Do changes in CO2, temperature, and precipitation interact to affect net ecosystem exchange in an old-field ecosystem?

Across the United States, abandoned agricultural land or old-fields cover approximately 50,000 km². Even though they represent a potentially significant pool for carbon storage, it is generally unknown how elevated [CO₂], warming and changing precipitation regimes will modify the capacity of old-fields to act as carbon sinks. Using open-top chambers (4-m diameter), the interactive effects of elevated [CO₂], warming, and precipitation on diurnal patterns of net ecosystem exchange (NEE) were examined in the OCCAM (Old-Field Community Climate and Atmospheric Manipulation) experiment at Oak Ridge National Laboratory in eastern Tennessee. Diurnal CO₂ fluxes were quantified with an open path infra red gas analyzer, monthly during the 2006 summer growing season. Simultaneous measurements of photosynthetically active radiation, soil temperature and moisture were also made. In general, interactions between elevated [CO₂], warming and drought increased CO₂ efflux from this ecosystem. This effect was strongest in August and September, when increased diurnal CO₂ efflux was measured in warm, dry and elevated [CO₂] plots. Midday net ecosystem carbon assimilation was consistently greater in wet relative to dry treatments. Elevated [CO₂] acted mainly to increase respiratory fluxes in the morning and evening, but there were few significant effects of warming on CO₂ fluxes. Old-fields may already be a source of C across a wide land area, and these results suggest that, in the warmer and drier climates predicted for the south eastern US, C efflux from these ecosystems will further increase.