How Can Soil Help Us Answer Geologic Questions?
Throughout the course of my summer internship, I spent countless days with Tropical Responses to Altered Climate Experiment (TRACE) field technician Laura Rubio Lebrón collecting soil cores (Figure 2). The soil samples originated from the Luquillo Long-Term Ecological Research Program based at El Verde Field Station in Río Grande and the TRACE experimental site at Sabana Field Research Station in Luquillo, Puerto Rico. The soil varied in texture, color, water content, nutrients, and root density but for me one question remained: where did this soil come from?
The TRACE project is located in the Northeast Igneous Province (Figure 1), which is naturally separated from the central region of Puerto Rico by the Cerro Mula Fault Zone. The Cerro Mula Fault zone is a region of left lateral displacement likely resulting from continuous hydrothermal intrusions (Joyce, 1992). TRACE sits atop the Río Piedras Siltstone series, which formed during the Lower Cretaceous period (Briggs, 1973). Yet, the igneous nature of Puerto Rico is a result of its inclusion in the presently extinct Lesser Antilles volcanic arc. The arc was previously active because of tectonic activity between the Caribbean and North American Plates from the late Jurassic to the Eocene era (Bosch et al., 2022).
The Luquillo Experimental Forest (LEF), also known as El Yunque National Forest, highlands are predominantly sedimentary silt and mudstone. The location of these sedimentary deposits is a result of submarine debris flow of volcanic ash from distant volcanoes in the Antilles arc (Joyce, 1992). The soil of the LEF results from the weathering of high content feldspar rocks. Feldspars are a mineral group containing calcium, sodium, or potassium, which comprise the basis of Earth’s crust (Zhuo, 2011). Feldspars are aluminosilicates which contain aluminum and silicon bonded together by oxygen. Gap sites within the crystalline structure are typically filled with sodium potassium, and calcium (Barton, 2002). The soil in the LEF however, is high in iron and aluminum oxides, but low in silica content (Zhuo, 2011).
For tropical soils, clays are the principal secondary minerals. Clay is classified as a phyllosilicate, which has a sheet-like layering. Between the atoms of a single sheet the bonds are strong, but the bonding between adjacent sheet layers is weak. As a result, clays have a high absorption ability for ions and other organic substances on their surface (Figure 3). The presence of these ions enables plant root matter to better bind to the clay particles (Barton, 2002).
Despite the 120 – 240 inches of rain each year, the erosion rates in the Luquillo Experimental Forest remain minimal. Therefore, understanding soil structure at TRACE is vital to ensure that root systems continue to anchor the soil. This is especially important because erosion threatens the forest's vulnerable freshwater ecosystems by increasing sedimentation in the stream channels. This in turn disturbs developed hydrologic patterns and increases the input from terrestrial ecosystems (Carpenter et al., 1992).
To better understand the geomorphologic properties of soil structure at TRACE, I assisted graduate students Kerry Mundy and Amrita Kadchha of Rutgers University in the use of tension infiltrometers (Figure 4). The tension infiltrometer utilizes the principles of Darcy’s Law to quantify the hydraulic properties of the soil. Darcy’s equation is used to determine the flow of groundwater infiltration in a porous medium. Hydraulic conductivity quantifies how easily water can travel through a porous medium, typically soil or rock. Hydrologist Robin A. Wooding then proceeded to use that foundation built by Darcy to further investigate and model hydraulic conductivity in unconfined soil. Unconfined soil is the texture most commonly found on the forest surface, characterized by minimal depth and subsequent lack of pressure stressors from overlying soil layers (Wooding, 1968).
In this case the porous medium is a base plate located at the bottom of the infiltrometer, collecting the rate of unconfined infiltration, from a 3D source. The volume of water entering the soil is then calculated using the drop in water level in the water reservoir over a set period, which is being recorded by a datalogger. The water flux can be taken at different tensions. Setting the water in the water reservoir to a negative value is ideal, so a pressure transducer can take the measurements of the water level and therefore the pressure (Wang et al., 1998). Using a three-dimensional infiltrometer provides measurements more quickly as the steady state infiltration rate is reached faster.
Steady-state infiltration is the maximum rate at which water enters the soil, which is dependent on how much water was in the soil before the measurement, and the regional hydrology. It is important to measure infiltration because it is dependent upon conditions such as the soil texture, vegetation, and weather patterns (Soil Measurement Systems LLC; Mark, n.d.). The infiltration measurements will also enable us to get a better understanding of fluctuations in the water availability for plants. This information will help us answer questions about plant physiology, reproduction, and overall life cycles within the Luquillo Experimental Forest.
References
Barton, C.D. and Karathanasis, A.D. (2002). Clay Minerals. In: Lal, R., Ed., Encyclopedia of Soil Science, Marcel Dekker, New York, USA, 187-192.
Bosch, D., Zami, F., Philippon, M., Lebrun, J.-F., Münch, P., Cornée, J.-J., et al. (2022). Evolution of the northern part of the Lesser Antilles arc—Geochemical constraints from St. Barthélemy Island lavas. Geochemistry, Geophysics, Geosystems, 23, e2022GC010482. https://doi.org/10.1029/2022GC010482
Briggs, R.P., 1973, The Lower Cretaceous Figuera Lava and Fajardo Formation in the stratigraphy of northeastern Puerto Rico, U.S. Geol. Survey Bull., 137-G, G1-10 https://pubs.usgs.gov/bul/1372g/report.pdf
Carpenter, S. R., Fisher, S. G., Grimm, N. B., & Kitchell, J. F. (1992). Global Change and Freshwater Ecosystems. Annual Review of Ecology and Systematics, 23, 119–139. http://www.jstor.org/stable/2097284
Joyce, J. (1992, April 24). Geology of the East Coast of Puerto Rico. Chautauqua Workshop, Puerto Rico. http://www-udc.ig.utexas.edu/external/plates/biblio/carib/guides/geol_e_coast_PR_chataugua.pdf
Kargas G, Koka D, Londra PA. Evaluation of Soil Hydraulic Parameters Calculation Methods Using a Tension Infiltrometer. Soil Systems. 2022; 6(3):63. https://doi.org/10.3390/soilsystems6030063
Krushensky R., Schellekens J., Geology of Puerto Rico. 1992.
Shirozu, H. Clay Mineralogy: The Basis of Clay Science, New ed.; Asakura Publishing: Tokyo, Japan, 2012; pp. 1–185.
Soil Measurement Systems LLC., & Mark, A. (n.d.). Tension Infiltrometer (8 cm) Users Manual.
Wang, D., Yates, S.R. and Ernst, F.F. (1998), Determining Soil Hydraulic Properties using Tension Infiltrometers, Time Domain Reflectometry, and Tensiometers. Soil Science Society of America Journal, 62: 318-325. https://doi.org/10.2136/sssaj1998.03615995006200020004x
Wooding, R.A. (1968). Steady Infiltration from a Shallow Circular Pond. Water Resources Research, 4, 1259-1273.
Zhuo, W. (2011). LCZO--Soil Mineralogy--Mineral Content of Soils 0-20 cm XRD--Northeastern Puerto Rico. Critical Zone Collaborative Network.
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