Displacement of grasses by woody plants (woody encroachment) is occurring in grasslands worldwide. Previous studies indicate that encroachment can alter subsurface carbon dioxide (CO2) concentrations and mineral weathering, though these impacts are still poorly understood. To address this knowledge gap, we sampled groundwater and stream water every three weeks during the 2022 water year from two watersheds at Konza Prairie Biological Station, a native tallgrass prairie underlain by limestone and mudrock units in Kansas, USA. Amounts of woody encroachment differ between the watersheds primarily because of differences in fire frequency. One watershed is burned annually and contains 6 % and 45 % woody plant coverage in its upland and riparian areas, respectively, whereas the other is burned every four years and contains 28 % and 74 % woody plant coverage, respectively. We expected to find higher CO2 levels in the more encroached watershed, assuming the deep roots of woody plants increase inputs of CO2 to bedrock. However, we found the opposite. Our results indicate that groundwater from a single limestone aquifer contained an average of 1.4 mM CO2 in the less encroached watershed and 1.0 mM CO2 in the more encroached watershed. Similarly, stream water CO2 concentrations at the outlet of the less encroached watershed (0.25 mM) were more than twice that of the more encroached watershed (0.12 mM) on average. Despite these differences in CO2 concentration, amounts of mineral weathering per liter of groundwater differed little between watersheds. We hypothesize that encroachment is causing differences in CO2 concentrations between watersheds by decreasing the proportion of mineral weathering that occurs under conditions that are open with respect to CO2 exchange. During open-system weathering, dissolved CO2 consumed by weathering reactions can be replaced from an adjacent gas phase, allowing CO2 concentrations to remain elevated as weathering progresses. In contrast, during closed-system weathering, CO2 is not replaced and decreases in concentration as weathering progresses. If weathering primarily occurs under open-system conditions within the study area soils, which are unsaturated, and closed-system conditions within the underlying bedrock, where pores are more commonly saturated, then woody encroachment has the potential to decrease the proportion of open-system weathering by increasing soil permeability and thus decreasing soil water residence times. This hypothesis is consistent with our findings and implies that a shortening of soil water residence time with woody encroachment lowers the proportion of CO2 delivered from the soil to the subsurface and creates a more aggressive weathering engine at depth and along deeper flow paths. Encroachment may also be altering soil CO2 production and/or venting, though these possibilities require further investigation.

中文翻译：

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草原木本侵蚀改变了碳酸盐系统中的地下矿物风化和地下水成分


木本植物取代草（木本侵占）正在世界各地的草原上发生。以前的研究表明，侵占可以改变地下二氧化碳 （CO2） 浓度和矿物风化，尽管这些影响仍然知之甚少。为了解决这一知识差距，我们在 2022 丰水年期间每三周从 Konza Prairie 生物站的两个流域采样地下水和溪水，Konza Prairie 生物站是美国堪萨斯州的一个原生高草草原，下面有石灰岩和泥岩单元。流域之间的木本侵占量不同，主要是因为火灾频率不同。一个流域每年燃烧一次，高地和河岸地区的木本植物覆盖率分别为 6% 和 45%，而另一个流域每四年燃烧一次，木本植物覆盖率分别为 28% 和 74%。我们预计在侵蚀程度更高的流域中会发现更高的 CO2 水平，假设木本植物的深根增加了对基岩的 CO2 输入。然而，我们发现情况恰恰相反。我们的结果表明，来自单个石灰岩含水层的地下水在侵占较少的流域中平均含有 1.4 mM CO2，在侵占较多的流域中平均含有 1.0 mM CO2。同样，侵占较少的流域出口处的溪水 CO2 浓度 （0.25 mM） 平均是侵占较多的流域 （0.12 mM） 的两倍多。尽管 CO2 浓度存在这些差异，但每升地下水的矿物风化量在不同流域之间几乎没有差异。我们假设侵占通过减少在 CO2 交换开放条件下发生的矿物风化比例，导致流域之间 CO2 浓度的差异。 在开放系统老化过程中，老化反应消耗的溶解 CO2 可以从相邻的气相中替代，从而使 CO2 浓度随着老化的进行而保持较高水平。相比之下，在闭路系统风化过程中，CO2 不会被取代，并且随着风化的进行而浓度降低。如果风化主要发生在研究区域土壤内的开放系统条件下（不饱和）和下层基岩内的封闭系统条件下（孔隙更普遍饱和），那么木质侵占有可能通过增加土壤渗透性来减少开放系统风化的比例，从而减少土壤水分的停留时间。这一假设与我们的发现一致，并意味着随着木本侵蚀而缩短土壤水分停留时间会降低从土壤输送到地下的 CO2 的比例，并在深处和沿更深的流动路径产生更具侵略性的风化引擎。侵占也可能改变土壤 CO2 的产生和/或通风，尽管这些可能性需要进一步调查。