The annual dynamics of carbon dioxide (CO) fluxes for irrigated and rainfed alfalfa ( L.) in the Southern Great Plains of the United States of America (USA) under different watering regimes are not yet fully understood. The main objective of this study was to examine the dynamics of eddy covariance (EC) measured CO fluxes in relation to various biophysical factors and hay harvests for irrigated and rainfed alfalfa in central Oklahoma, USA. The study also aimed to investigate the relationship between CO fluxes and satellite-derived enhanced vegetation index (EVI) at different spatiotemporal scales and to assess the temporal variability in CO fluxes and EVI to variable growing conditions and hay harvests. The cumulative hay yields were 7.15 t ha (two harvests in 2019) in the rainfed field and ∼9 t ha (4–5 harvests in 2020 and 2021) in the irrigated field. Having sufficient rainfall during April and May was crucial to achieve economically feasible yields of alfalfa during the first harvest in May. The availability of water strongly regulated the potential for regrowth and carbon uptake of alfalfa following harvesting. 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 (cumulative annual net ecosystem CO exchange, NEE, up to −578 g C m in irrigated field) on an annual scale. When accounting for the loss of carbon due to the removal of hay from the fields, the carbon balance of the alfalfa fields varied from small carbon sinks to small carbon sources, depending on the amount of hay harvested annually and the growing conditions. In general, the temporal patterns of CO fluxes and EVI were similar in relation to growing conditions and hay harvests. However, some discrepancies and time lags were observed due to the coarse spatiotemporal resolution of the EVI products. Thus, it is essential to integrate two or more satellite products with different temporal and spatial resolutions to accurately monitor the frequent and varying sizes of hay harvests and vegetation regrowth after harvesting, and to simulate continuous time-series CO fluxes.

中文翻译：

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美国南部大平原灌溉和雨养苜蓿的二氧化碳通量


美国南部大平原不同浇水方式下灌溉和雨养苜蓿 (L.) 二氧化碳 (CO) 通量的年度动态尚未完全了解。本研究的主要目的是研究美国俄克拉荷马州中部灌溉和雨养苜蓿的涡流协方差（EC）测量的二氧化碳通量与各种生物物理因素和干草收成的动态关系。该研究还旨在调查不同时空尺度下二氧化碳通量和卫星增强植被指数（EVI）之间的关系，并评估二氧化碳通量和EVI随生长条件和干草收成变化的时间变化。雨养田的干草累计产量为 7.15 吨公顷（2019 年收获两次），灌溉田的累计干草产量为 ∼9 吨公顷（2020 年和 2021 年收获 4-5 次）。四月和五月充足的降雨对于在五月第一次收获期间实现苜蓿经济上可行的产量至关重要。水的可用性强烈调节了苜蓿收获后的再生和碳吸收潜力。 1月至3月中旬苜蓿田接近碳中性或少量碳源，3月中旬开始营养生长后出现碳汇。苜蓿田在年度规模上是强大的碳汇（每年累积的净生态系统二氧化碳交换，NEE，在灌溉田中高达-578 g C m ）。当考虑到因从田间清除干草而造成的碳损失时，苜蓿田的碳平衡从小型碳汇到小型碳源不等，具体取决于每年收获的干草量和生长条件。 一般来说，CO 通量和 EVI 的时间模式与生长条件和干草收成的关系相似。然而，由于 EVI 产品的时空分辨率较粗，观察到了一些差异和时间滞后。因此，有必要集成两个或多个具有不同时间和空间分辨率的卫星产品，以准确监测频繁且不同规模的干草收割和收割后植被的再生，并模拟连续时间序列的二氧化碳通量。