RESEARCH
My research program focuses on the quantification of soil formation and erosion processes to understand landscape evolution in response to land-use and climate change. Specifically, my research goals are:
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Understanding how environmental factors control physical, chemical, and biotic processes leading to soil development and geomorphic stabilization;
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Evaluating the impact of land-use change on erosion and sedimentation rates, and
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Testing the suitability of the 239+240 Pu technique to assess soil erosion processes in a variety of environments.
I combine soil science with geomorphology, isotopic geochemistry, sedimentology, and geochronology to understand soil-landscape relationships across timescales. My projects are field-oriented and include laboratory analyses and modeling approaches to quantify chemical weathering rates, soil formation, soil erosion, and sedimentation rates. I use soil geochemistry, mineralogy, 239+240Pu, and 10Be, as erosion tracers and dating techniques such as 10Be, and dendrochronology.
Most recent and current research projects
The impact of deforestation on soil redistribution rates at Hubbard Brook Experimental Forest, NH, USA
Accelerated soil erosion poses a significant threat to soil health and ecosystem services, making it crucial to understand the impact of deforestation on erosional processes for sustainable soil management. In this research project, we focus on assessing soil erosion using 239+240Pu to evaluate the impact of whole-tree harvesting on erosion and deposition rates at the Hubbard Brook Experimental Forest (HBEF) in NH.
We quantified erosion and deposition rates using depth profile patterns of 239+240Pu from backslope Spodosols in a reference watershed (W3) and compared them to those in W5 backslopes and toeslopes Spodosols. Our preliminary results show alarming soil erosion rates in the backslopes and deposition rates in the toeslopes, indicating an increase in erosion and deposition rates due to whole-tree harvesting. Furthermore, the toeslope soil shows an increase in both soil surface and depth profiles, suggesting the transportation of Pu isotopes by surficial processes and lateral podzolization.
In the second phase of our project, we are investigating the mobility of Pu isotopes along slopes in watersheds 3 and 5. We sampled pedons from different positions on the slopes to better understand soil redistribution rates and gain insights into the behavior of Pu isotopes at HBEF. Our goal is to test their suitability as erosion tracers for hardwood forests affected by lateral podzolization. By determining the magnitude of soil erosion rates resulting from land use changes in forest ecosystems, our research can provide valuable information for forest management practices.
Collaborators: Eli Gundersen (Bates College), Dr. Diogo Spinola (University Alaska Fairbanks), Prof. Michael Ketterer (Northern Arizona University), Scott Bailey (Virginia Tech).
Soil development and slope stabilization on recent post-glacial landscapes of Southeast Alaska
Southeast Alaska is experiencing one of the fastest rates of glacier retreat and soil formation on newly exposed surfaces in the world. As part of a collaborative program, my research aims to understand the evolution of rapidly changing landscapes and predict future soil-landscape relationships as a result of climate change. Specifically, I am studying two soil chronosequences ranging in age from 40 to 270 years old to evaluate how plant succession controls chemical weathering and soil development on moraines.
In a previous project, I led research on the interplay between soil formation and erosion on moraine hillslopes using 239+240Pu isotopes as erosion tracers. Our results revealed that erosion rates decrease sharply as soils develop, and we estimated that hillslopes achieve stabilization after 400 years of soil formation (Portes et al., in prep). This rate of stabilization is remarkably faster compared to drier areas as found in my previous study (Portes et al., 2018).
The combination of high precipitation and fast plant colonization drives faster landscape stabilization and accelerates soil formation in the coastal rainforest of Alaska. Understanding the mechanisms behind this process can help us predict how rapidly changing landscapes will evolve in the future and how they will impact soil-landscape relationships.
Collaborators: Alana Margerum (Bates College), Dr. Diogo Spinola (University Alaska Fairbanks), Dr. David DAmore, Frances Biles , Randy Hesser (Juneau Forestry Science lab), Prof. Michael Ketterer (Northern Arizona University), Prof. Rebecca Lybrand (UC Davis) and Dr. Yakun Zhang (U Wisconsin-Madison).
