Winter carbon dioxide fluxes in humid-temperate pastures
Introduction
Human induced increases in atmospheric CO2 through the burning of fossil fuels and deforestation are considered to be a primary cause of rising global temperatures and will likely continue to contribute to future global warming. However, carbon sequestration by terrestrial ecosystems has reduced the rate of CO2 accumulation in the atmosphere, although the exact size and location of this sink has not been determined (Janssens et al., 2003). 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, CO2 efflux following defoliation (McGinn and King, 1990), during periods of drought, and during winter months can significantly reduce annual sequestration, frequently turning grazing lands into net carbon sources.
Ecosystem respiration during the winter months is primarily a function of soil temperature and can occur even when soil temperatures are well below 0 °C. Frank et al. (2002) suggested that the minimum temperature for soil respiration in a northern semiarid grassland was −12 °C. Grogan et al. (2001) observed low soil respiration rates of about 0.2 g CO2 m−2 day−1 at soil temperatures between −5 and −9 °C in a sub-arctic heath tundra. Wintertime losses are also closely related to the length of the non-growing season, and can be substantial in northern latitudes. In the western USA and Canada, winter respiration ranged from 11% of growing-season CO2 uptake in Oklahoma (Suyker and Verma, 2001) to more than 100% in Alberta (Flanagan et al., 2002). At a latitude similar to Pennsylvania but with vastly different vegetation cover and precipitation patterns, 43% of growing-season CO2 uptake on a sagebrush-steppe site in Idaho was lost during winter months (Gilmanov et al., 2003).
In extreme environments, where daily minimum temperature remains below 0 °C throughout the winter, photosynthesis can be completely inhibited until spring. However, where frost-free periods often occur during winter the potential exists for evergreen conifers and winter annuals to resume CO2 uptake during periods of mild weather (Oquist and Martin, 1986). Photosynthetic activity of arctic and alpine plants can occur at sub-zero temperatures as long as temperatures are greater than the freezing point of water in the leaves. For many species, the minimum temperature for photosynthesis was close to the freezing temperature of the leaf (Pisek, 1973). The minimum temperature for photosynthetic uptake can vary based on a variety of biotic and abiotic factors, but can range from about −3 to −10 °C (Pisek, 1973, Terry et al., 2000, Starr and Oberbauer, 2003). Photosynthetic CO2 uptake, at a rate of about 5 μmol m−2 s−1, occurred in a Colorado sub-alpine forest when air temperature was near −2 °C (Huxman et al., 2003). More limited uptake, on the order of 1.0 μmol m−2 s−1, was observed for dwarf arctic shrubs in Alaska when mean springtime leaf temperature was −4.7 °C (Starr and Oberbauer, 2003). Although many perennial temperate cool-season grasses have excellent winter hardiness, little information is available on their ability to maintain photosynthetic activity during the winter.
The goal of this research was to quantify the magnitude of wintertime respiratory CO2 loss and photosynthetic uptake for humid-temperate pastures in the northeastern USA.
Section snippets
Materials and methods
The study was conducted on two pastures at the Pennsylvania State University Haller Research Farm located about 10 km northeast of State College, Pennsylvania. The first eddy covariance system was installed in May 2002 on a grass-dominated permanent pasture that has been traditionally cut once in the spring for hay then rotationally grazed by beef cattle until early-November. The second system, installed in January 2003, was on an adjacent, alfalfa-dominated pasture managed for hay production
Results
Mean monthly air temperature during winter months (1 December to 31 March) ranged from −7.0 to 4.0 °C (Table 1). The lowest daily air temperature each year was −17 °C in March 2003, −19 °C in January 2004, and −20 °C in January 2005. Daytime air temperatures throughout the winter were frequently above freezing and occasionally exceeded 20 °C. In the grass pasture, soil temperature at 3 cm decreased to near 0 °C by late-December each year and generally remained within a degree of freezing until early-
Discussion
Averaged over years and sites, daily flux during the winter for these pasture systems, where soil temperature averaged about 1 °C, was 2.88 g CO2 m−2 day−1. Daily gross photosynthesis averaged about 0.9 g CO2 m−2 day−1 suggesting that respiration rates were on the order of about 3.8 g CO2 m−2 day−1. A global analysis by Raich and Potter (1995) of several studies reporting on CO2 emissions from soils found soil CO2 efflux rates of about 4–6 g CO2 m−2 day−1 when mean air temperature equaled 0 °C. However, they
Conclusions
Significant losses of CO2 can occur from temperate pastures during the winter. Averaged over years and sites, wintertime daily flux for these pasture systems was 2.88 g CO2 m−2 day−1. The alfalfa pasture was less of a CO2 source to the atmosphere during winter months 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. Eddy covariance, canopy chamber, and leaf chamber measurements all indicated that photosynthetic CO2 uptake occurred at
Acknowledgments
The author thanks D. Genito and S. LaMar for assistance in data collection and A. Frank, V. Baron and several anonymous reviewers for helpful comments on earlier versions of the manuscript.
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