Deepening roots can enhance carbonate weathering by amplifying CO2-rich recharge
Deepening roots can enhance carbonate weathering by amplifying CO2-rich recharge
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DOI：
10.5194/bg-18-55-2021
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发表时间：
2021-01
期刊：
影响因子：
4.9
通讯作者：
H. Wen;P. Sullivan;G. Macpherson;S. Billings;Li Li-Li
中科院分区：
文献类型：
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作者：
H. Wen;P. Sullivan;G. Macpherson;S. Billings;Li Li-Li
抽象。碳酸盐风化作用在世纪尺度上对大气CO2和碳循环的调节是必不可少的.植物根系通过呼吸作用提高土壤CO2含量，加速风化.然而，人们仍然不太了解如何以及有多少生根特性（例如，深度和密度分布）改变流动路径和风化。我们解决这一知识差距，使用现场数据和反应性运输数值实验在康扎草原生物站（康扎），堪萨斯（美国），一个网站，木本侵入草原被推测加深根。结果表明，加深根可以通过两种方式增强风化。首先，加深的根可以控制碳酸盐溶解的热力学限制，通过调节有多少CO2垂直向下输送到更深的富碳酸盐带。Konza的基本数据和模型表明，Ca和溶解性无机碳（DIC）的浓度受季节性土壤呼吸驱动的土壤pCO 2的调节。这种关系可以封装在本工作中描述的Ca和DIC对温度和土壤CO2的依赖关系推导出的方程。这种关系可以解释泉水钙和DIC浓度从多个碳酸盐为主的集水区。其次，数值实验表明，根控制风化速率调节补给（或垂直水通量）到更深的碳酸盐岩带和出口反应产物溶解平衡。数值试验探索了在林地中将40%的渗透水分配到深度的潜在影响，而在草地中仅为5%。土壤CO2数据表明，土壤CO2分布在深度上相对相似，如果两种土地覆盖之间的流量分配保持相同，则林地和草地的风化速率仅相差1%至12%。相比之下，当渗透率从3.7 × 10 −2增加到3.7 m/a时，加深的根系可以使风化增加17%到200%。然而，在这些情况下的风化率比根本没有根的情况高出一个数量级以上，强调了根的基本作用。数值试验还表明，300年后，林地中的风化锋以0.37 m/a的入渗速率比草地深2倍以上。这些风化锋的差异最终是由充CO2水与深层地下碳酸盐接触时间的差异造成的。在模拟试验的限制下，这些数据和数值试验提示了以下假设：（1）加深林地中的根系可以通过促进深层地下的补给和CO2-碳酸盐接触来增强碳酸盐风化;（2）在调节风化速率方面，根系特征的水文影响可能比土壤CO2分布的影响更大。我们呼吁根，地下结构和土壤CO2水平，以及它们与水和水化学的联系的同位表征。这些测量对于阐明土地覆盖变化、化学风化、全球碳循环和气候的反馈机制至关重要。
Abstract. Carbonate weathering is essential in regulating atmospheric CO 2 and carbon cycle at the century timescale. Plant roots accelerate weathering by elevating soil CO 2 via respiration. It however remains poorly understood how and how much rooting characteristics (e.g., depth and density distribution) modify flow paths and weathering. We address this knowledge gap using field data from and reactive transport numerical experiments at the Konza Prairie Biological Station (Konza), Kansas (USA), a site where woody encroachment into grasslands is surmised to deepen roots. Results indicate that deepening roots can enhance weathering in two ways. First, deepening roots can control thermodynamic limits of carbonate dissolution by regulating how much CO 2 transports vertical downward to the deeper carbonate-rich zone. The base-case data and model from Konza reveal that concentrations of Ca and dissolved inorganic carbon (DIC) are regulated by soil p CO 2 driven by the seasonal soil respiration. This relationship can be encapsulated in equations derived in this work describing the dependence of Ca and DIC on temperature and soil CO 2 . The relationship can explain spring water Ca and DIC concentrations from multiple carbonate-dominated catchments. Second, numerical experiments show that roots control weathering rates by regulating recharge (or vertical water fluxes) into the deeper carbonate zone and export reaction products at dissolution equilibrium. The numerical experiments explored the potential effects of partitioning 40 % of infiltrated water to depth in woodlands compared to 5 % in grasslands. Soil CO 2 data suggest relatively similar soil CO 2 distribution over depth, which in woodlands and grasslands leads only to 1 % to ∼ 12 % difference in weathering rates if flow partitioning was kept the same between the two land covers. In contrast, deepening roots can enhance weathering by ∼ 17 % to 200 % as infiltration rates increased from 3.7 × 10 −2 to 3.7 m/a. Weathering rates in these cases however are more than an order of magnitude higher than a case without roots at all, underscoring the essential role of roots in general. Numerical experiments also indicate that weathering fronts in woodlands propagated > 2 times deeper compared to grasslands after 300 years at an infiltration rate of 0.37 m/a. These differences in weathering fronts are ultimately caused by the differences in the contact times of CO 2 -charged water with carbonate in the deep subsurface. Within the limitation of modeling exercises, these data and numerical experiments prompt the hypothesis that (1) deepening roots in woodlands can enhance carbonate weathering by promoting recharge and CO 2 –carbonate contact in the deep subsurface and (2) the hydrological impacts of rooting characteristics can be more influential than those of soil CO 2 distribution in modulating weathering rates. We call for colocated characterizations of roots, subsurface structure, and soil CO 2 levels, as well as their linkage to water and water chemistry. These measurements will be essential to illuminate feedback mechanisms of land cover changes, chemical weathering, global carbon cycle, and climate.