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ARS Home » Plains Area » El Reno, Oklahoma » Oklahoma and Central Plains Agricultural Research Center » Livestock, Forage and Pasture Management Research Unit » Research » Publications at this Location » Publication #401562
Research Project: Integrated Agroecosystem Research to Enhance Forage and Food Production in the Southern Great Plains

Location: Livestock, Forage and Pasture Management Research Unit

Title: Carbon dioxide fluxes over irrigated and rainfed alfalfa in the Southern Great Plains, USA

Author
item Wagle, Pradeep
item ZHOU, YUTING - Oklahoma State University
item Northup, Brian
item Moffet, Corey
item Gunter, Stacey

Submitted to: European Journal of Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/22/2024
Publication Date: 6/25/2024
Citation: Wagle, P., Zhou, Y., Northup, B.K., Moffet, C., Gunter, S.A. 2024. Carbon dioxide fluxes over irrigated and rainfed alfalfa in the Southern Great Plains, USA. European Journal of Agronomy. 159. Article 127265. https://doi.org/10.1016/j.eja.2024.127265.
DOI: https://doi.org/10.1016/j.eja.2024.127265

Interpretive Summary: This study quantified the magnitudes and annual dynamics of eddy covariance (EC) measured CO2 fluxes and investigated the temporal variability in CO2 fluxes and satellite-derived enhanced vegetation index (EVI) with respect to biophysical factors and hay harvests for rainfed and irrigated alfalfa in Oklahoma, USA. Adequate rainfall during April and May was important to achieving high forage yields for the first harvest in May. The regrowth of alfalfa was strongly regulated by the amount and timing of rainfall or irrigation. Alfalfa fields were strong carbon sinks with seasonal (April-October) sums of net ecosystem CO2 exchange (NEE) ranging approximately from -280 to -490 g C m-2. Considering carbon losses from hay harvest, the alfalfa fields could vary from a small carbon sink to a small carbon source at annual scales depending on annually harvested hay yields and climatic conditions (i.e., wet or dry years and timing of rainfall). A strong relationship between CO2 fluxes and satellite-derived greenness index (enhanced vegetation index, EVI) offered the potential of simulating CO2 fluxes in alfalfa with remote sensing data. However, the Moderate Resolution Imaging Spectroradiometer (MODIS)-derived EVI at 250 m resolution showed stronger relationships with CO2 fluxes than did the MODIS-derived EVI at 500 m and Landsat-derived EVI integrated for the entire field scale, due to better matching between EC footprint areas and remote sensing pixels. Thus, satellite-based modeling should select remote sensing products of spatial resolutions that are comparable to EC footprints to improve the performance of models. Analyses of multiple site-years of flux data in this study provide a comprehensive understanding of the dynamics of CO2 fluxes with respect to biophysical factors and management practices for alfalfa in the Southern Great Plains, USA.

Technical Abstract: Annual dynamics of carbon dioxide (CO2) fluxes for irrigated and rainfed alfalfa (Medicago sativa L.) in the Southern Great Plains of the United States of America (USA) under different watering regimes are not fully understood. This study analyzes multi-years of eddy covariance (EC) measured CO2 fluxes over irrigated and rainfed alfalfa in central Oklahoma, USA. The cumulative hay yields were 7.15 t ha-1 (two harvests in 2019) in the rainfed field and ~9 t ha-1 (4-5 harvests in 2020 and 2021) in the irrigated field. Adequate rainfall during April and May was important to produce viable yields of alfalfa at the first harvest in May. The regrowth and carbon uptake potential of alfalfa after harvesting was strongly regulated by water availability. The alfalfa fields were near carbon neutral or a small carbon source from January to mid-March and carbon sink after the initiation of vegetative growth in mid-March. The alfalfa fields were strong carbon sinks (as high as -578 g C m-2) on an annual scale. The alfalfa fields varied from small carbon sinks to small carbon sources depending on the quantity of annually harvested hay and growing conditions when the loss of carbon by removing the hay from the field was accounted for. Overall, CO2 fluxes and satellite-derived enhanced vegetation index (EVI) showed similar temporal patterns with respect to growing conditions and hay harvests, except for some discrepancies and time lags due to coarse spatiotemporal resolutions of EVI products. Thus, it is necessary to combine two or more satellite products with different temporal and spatial resolutions to more precisely track the frequent and variable sizes of hay harvests and vegetation regrowth after harvesting and to simulate continuous time-series CO2 fluxes.