Water scarcity in semi-arid California compromises perennial alfalfa’s high yield and carbon sinking potentials

Alfalfa (Medicago sativa L.), a C3 and nitrogen fixing crop, offers high yields and carbon sinking potential due to its perennial nature. As a prolific water user, its ability to thrive in warmer climates, such as semi-arid regions like California, is further enhanced by ample solar energy and irrigation. However, water scarcity in these regions poses increasing challenges to alfalfa’s yield and carbon sinking potential. In this study, we used long-term eddy covariance data (2017–2023) to assess the trade-off between yield potential and water availability via energy, water, and carbon dynamics at an irrigated alfalfa site in California. Over the 7-year period, the average net ecosystem exchange (NEE) was −544 ± 201 g C m−2 y−1, accompanied by an average gross primary productivity (GPP) of −2281 ± 280 g C m−2 y−1 and an average of evapotranspiration (ET) of 861 ± 71 mm y−1. However, a notable decline in NEE (-175 g C m−2 y−1) was observed in 2022, which coincided with reduction in GPP (-1794 g C m−2 y−1) and ET (722 mm y−1). From our analysis, the lower evaporative fraction and higher sensible heat flux in 2022 indicated potential water stress, which was further reflected in a decrease water use efficiency and a lower canopy omega coefficient (i.e., higher stomatal closure). This water stress was a result of record-low springtime precipitation and irrigation curtailments, leading to a significant reduction in NEE and ET in 2022. Our results demonstrated that despite alfalfa’s potential for high yields and carbon sequestration, its effectiveness is compromised when water availability is limited. This highlights the complex dynamics between high yield and water consumption in alfalfa cultivation, especially under conditions of water scarcity. Our findings underscore the need for broader assessments of the trade-off between the high yield and water consumption in alfalfa cultivation, raising questions about the sustainability of growing alfalfa in drought-prone arid and semi-arid regions.