plants take up CO2 through the same pores (stomata) from which they lose water, leading to a balance between CO2 uptake and water loss. Rising CO2 increases carbon uptake per unit of water lost, allowing plants to close their stomata and therefore become more efficient in water usage (see Box 17.1, Short-Term Physiological Effects of CO2 on Plants). These physiological effects play out differently in different types of plants and under different environmental conditions. Twenty years of CO2-enrichment experiments have shown that elevated CO2 enhances photosynthetic carbon gain over the long term for certain ecosystem types but only over the short term for others (Leakey et al., 2009; Leuzinger et al., 2011; Norby and Zak 2011). Plant communities dominated by trees and grasses generally show greater stimulation of photosynthetic carbon uptake compared to that of legumes, shrubs, and nonleguminous crops (Ainsworth and Rogers 2007).
Net primary production (NPP) is calculated as either the balance between carbon gained through photosynthesis and lost through respiration or the sum of all growth over a year. With increased CO2, NPP is enhanced by ~23% across a broad range of early successional forests (Norby et al., 2005). These results probably are not indicative of all forests, and smaller responses have been observed in the limited number of studies carried out in old-growth temperate, boreal, and tropical forests (Hickler et al., 2008; Körner et al., 2005). Also clear is that the temporal pattern of NPP responses to elevated CO2 differs among forests (e.g., McCarthy et al., 2010; Norby et al., 2010).
Plants balance carbon gain and water loss. Stomatal conductance is depressed at elevated CO2, so plants may reduce water loss without reducing carbon gain, an observation which has been noted at the leaf and canopy scales (Keenan et al., 2013; Leakey et al., 2009; Peñuelas et al., 2011). Observations of decreased canopy evapotranspiration at elevated CO2 are therefore coupled with those of increased soil moisture. Crop carbon accumulation and water-use efficiency can be enhanced under drought conditions (Blum 2009; Morison et al., 2008), but extreme droughts may reduce or eliminate these enhancements (Gray et al., 2016).
Plant growth over years is not limited by CO2 alone (Körner 2015). If another environmental factor limits growth, then experimentally increasing CO2 causes diminished enhancement of photosynthesis and plant production (Ainsworth and Long 2005; Ainsworth and Rogers 2007). For example, nitrogen is sequestered in long-lived biomass and soil pools and may not always be readily available to plants. In this case, nitrogen limitation inhibits increases in plant production associated with elevated CO2, a process which is referred to as a negative feedback. In systems where nitrogen cycling did not reduce sink strength, the effects of CO2 fertilization on increasing NPP persisted (Drake et al., 2011; Finzi et al., 2006). The effects of rising CO2 on tree biomass may be inferred from tree-ring records, but results are mixed; some studies show no effect from changing CO2, and others show increased growth or water-use efficiency (Andreu-Hayles et al., 2011; Cole et al., 2009; Knapp and Soulé 2011; Koutavas 2013).
Because of these complications, whether rising CO2 will lead to larger standing biomass and carbon storage is unclear, in part because of the enormous complexity of the entire system (Norby and Zak 2011; Leuzinger and Hattenschwiler 2013). While instantaneous and annual fluxes of carbon are well studied in the FACE literature, the allocation of carbon to stems, roots, and leaves, for example, varies among experiments (DeLucia et al., 2005), and enhancement of multidecadal carbon stocks (e.g., woody biomass and soil organic matter) is not well studied (Leuzinger and Hattenschwiler 2013; Norby and Zak 2011). Increased carbon supply from plants can lead to heightened activity of soil fauna and more rapid cycling of carbon rather than increased carbon storage in soils (Phillips et al., 2012; van Groenigen et al., 2011, 2014). Because observed changes in soil carbon were small over the timescale of the FACE studies (3 to 16 years), firm conclusions about the impact of elevated CO2 on soil carbon remain elusive (Luo et al., 2011). In general, research suggests that large effects of rising CO2 on carbon storage in soils are limited (Schlesinger and Lichter 2001), although the combined effects of CO2 and nitrogen deposition and rising temperatures may significantly affect soil carbon loss (Zhou et al., 2016).
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