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Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 11/20/2018 Publication Date: 11/26/2018 Citation: Wagle, P., Gowda, P.H., Northup, B.K. 2018. Annual dynamics of carbon dioxide fluxes over a rainfed alfalfa field in the U.S. Southern Great Plains. Agricultural and Forest Meteorology. 265:208-217. https://doi.org/10.1016/j.agrformet.2018.11.022. DOI: https://doi.org/10.1016/j.agrformet.2018.11.022 Interpretive Summary: Annual dynamics of carbon dioxide (CO2) fluxes with respect to major controlling factors and harvesting is lacking for rainfed alfalfa, a high quality perennial legume forage. An eddy covariance system was used to measure CO2 fluxes for two years (April 2016 - March 2018) over a rainfed alfalfa field in El Reno, Oklahoma, USA. Alfalfa forage production was strongly regulated by amounts and the frequency of rainfall. As a result, cumulative dry forage yield was higher in 2017 (wet year, ~10 t ha-1) than in 2016 (dry year, ~7.5 t ha-1). Results showed that cumulative rainfall of around 250 mm for the period of January-April was sufficient to produce optimum level of forage (~3 t ha-1) for harvesting in May. Optimum air temperature (Ta) and vapor pressure deficit (VPD) were approximately 25 ºC and 2.2 kPa, respectively, for net ecosystem CO2 exchange (NEE). Growth stages of alfalfa and climatic conditions (i.e., dry or normal/wet periods) influenced the response of NEE to photosynthetically active radiation (PAR). Daily (8-day averages) net ecosystem CO2 exchange (NEE) and gross primary production (GPP) reached up to -8.17 and 16.69 g C m-2 d-1, respectively. The maximum 8-day composite ecosystem light use efficiency (ELUE) was 0.36 g C mol-1 PAR. Results showed that rainfed alfalfa, even though harvested for hay periodically (4-5 times a year), can be a large sink (i.e., gain of carbon) of carbon (cumulative NEE of -454 g C m-2 in 2017) in the Southern Great Plains. A strong correspondence of GPP and ELUE with MODIS-derived vegetation indices offered the potential of upscaling site-level observations of CO2 fluxes to larger spatial scales for alfalfa. Technical Abstract: Thorough investigation of annual dynamics of carbon dioxide (CO2) fluxes with respect to major controlling factors and harvesting is lacking for rainfed alfalfa (Medicago sativa L.), a high quality perennial legume forage. To address this knowledge gap, this study reports two years (April 2016 - March 2018) of eddy covariance measurements of CO2 fluxes over a rainfed alfalfa field in El Reno, Oklahoma, USA. Alfalfa forage yield was strongly regulated by amounts and the frequency of rainfall. As a result, cumulative dry forage yield was ~7.5 t ha-1 (four harvests) in 2016 (dry year) and ~10 t ha-1 (five harvests) in 2017 (wet year). Thresholds of air temperature (Ta) and vapor pressure deficit (VPD) were approximately 25 ºC and 2.2 kPa, respectively, for net ecosystem CO2 exchange (NEE). The response of NEE to photosynthetically active radiation (PAR) varied with growth stages of alfalfa and climatic conditions (i.e., dry or normal/wet periods). Daily (8-day averages) NEE and gross primary production (GPP) reached up to -8.17 and 16.69 g C m-2 d-1, respectively. Magnitudes of GPP (GPPMOD) derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) were less than 50% of tower-derived GPP due to a smaller value (0.15 g C mol-1 PAR) of light use efficiency in the GPPMOD algorithm. The observed 8-day composite ecosystem light use efficiency (ELUE) was up to 0.36 g C mol-1 PAR in this study. Results showed that the rainfed alfalfa field, which was harvested 4-5 times a year for hay, can be a large sink of carbon (e.g., cumulative NEE of -454 g C m-2 in 2017) at an annual scale. The GPP and ELUE showed a strong correspondence with MODIS-derived vegetation indices, indicating the potential of satellite remote sensing for upscaling site-level observations of CO2 fluxes to larger spatial scales for alfalfa. |
