Abstract
The earth's future climate state is highly dependent upon changes in terrestrial C storage in response to rising concentrations of atmospheric CO₂. Here we show that consistently enhanced rates of net primary production (NPP) are sustained by a C-cascade through the root-microbe-soil system; increases in the flux of C belowground under elevated CO₂ stimulated microbial activity, accelerated the rate of soil organic matter decomposition and stimulated tree uptake of N bound to this SOM. This process set into motion a positive feedback maintaining greater C gain under elevated CO₂ as a result of increases in canopy N content and higher photosynthetic N-use efficiency. The ecosystem-level consequence of the enhanced requirement for N and the exchange of plant C for N belowground is the dominance of C storage in tree biomass but the preclusion of a large C sink in the soil.
Highlights
Predictions of the earth’s future climate state are highly sensitive to the C-cycle response of ecosystems to rising concentrations of atmospheric CO2 (Friedlingstein et al 2006; Meehl et al 2007)
Ecosystem responses to experimental increases in atmospheric CO2 concentrations vary widely, from ecosystems in which low soil-N availability precludes an enhancement of net primary productivity (NPP) in response to elevated CO2 (Oren et al 2001; Menge & Field 2007) to experiments that, in the absence of N fertilization, show only a transient response of NPP to elevated CO2 (Reich et al 2006; Seiler et al 2009; Norby et al 2010), to demonstrably N limited ecosystems where the enhancement in NPP is sustained through time (Langley et al 2009; McCarthy et al 2010)
Among elevated CO2 experiments, the Duke Forest free-air CO2 enrichment (FACE) experiment is unique in that NPP in this N-limited system has remained consistently and significantly higher under elevated compared with ambient CO2 for over a decade in the absence of nutrient amendment, even in the face of extreme climate events such as droughts and ice storms (McCarthy et al 2010)
Summary
Predictions of the earth’s future climate state are highly sensitive to the C-cycle response of ecosystems to rising concentrations of atmospheric CO2 (Friedlingstein et al 2006; Meehl et al 2007). Among elevated CO2 experiments, the Duke Forest free-air CO2 enrichment (FACE) experiment is unique in that NPP in this N-limited system has remained consistently and significantly higher under elevated compared with ambient CO2 for over a decade in the absence of nutrient amendment, even in the face of extreme climate events such as droughts and ice storms (McCarthy et al 2010) This site is at one end of the NPP-CO2 response gradient raising the question of what processes sustain higher productivity under elevated CO2 and how the responses here may apply to other ecosystems. Fine root production decreased by only c. 12% (Jackson et al 2009) suggesting that the remaining 13% reduction in University, Ames, IA, USA 5Department of Biology, Duke University, Durham, NC 27708, USA 6Department of Ecology and Evolutionary Biology and Kansas Biological Survey, University of Kansas, Lawrence, KS 66047, USA 7Nicholas School of the Environment, Duke University, Durham, NC 27708, USA 8US Forest Service Southern Research Station, Durham, NC 27708, USA 9Biology and Environmental Studies, Bowdoin College, Brunswick,
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