Data from: Asymmetric sensitivity of ecosystem carbon and water processes in response to precipitation change in a semiarid steppe

1.Semiarid ecosystems play an important role in regulating the dynamics of the global terrestrial CO2 sink. These dynamics are mainly driven by increasing inter-annual precipitation variability. However, how ecosystem carbon processes respond to changes in precipitation is not well understood, due to a lack of substantial experimental evidence that combines increased and decreased precipitation treatments.

2.This study, a 3-year field manipulation experiment with 5 precipitation levels conducted in a semiarid steppe, examined the impacts of increased and decreased precipitation on ecosystem CO2 (GEP: gross ecosystem photosynthesis; ER: ecosystem respiration; NEE: net ecosystem CO2 exchange), water exchange (ET: evapotranspiration), and resource use efficiency (CUE: carbon use efficiency; WUE: water use efficiency).

3.We found that decreased precipitation reduced ecosystem CO2, water exchange and resource use efficiency significantly, while increased precipitation did not cause significant influence on them. That is, they responded more sensitively to decreased precipitation. Soil water availability was the most important driver determining changes in GEP, ER and ET. Changes in NEE, CUE and WUE were predominately regulated by soil temperature. Photosynthesis at leaf and ecosystem levels showed significantly greater sensitivity to changed precipitation than respiration and ET, and therefore determined the trends of net carbon uptake and resource use efficiency.

4.This study highlighted an asymmetric response of ecosystem carbon and water processes to altered precipitation. This is potentially important for improving our understanding of how possible future changes in precipitation will affect the carbon cycle in this ecosystem. Taking this asymmetric response into consideration will inevitably reduce uncertainties in predicting the dynamics of the global carbon cycle.

1. 半干旱生态系统（Semiarid ecosystems）在调控全球陆地碳汇的动态过程中发挥着关键作用。这类生态系统的动态主要由年际降水变异性增强所驱动。然而，由于缺乏结合增减降水处理的扎实实验证据，学界对生态系统碳过程如何响应降水变化的机制仍缺乏充分认知。
2. 本研究在半干旱草原（semiarid steppe）设置5个降水梯度，开展了为期3年的野外控制实验，探究了增减降水对生态系统CO2交换（总生态系统光合作用（GEP: gross ecosystem photosynthesis）、生态系统呼吸（ER: ecosystem respiration）、净生态系统CO2交换（NEE: net ecosystem CO2 exchange））、水交换（蒸散量（ET: evapotranspiration））以及资源利用效率（碳利用效率（CUE: carbon use efficiency）、水分利用效率（WUE: water use efficiency））的影响。
3. 研究结果显示，降水减少会显著降低生态系统的CO2交换、水交换与资源利用效率，而降水增加则未对上述指标产生显著影响，即生态系统过程对降水减少的响应更为敏感。其中，土壤水分有效性是调控总生态系统光合作用、生态系统呼吸与蒸散量变化的核心驱动因子；净生态系统CO2交换、碳利用效率与水分利用效率的变化则主要受土壤温度调控。叶片与生态系统尺度的光合作用对降水变化的响应敏感性显著高于呼吸作用与蒸散量，因此决定了净碳吸收与资源利用效率的变化趋势。
4. 本研究强调了生态系统碳、水过程对降水改变的非对称响应，这一发现有助于增进我们对未来降水变化如何影响该生态系统碳循环的理解。将该非对称响应纳入预测模型，势必能降低全球碳循环动态预测的不确定性。