Priming increases mean age of carbon respired in incubated soils

The most important human perturbation to soil carbon stocks on decadal timescales may come from the effects of global change on vegetation. Apart from changing soil carbon stocks directly, through inputs of biomass to soils, vegetation change may alter the turnover time of native soil organic matter, resulting from changes in the quality and quantity of carbon inputs belowground. One possible mechanism for this change is acceleration of decomposition from increased inputs of easily decomposable carbon substrate (the priming effect). We investigated the effect of adding easily decomposed carbon substrate (in this case, isotopically-labeled sucrose) on the age of carbon respired using measurements of 14C-CO2 respired during short- and long-term incubation of soils from the Free Air CO2 Enrichment (FACE) experiments. Soils from FACE experimental ecosystems are depleted in both 13C and 14C due to decade-long addition of fossil-derived CO2, making young (FACE-labeled C, <10 y) and old (pre-FACE fumigation, >10 y) soil carbon easily distinguished by their isotopic signatures. Soils were incubated at the site mean annual temperature and with a warming treatment. We found that priming and warming increased fluxes of CO2 by a similar magnitude (30% to 190% increase), however, isotopes show that priming-induced flux increases originate from different source pools than fluxes from the controls. In non-FACE treated soils, priming promoted the loss of CO2 enriched in bomb-14C, increasing the mean age of carbon respired by 10 years on average at the control temperature, and by 5 years in warmed soils. Other global change factors, such as elevated CO2 and nitrogen deposition had no statistically significant effect on fluxes or age of respired CO2. If belowground carbon allocation increases due to global change, loss of older soil carbon pools may be accelerated through priming.