Arid and semiarid ecosystems comprise approximately 40 % of the global terrestrial surface and play an important role in the carbon-climate system. However, despite their large geographic extension in South America, they are largely under-represented in studies of ecosystem carbon fluxes. Eddy-covariance measurements of net ecosystem carbon exchange (NEE) were carried out in a Patagonian steppe co-dominated by grasses and shrubs in southern Argentina, for four years, including relatively dry and wet years. We evaluated the seasonal and annual variation of NEE, gross primary productivity (GPP) and ecosystem respiration (Reco), and their environmental controls. This ecosystem exhibits large seasonal fluctuations in global radiation (Rg), air temperature, vapor pressure deficit, soil moisture and leaf area index (LAI). This steppe was a net carbon sink with a mean annual cumulative NEE of -249 g C m−2 yr−1. The strength as carbon sink was higher (-283.9 g C m−2 yr−1) in the wet year. NEE was tightly coupled to GPP with a marked peak at the middle of the growing season when LAI achieved its maximum value. Reco exhibited less fluctuation throughout time than GPP. A random forest machine-learning analysis indicated that Rg, LAI and deep soil moisture are the main drivers contributing to variability in daily and monthly cumulative NEE, GPP and Reco during the growing season and across the entire study period. Results suggest that deep-rooted shrubs contribute substantially to C fluxes during the growing season (low precipitation and high VPD period) by acceding to deep and more stable water sources which allow extending the period of net carbon capture. This study helps to understand how this ecosystem functions and to predict how this steppe will respond to future climate. In addition, this research highlights the need for conservation of this ecosystem due to its large extension and carbon sequestration capacity.

中文翻译：

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巴塔哥尼亚草原碳固存的年度和季节动态


干旱和半干旱生态系统约占全球陆地表面的40%，在碳气候系统中发挥着重要作用。然而，尽管它们在南美洲的地理范围很广，但它们在生态系统碳通量研究中的代表性基本上不足。在阿根廷南部以草和灌木为主的巴塔哥尼亚草原上，对净生态系统碳交换（NEE）进行了四年的涡度协方差测量，包括相对干燥和潮湿的年份。我们评估了 NEE、总初级生产力 (GPP) 和生态系统呼吸 (Reco) 的季节和年度变化及其环境控制。该生态系统在全球辐射（Rg）、气温、蒸气压赤字、土壤湿度和叶面积指数（LAI）方面表现出较大的季节性波动。该草原是一个净碳汇，年平均累积NEE为-249 g C m−2 yr−1。丰水年碳汇强度较高（-283.9 g C m−2 yr−1）。 NEE 与 GPP 紧密耦合，在生长季节中期达到明显峰值，此时 LAI 达到最大值。 Reco 在整个时间内表现出的波动比 GPP 更小。随机森林机器学习分析表明，Rg、LAI 和深层土壤湿度是生长季节和整个研究期间每日和每月累积 NEE、GPP 和 Reco 变化的主要驱动因素。结果表明，深根灌木通过进入深层且更稳定的水源，从而延长净碳捕获期，对生长季节（低降水量和高 VPD 时期）的碳通量做出了重大贡献。这项研究有助于了解该生态系统如何运作，并预测该草原将如何应对未来的气候。 此外，这项研究强调了由于该生态系统的巨大扩展和碳封存能力而需要对其进行保护。