RUI: Landscape-scale Implications of Mineral Reaction Rates and Mechanisms in Tropical Soils: Insights from Soil Chronosequences and Synthesis Experiments

RUI：热带土壤中矿物质反应速率和机制的景观规模影响：土壤年代序列和合成实验的见解

• 批准号: 1226494
• 负责人: Peter Ryan
• 金额: $ 13.76万
• 依托单位: Middlebury College
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-09-15 至 2016-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1226494&HistoricalAwards=false
• 关键词: RUI; Landscape; scale; Implications; Mineral

Tropical soils are not uniformly kaolinite-rich and nutrient-depleted, and this project aims to examine rates and mechanisms of mineral reactions (and associated chemical changes) that occur with maturation of tropical soils. Ultimately, pronounced leaching of base cations and silica drives humid tropical soil solutions towards a nutrient-poor, low pH, Al-rich composition that favors crystallization of kaolinite; however, data of the Food and Agricultural Organization of the United Nations indicate that 65 % of tropical soils do not fit this end-member Oxisol state. Pedogenic Fe-smectite (S) forms in many tropical soils as a metastable precursor to kaolinite (K), and a recent study of a humid tropical soil sequence by the PI and collaborator in Costa Rica indicates that the transformation of S to K occurs over time spans on the order of about 100,000 years via intermediate kaolinite-smectite (K-S) phases. The reaction occurs on a layer-by-layer basis, and the preservation of S layers within K-S crystals enables Al to be sequestered in expandable S interlayers, thus limiting plant-toxic Al in solution. Furthermore, the cation exchange capacity of K-S is about 5 times greater than K, further underscoring the need to better constrain these reactions. Accordingly, this project seeks to test the hypothesis that humid tropical soils, regardless of mean annual precipitation (MAP), undergo step-wise transformation from S to K-S before ultimately producing K. This hypothesis will be tested by examining the roles of time and climate (as MAP) on the rates and mechanisms of the S to K-S to K reaction in soil chronosequences from three separate climate zones along the Pacific coast of Costa Rica. Preliminary data indicate that rate is positively correlated to MAP. Mineral reaction mechanisms will also be examined by carrying out synthesis reactions where solution composition and time are controlled. Middlebury College is a residential liberal arts college with a long tradition of involving students in hands-on, experiential research. This is particularly true in the Geology Department, which over many decades has cultivated a critical-thinking, problem-oriented curriculum that includes a required year-long senior thesis. Positive demographic changes in the Geology Dept provide a much greater opportunity for this and all projects in the department to extend to typically underrepresented students. Graduating geology majors from the classes of 2008 to 2013 are comprised of 15 % (8/53) American students of color, whereas the geology majors from 2000 to 2007 consisted of less than 2 % American students of color (1/65). Female students comprise 46 % of geology majors over the past 15 years, and in the classes of 2012 and 2013 combined, 13/19 students are female. 15% of Middlebury geology theses result in co-authored publications with faculty, and 50 % of Middlebury geology majors attend graduate school, so these demographic shifts present the opportunity to add diversity to the broader geologic community. Thus, this project will target a diverse group of students in a strong program with a proven record of success in undergraduate research. The project also has an international undergraduate scope, which includes a field excursion to the Osa Peninsula to do field work with University of Costa Rica (UCR) geologist Guillermo Alvarado and undergraduate students from UCR and Middlebury College, as well as analytical and experimental research at the Instituto Andaluz de Ciencias de la Tierra in Granada Spain, where the PI and students will collaborate on detailed mineralogical and geochemical analysis of tropical soils.The NSF Office of International Science and Engineering contributed to the international component of this project.

热带土壤的高岭石含量并不均匀，养分缺乏，本项目旨在研究随着热带土壤成熟而发生的矿物反应(以及相关的化学变化)的速率和机制。最终，盐基阳离子和二氧化硅的显著淋溶促使湿润的热带土壤溶液转向营养贫乏、低pH、富铝的成分，有利于高岭石的结晶；然而，联合国粮食及农业组织的数据表明，65%的热带土壤不适合这种最终成员棕壤状态。成土型铁蒙皂石(S)存在于许多热带土壤中，是高岭石(K)的亚稳前体，最近哥斯达黎加的Pi和合作者对热带潮湿土壤序列的研究表明，S到K的转变是通过中间的高岭石-蒙皂石(K-S)相发生的，时间跨度约为10万年。该反应是逐层进行的，而K-S晶体中S层的保存使铝能够被隔离在可膨胀的S层间，从而限制了溶液中的植物毒性铝。此外，K-S的阳离子交换容量大约是K的5倍，进一步强调了更好地限制这些反应的必要性。因此，这个项目试图检验这样的假设，即无论年平均降雨量(MAP)如何，潮湿的热带土壤在最终产生K之前会经历从S到K-S的逐步转化。该假说将通过检验时间和气候(如MAP)对哥斯达黎加太平洋沿岸三个不同气候带土壤时间序列中S和K-S对K反应的速率和机制的作用来检验。初步数据显示，心率与MAP呈正相关。矿物反应机理也将通过进行合成反应来研究，在合成反应中，溶液组成和时间受到控制。米德尔伯里学院是一所寄宿文理学院，有着让学生参与实践经验研究的悠久传统。地质系的情况尤其如此，几十年来，该系培养了一门批判性思维、以问题为导向的课程，其中包括一份为期一年的必修高中论文。地质系积极的人口结构变化为该系的这一项目和所有项目提供了一个更大的机会，以扩大到通常代表人数较少的学生。2008至2013届地质专业毕业生中，美国有色人种学生占15%(8/53)，而2000至2007届地质专业毕业生中，美国有色人种学生不到2%(1/65)。在过去的15年里，地质学专业的女生占到了46%，2012和2013届加起来，有13/19的学生是女生。15%的米德尔伯里地质学论文是与教师合著的论文，50%的米德尔伯里地质学专业的学生在读研究生，因此这些人口结构的变化为更广泛的地质界提供了增加多样性的机会。因此，这个项目将以一个强大的项目中的不同群体的学生为目标，这些学生在本科研究中取得了成功的记录。该项目还有一个国际本科项目，其中包括与哥斯达黎加大学(UCR)地质学家吉列尔莫·阿尔瓦拉多和UCR和米德尔伯里学院的本科生一起到奥萨半岛进行实地考察，以及在西班牙格拉纳达的Andaluz de Ciencias de la Tiera研究所进行分析和实验研究，在那里，PI和学生将合作对热带土壤进行详细的矿物学和地球化学分析。