Addressing a significant knowledge gap in fluvial system atmospheric CO2 efflux: the contribution from karst landscapes

喀斯特地貌覆盖了地球表面的很大一部分：约12%的喀斯特地貌代表着陆地上重要的碳库，也被认为在气候调节中发挥着重要作用，在化学风化过程中消耗大气中的二氧化碳。然而，如果我们不知道排出喀斯特地貌的河流将风化的碳以二氧化碳的形式重新转移到大气中的效率，我们就不能确定这个汇有多有效。此外，尽管有大量关于风化岩溶系统带走了多少碳的研究，但研究人员还没有通过直接测量来量化从河流返回大气的碳。我们建议测量喀斯特系统河流中二氧化碳的脱气情况，并使用化学示踪剂来区分河流中不同的碳来源，量化来自不同来源的碳的数量，以及这些贡献如何随着不同土地利用的变化，或随着季节和河流流量的变化而变化。我们将通过在中国的一个喀斯特集水区进行测量来做到这一点。这块土地的位置是这项研究的关键，因为人们对风化造成的碳损失以及集水区不同的土地利用(例如，裸露的岩石和水稻土)如何影响风化过程已经有了很多了解。在研究的第一年，我们将进行实地工作，测量泉水和地表水的二氧化碳外流，确保我们捕捉到由于河流流量和季节温度差异而导致的外流自然变化。在第二年，基于我们对二氧化碳外流的集水区控制的详细了解，我们将使用碳(同位素)来源的专门示踪剂来量化脱气二氧化碳中有多少碳来自不同的来源。有了这些知识，我们将产生一个基于过程的定量模型来估计二氧化碳外流，该模型依赖于流域和河流特征的描述符。最后，我们将在其他流域测试这个模型，以评估它对更广泛的岩溶排水系统二氧化碳外流的描述情况，并从这次测试中改进模型/确定如何改进它。该模型很重要，因为它将支持岩溶流域系统二氧化碳外流的放大，这些信息对于那些模拟全球C循环并试图了解大气中二氧化碳浓度的变化是如何受到从地球表面到大气的通量的影响的人是有价值的。

Karst landscapes cover a large proportion of the Earth's land surface: ~ 12% They represent an important C store on land, and also are considered to play an important role in climate regulation by consuming atmospheric CO2 during chemical weathering. However, we cannot be certain how effective this sink is if we do not know how efficiently the rivers draining karst landscapes remobilise weathered C to the atmosphere as CO2. Further, although there is a large body of research on how much C is carried away from weathered karst systems, researchers have not yet quantified by direct measurement the C return to the atmosphere from rivers. We propose to measure this CO2 degassing from rivers in karst systems and using chemical tracers that can distinguish between different source of carbon in the rivers, quantify how much C comes from the different sources and how these contributions change with different land use change, or as season and river flow change. We will do this by undertaking measurements in a karst catchment in China. This field location is key to the research as there is much known about C loss by weathering and how different land use in the catchment (e.g. bare rock vs. paddy soils) affects the weathering process. In the first year of the research we will undertake fieldwork to measure CO2 efflux from springs and surface waters, ensuring we capture natural variation in efflux due to differences in river flow and seasonal temperature. In year 2 based on our detailed understanding of the catchment controls on CO2 efflux, we will use specialised tracers of the source of the C (isotopes) to quantify how much C in the CO2 degassed comes from the different sources. With this knowledge we will produce a quantitative process-based model to estimate CO2 efflux, that relies on descriptors of catchment and river characteristics. Finally we will test this model in other catchments to assess how well it describes more widely CO2 efflux from karst drainage systems, and from this testing refine the model / identify how it can be improved.The model is important as it will support up-scaling of CO2 efflux from karst drainage systems and this information is valuable to those modelling the global C cycle and trying to understand how changes in atmospheric CO2 concentration are affected by fluxes to the atmosphere from the Earth's surface.