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 (O3) or vary based on species-specific growth responses to elevated CO2. To understand how projected increases in atmospheric CO2 and O3 alter SOM formation, we used physical fractionation to characterize soil C and N at the Rhinelander Free Air CO2–O3 Enrichment (FACE) experiment. Tracer amounts of 15NH4+ were applied to the forest floor of Populus tremuloides, P. tremuloides–Betula papyrifera and P. tremuloides–Acer saccharum communities exposed to factorial CO2 and O3 treatments. The 15N tracer and strongly depleted 13C–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 O3 significantly altered 15N 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 O3 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.