Carbon losses in soils previously exposed to elevated atmospheric CO2 in a chaparral ecosystem: Potential implications for a sustained biospheric C sink

Between 1996 and 2001 an experimental set up in a chaparral community near San Diego, CA, examined various plant and ecosystem responses to CO2 concentrations ranging from 250 to 750 μl l− 1. These experiments indicated a significant increase in soil C sequestration as CO2 rose above the ambient levels. In 2003, two years after the cessation of the CO2 treatments, we returned to this site to examine soil C dynamics with a particular emphasis on stability of specific pools of C. We found that in as little as two years, C content in the surface soils (0–15 cm) of previously CO2 enriched plots had dropped to levels below those of the ambient and pretreatment soils. In contrast, C retained in response to CO2 enrichment was more durable in the deeper soil layers (> 25 cm deep) where both organic and inorganic C were on average 26% and 55% greater, respectively, than C content of ambient plots. Using stable isotope tracers, we found that treatment C represented 25% of total soil C and contributed to 55% of soil CO2 efflux, suggesting that most of treatment C is readily accessible to decomposers. We also found that, C present before CO2 fumigation was decomposed at a faster rate in the plots that were exposed to elevated CO2 than in those exposed to ambient CO2 levels. To our knowledge, this is the first report that allows for a detail accounting of soil C after ceasing CO2 treatments. Our study provides a unique insight to how stable the accrued soil C is as CO2 increases in the atmosphere.