Annual dynamics of carbon dioxide fluxes over a rainfed alfalfa field in the US Southern Great Plains

A thorough investigation of annual dynamics of carbon dioxide (CO2) fluxes with respect to major controlling factors and harvest management 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 central Oklahoma, USA. Alfalfa yields were strongly regulated by amount and timing of rainfall. As a result, cumulative dry forage yield was similar to 7.5 t ha(-1) (four harvests) in 2016 (dry year) and similar to 10t ha(-1) (five harvests) in 2017 (wet year). An optimum air temperature (TO and vapor pressure deficit (VPD) for net ecosystem CO2 exchange (NEE) was approximately 25 degrees C and 2.2 kPa, respectively. The response of gross primary production (GPP) to photosynthetically active radiation (PAR) varied with growth stage of alfalfa and climatic conditions (i.e., dry or normal/wet periods). Daily (8-day averages) NEE and gross primary production (GPP) reached -8.17 and 16.69 g C m(-2) d(-1), respectively. Magnitudes of GPP (GPP(MOD)) derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) were < 50% of tower-derived GPP, most likely due to a smaller value (0.15 g C mol(-1) PAR) for light use efficiency in the GPP(MOD) algorithm. The observed 8-day composite ecosystem light use efficiency (ELUE) was up to 0.36 g C mol(-1) PAR in this study. The rainfed alfalfa field with 4-5 hay harvests per year showed large carbon uptake potential (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 applying satellite remote sensing to upscale site-level observations of CO2 fluxes for alfalfa to larger spatial scales.