Carbon dioxide and water vapor fluxes in winter wheat and tallgrass prairie in central Oklahoma

Abstract

Winter wheat (Triticum aestivum L.) and tallgrass prairie are common land cover types in the Southern Plains of the United States. During the last century, agricultural expansion into native grasslands was extensive, particularly managed pasture or winter wheat. In this study, we measured carbon dioxide (CO₂) and water vapor (H₂O) fluxes from winter wheat and tallgrass prairie sites in Central Oklahoma using the eddy covariance in 2015 and 2016. The objective of this study was to contrast CO₂ and H₂O fluxes between these two ecosystems to provide insights on the impacts of conversion of tallgrass prairie to winter wheat on carbon and water budgets. Daily net ecosystem CO₂ exchange (NEE) reached seasonal peaks of -9.4 and -8.8 g C m^-2 in 2015 and -6.2 and -7.5 g C m^-2 in 2016 at winter wheat and tall grass prairie sites, respectively. Both sites were net sink of carbon during their growing seasons. At the annual scale, the winter wheat site was a net source of carbon (56 ± 13 and 33 ± 9 g C m^-2 year^-1 in 2015 and 2016, respectively). In contrast, the tallgrass prairie site was a net sink of carbon (-128 ± 69 and -119 ± 53 g C m^-2 year^-1 in 2015 and 2016, respectively). Daily ET reached seasonal maximums of 6.0 and 5.3 mm day^-1 in 2015, and 7.2 and 8.2 mm day^-1 in 2016 at the winter wheat and tallgrass prairie sites, respectively. Although ecosystem water use efficiency (EWUE) was higher in winter wheat than in tallgrass prairie at the seasonal scale, summer fallow contributed higher water loss from the wheat site per unit of carbon fixed, resulting into lower EWUE at the annual scale. Results indicate that the differences in magnitudes and patterns of fluxes between the two ecosystems can influence carbon and water budgets.

Keywords: Carbon sink; Ecosystem water use efficiency; Evapotranspiration; Land use change; Net ecosystem exchange.