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Terrestrial ecosystems remove about 30 per cent of the carbon dioxide (CO2) emitted by human activities each year1, yet the persistence of this carbon sink depends partly on how plant biomass and soil organic carbon (SOC) stocks respond to future increases in atmospheric CO2 (refs. 2,3). Although plant biomass often increases in elevated CO2 (eCO2) experiments4–6, SOC has been observed to increase, remain unchanged or even decline7. The mechanisms that drive this variation across experiments remain poorly understood, creating uncertainty in climate projections8,9. Here we synthesized data from 108 eCO2 experiments and found that the effect of eCO2 on SOC stocks is best explained by a negative relationship with plant biomass: when plant biomass is strongly stimulated by eCO2, SOC storage declines; conversely, when biomass is weakly stimulated, SOC storage increases. This trade-off appears to be related to plant nutrient acquisition, in which plants increase their biomass by mining the soil for nutrients, which decreases SOC storage. We found that, overall, SOC stocks increase with eCO2 in grasslands (8 ± 2 per cent) but not in forests (0 ± 2 per cent), even though plant biomass in grasslands increase less (9 ± 3 per cent) than in forests (23 ± 2 per cent). Ecosystem models do not reproduce this trade-off, which implies that projections of SOC may need to be revised. A synthesis of elevated carbon dioxide experiments reveals that when plant biomass is strongly stimulated by elevated carbon dioxide levels, soil carbon storage declines, and where biomass is weakly stimulated, soil carbon accumulates.

Terrestrial ecosystems remove about 30 per cent of the carbon dioxide (CO

The current rise in atmospheric [CO2], a consequence of human activities such as fossil fuel burning and deforestation, is thought to stimulate plant growth in many ecosystems (Bazzaz and Fajer 1990). Gifford (1994) suggested that the resulting increase in C assimilation by plants and its subsequent sequestration in the soil could counterbalance CO2 emissions. However, higher plant growth rates in a CO2-rich world can only be sustained if the soil supplies plants with additional nutrients (Zak et al. 2000; Luo et al. 2004). Therefore, the effect of elevated (e)[CO2] on soil N availability is of key importance when predicting the potential for C storage in terrestrial ecosystems. In short-term experiments, soil N availability can decrease (Diaz et al. 1993) or increase (Zak et al. 1993) under e[CO2], depending on the response of the soil microbial community. Moreover, plants under e[CO2] can increase N uptake at the expense of microbial N consumption (Hu et al. 2001). Clearly, the impact of higher [CO2] levels on C and N dynamics in terrestrial ecosystems depends on a set of complex interactions between soil and plants. Also, the establishment of equilibrium between soil organic matter (SOM) input and decomposition can take up to decades or longer. Therefore, we need longterm experiments under realistic field situations to predict changes in ecosystems under future [CO2]. The use of open-top chambers (OTC) and free-air carbon dioxide enrichment (FACE) techniques allowed for CO2 fumigation studies under far more realistic conditions than before (Rogers et al. 1983; Hendrey 1993; Chapter 2). Over the past two decades, many OTC and FACE experiments have been conducted, covering a wide range of terrestrial ecosystems. Soil characteristics