Changes in forest soil organic matter pools after a decade of elevated CO2 and O3

The impact of rising atmospheric carbon dioxide (CO2) may be mitigated, in part, by enhanced rates of net primary production and greater C storage in plant biomass and soil organic matter (SOM). However, C sequestration in forest soils may be offset by other environmental changes such as increasing tropospheric ozone (O-3) or vary based on species-specific growth responses to elevated CO2. To understand how projected increases in atmospheric CO2 and O-3 alter SOM formation, we used physical fractionation to characterize soil C and N at the Rhinelander Free Air CO2-O-3 Enrichment (FACE) experiment. Tracer amounts of (NH4+)-N-15 were applied to the forest floor of Populus tremuloides, P. tremuloides-Betula papynlera and P. tremuloides-Acer saccharum communities exposed to factorial CO2 and O-3 treatments. The N-15 tracer and strongly depleted C-13-CO2 were traced into SOM fractions over four years. Over time, C and N increased in coarse particulate organic matter (cPOM) and decreased in mineral-associated organic matter (MAOM) under elevated CO2 relative to ambient CO2. As main effects, neither CO2 nor O-3 significantly altered N-15 recovery in SOM. Elevated CO2 significantly increased new C in all SOM fractions, and significantly decreased old C in fine POM (fPOM) and MAOM over the duration of our study. Overall, our observations indicate that elevated CO2 has altered SOM cycling at this site to favor C and N accumulation in less stable pools, with more rapid turnover. Elevated O-3 had the opposite effect, significantly reducing cPOM N by 15% and significantly increasing the C:N ratio by 7%. Our results demonstrate that CO2 can enhance SOM turnover, potentially limiting long-term C sequestration in terrestrial ecosystems; plant community composition is an important determinant of the magnitude of this response. (C) 2011 Elsevier Ltd. All rights reserved.