Winter carbon dioxide fluxes in humid-temperate pastures

Because of their vast size, grazing lands have the potential to sequester significant quantities of carbon, slowing the increase in atmospheric CO2 and reducing the risk of global warming. Although CO2 uptake during the growing season can be substantial, losses during winter months reduce annual sequestration, potentially turning grazing lands into net carbon sources. The goal of this research was to quantify the magnitude of winter fluxes for humid-temperate pastures in the northeastern USA. The study was conducted from 2003 to 2005 on two pastures in the ridge and valley region of central Pennsylvania, one dominated by a mix of cool-season grasses and the other transitioning from an alfalfa to mixed-grass pasture. Pasture-scale CO2 fluxes were quantified using eddy covariance techniques. The alfalfa pasture was less of a CO2 source to the atmosphere during winter months (1 December to 31 March) than the grass pasture, averaging 2.68 g CO2 m−2 day−1 compared with 3.09 g CO2 m−2 day−1 for the grass pasture. Cumulative efflux for the winter months averaged 326 g CO2 m−2 (88 g C m−2) for the alfalfa and 375 g CO2 m−2 (101 g C m−2) for the grass pasture. In the absence of snow cover, eddy covariance measurements estimated that photosynthetic CO2 uptake occurred at temperatures below 0 °C. Canopy and leaf chamber measurements in the field and in controlled environments suggested minimum temperatures for photosynthetic CO2 uptake of about −4 °C. Even when daytime uptake occurred, nighttime efflux from the system was greater so that the pastures remained CO2 sources throughout the winter.