Abstract
The flux of carbon dioxide (CO2) between terrestrial ecosystems and the atmosphere may ameliorate or exacerbate climate change, depending on the relative responses of ecosystem photosynthesis and respiration to warming temperatures, rising atmospheric CO2, and altered precipitation. The combined effect of these global change factors is especially uncertain because of their potential for interactions and indirectly mediated conditions such as soil moisture. Here, we present observations of CO2 fluxes from a multi-factor experiment in semi-arid grassland that suggests a potentially strong climate – carbon cycle feedback under combined elevated [CO2] and warming. Elevated [CO2] alone, and in combination with warming, enhanced ecosystem respiration to a greater extent than photosynthesis, resulting in net C loss over four years. The effect of warming was to reduce respiration especially during years of below-average precipitation, by partially offsetting the effect of elevated [CO2] on soil moisture and C cycling. Carbon losses were explained partly by stimulated decomposition of soil organic matter with elevated [CO2]. The climate – carbon cycle feedback observed in this semiarid grassland was mediated by soil water content, which was reduced by warming and increased by elevated [CO2]. Ecosystem models should incorporate direct and indirect effects of climate change on soil water content in order to accurately predict terrestrial feedbacks and long-term storage of C in soil.
Highlights
Models predict declining C sequestration in the coming century [1,2,3] as ecosystem respiration (Reco) is preferentially stimulated over ecosystem photosynthesis (Peco), but experimental tests for these predictions are lacking [4]
Our data suggest that the semi-arid grassland of the Prairie Heating And CO2 Enrichment (PHACE) experiment site ranged from being a slight C sink to a slight source to the atmosphere over 2006–2010; with growing season (April–October) net ecosystem production (NEP, the sum of net ecosystem exchange (NEE)) losses averaging +33 g m22 over this period (Fig. 1C; positive fluxes indicate mass transfer from the ecosystem to the atmosphere, and negative fluxes indicate mass transfer from the atmosphere to the ecosystem)
Elevated [CO2] reduced net C uptake in 2006, and caused significantly greater net C losses than ambient in 2009 and 2010 (Fig. 1C; ANOVAR P,0.01 in all 3 years; n = 5)
Summary
Models predict declining C sequestration in the coming century [1,2,3] as ecosystem respiration (Reco) is preferentially stimulated over ecosystem photosynthesis (Peco), but experimental tests for these predictions are lacking [4]. Reco, composed of both autotrophic (Ra) and heterotrophic (Rh) respiration, responds more strongly to temperature than does photosynthesis [9], underpinning model predictions that ecosystem C storage will gradually decrease in a future warmer world [1,10]. Multifactor climate change experiments are needed to test the predictions of global C cycle models and identify the strength of interactive effects on ecosystem C uptake and loss [4]
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