Accelerated belowground C cycling in a managed agriforest ecosystem exposed to elevated carbon dioxide concentrations

Abstract

We investigated the effects of three elevated atmospheric CO 2 levels on a Populus deltoides plantation at Biosphere 2 Laboratory in Oracle Arizona. Stable isotopes of carbon have been used as tracers to separate the carbon present before the CO 2 treatments started (old C), from that fixed after CO 2 treatments began (new C). Tree growth at elevated [CO 2 ] increased inputs to soil organic matter (SOM) by increasing the production of fine roots and accelerating the rate of root C turnover. However, soil carbon content decreased as [CO 2 ] in the atmosphere increased and inputs of new C were not found in SOM. Consequently, the rates of soil respiration increased by 141% and 176% in the 800 and 1200 lL L À1 plantations, respectively, when compared with ambient [CO 2 ] after 4 years of exposure. However, the increase in decomposition of old SOM (i.e. already present when CO 2 treatments began) accounted for 72% and 69% of the increase in soil respiration seen under elevated [CO 2 ]. This resulted in a net loss of soil C at a rate that was between 10 and 20 times faster at elevated [CO 2 ] than at ambient conditions. The inability to retain new and old C in the soil may stem from the lack of stabilization of SOM, allowing for its rapid decomposition by soil heterotrophs.

Carbon allocation and carbon isotope fluxes in the plant-soil-atmosphere continuum: a review

“…However, recent studies have shown the importance of soil C availability as a driver of heterotrophic respiration (Vance and Chapin, 2001;Trueman and Gonzalez-Meler, 2005;Scott-Denton et al, 2006). There is evidence that fresh C input into soil can increase, decrease or have little or no effect on R h Fontaine et al, 2007).…”

Interactive Effects of Time, CO2, N, and Diversity on Total Belowground Carbon Allocation and Ecosystem Carbon Storage in a Grassland Community

“…First, we hypothesized that more diverse systems would have greater TBCA because complementary resource use results in greater C fixation and productivity (Tilman and others 1996;Fargione and others 2007). Second, we hypothe-sized that because root production, respiration, exudation, and mycorrhizal allocation tend to increase at elevated CO 2 , likely due to increased photosynthate availability (Matamala and Schlesinger 2000;Treseder and Allen 2000;Pendall and others 2004;Allen and others 2005;Trueman and Gonzalez-Meler 2005), TBCA would increase at elevated CO 2 . However, over time, the CO 2 fertilization effect may create feedbacks that reduce N availability and thus also reduce the stimulation of biomass by elevated CO 2 (progressive N limitation, PNL; Reich and others 2006b).…”

Our Eclectic Adventures in the Slower Eras of Photosynthesis: From New England Down Under to Biosphere 2 and Beyond

“…Analysis of δ 13 C in respiratory CO 2 showed that, rather than the anticipated increase of soil sequestration of carbon (163), stimulation of aboveground carbon influx by elevated [CO 2 ] actually primed respiration of residual soil carbon (164), without significantly altering the soil microbial biota (82). Among other things, leaf-canopy-ecosystem experiments in B2L made a unique contribution to the debate about the CO 2 sequestration capacity of fast-growing woody plants to mitigate increasing atmospheric [CO 2 ].…”