Warming Reduces Carbon Losses from Grassland Exposed to Elevated Atmospheric Carbon Dioxide

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.

Disentangling root responses to climate change in a semiarid grassland

“…Phace modelling predictions by Parton et al (2007b) showed a gradual decrease of soil c with elevated cO 2 despite increased production, due to more rapid decomposition. Our observations of enhanced mass loss are consistent with ecosystem-level c exchange measurements indicating net c loss with elevated cO 2 (Pendall et al 2013). In addition, there are no indications of increases in the total or resistant soil organic c at Phace with elevated cO 2 (carrillo et al 2011 and recent unpublished data), despite greater production.…”

“…Warming increased soil temperature at 3-cm depth by 2.8 °c on average across the elevated cO 2 treatments (Table 1). These treatment effects on overall averages are reflective of dominant effects across seasons (see carrillo et al (2012) and Pendall et al (2013)). Prsavailable n (nO 3 − + nh 4 + ) decreased with elevated cO 2 and increased with warming under ambient cO 2 , but was not affected by warming under elevated cO 2 or by irrigation (Table 1).…”

Drought and Carbon Cycling of Grassland Ecosystems under Global Change: A Review

“…Effects from global change drivers and droughts may interact within ecological diversity and in the composition of natural communities [164]. C cycling may mitigate or exacerbate climate change, depending upon the relative responses of grassland C sequestration and emissions to global change factors and droughts [165].…”

“…Also, anthropogenic pressures such as grazing and land-use change could be key drivers of biodiversity loss, with serious consequences for ecosystem functioning [12]. Simultaneously, the combined effects of environmental factors have great potential to interact and indirectly or directly mediate soil moisture, affecting the main process of C cycling [165].…”

Patterns of longer-term climate change effects on CO<sub>2</sub> efflux from biocrusted soils differ from those observed in the short-term

“…While moss 242 died off quickly in the watered plots, the negative effects of temperature took longer to emerge 243 ). Indeed, increased CO2 efflux with warming was clearer in the following 244 effects would also be consistent with drying from the infrared heat lamps, a mechanism that was 266 supported in a Wyoming grassland experiment (Pendall et al, 2013). Our soil moisture data 267 showed little evidence of such drying effects (Supplemental Fig.…”