Plant succession and soil development on moraines of the Mendenhall Glacier valley, Juneau, Alaska. Based on Alexander and Burt (1996) and Crocker and Dickson (1957). Illustrated by Gabriela Andrade
Spodosols development and slope stability in old-growth temperate rainforest of SE Alaska
This research focuses on how Spodosols development controls soil erosion rates and slope stability in the temperate rainforest of SE Alaska. We are using Pu isotopes as a tracer for quantifying and evaluate soil erosion/deposition rates.
The quantification of soil redistribution rates (erosion and deposition) has been crucial for understanding erosion processes and landscape evolution in natural areas worldwide. Soil redistribution assessments have mostly focused on the effect of topography, climate, and vegetation, but few explored how soil development influences soil redistribution rates and slope stability. In Southeast Alaska, deep, well-developed Spodosols formed in colluvial deposits are found on steep backslope positions (37- 60% slope gradient), where shallower and less-developed soils would be expected due to high erosion rates from flowing water. We hypothesized that deep well-developed Spodosols formed on different parent materials in mid backslopes in SE Alaska have negligible soil redistribution rates and surfaces are stable. To test this hypothesis, we selected Spodosols in forested hilly and mountainous areas in Juneau (SE Alaska) formed on distinct lithologies of the region (i.e. tonalite, slate, greywacke, phyllite). We quantified soil erosion and deposition rates using profile patterns of 239+240Pu radionuclides in slopes with similar slope form, aspect, inclination, and length. Pedons show a sequence of O, E, Bhs, and Bs horizons and share similar morphological properties: depth varying from 87 to 158 cm deep, thick O horizons (5-15 cm), gravelly textures, moderate medium subangular blocky structure, and abundant fine to coarse roots in surface horizons. Results from 239+240 Pu measurements reveal negligible soil redistribution rates for all pedons, ranging from erosion rates of 0.42 t/ha/y to deposition rates of 0.36 t/ha/y. These results suggest that, regardless of soil parent material, well-developed Spodosols are in a steady-state condition in terms of soil redistribution processes and indicates geomorphological stability of steep, forested slopes in SE Alaska. These findings shed light on the co-evolution of soils and slopes towards geomorphological stability in a temperate rainforest environment. Moreover, it also provides forest managers additional information, based on soil properties, to differentiate stable and unstable areas for better soil management.
Collaborators: Dr. David D´Amore (Juneau Forestry Science lab), Dr. Diogo Spinola and Prof. Thomas Trainor (University Alaska Fairbanks), Prof. Michael Ketterer (Northern Arizona University), Prof. Markus Egli (University of Zurich), Prof. Rebecca Lybrand and Jennifer Fedenko (Oregon State University).
In parallel to my research, I have been collaborating with other colleagues from the University of Alaska Fairbanks, Penn State University, and Oregon State University on a Critical Zone project that focuses on chemical weathering and mineral transformation in soils of SE Alaska.
Past research projects
Soil formation and landscape evolution using isotope geochemistry techniques
I held a postdoc grant by the Swiss Government Excellence Scholarship to conduct my research at Prof. Markus Egli`s geochronology laboratory at the University of Zürich. My postdoc research provides the first quantitative evaluation of how soil erosion rates have evolved over time in an alpine area (Portes, et al., 2018 - Earth & Planet. Sci. Lett.). We studied soil chronosequence on moraines in the Central Rocky Mountains using 10Be dating, Pu isotopes and δ13C. Our results revealed that erosion rates strongly decrease with time as soils develop. After 15 ka of soil formation, soil erosion rates reach a steady-state and slopes stabilize.
As a co-author of Dahms, et al., (2018), I provided some 10Be ages to a study of the Quaternary glacial succession and post-LGM recession in the Central Rockies. In parallel to my postdoc, I collaborated in studies that used 239+240Pu and 10Be for medium and long-term soil erosion assessment in agricultural and natural landscapes (Calitri et al., 2019) and (Raab et al., 2018). I also worked in studies that focus on Holocene surface processes and landscape evolution in the Swiss Alps (Boxleitner et al., 2017) and the chronology of volcanic ash and Quaternary soil genesis in Italy (Raab et al., 2017). In addition, I collaborated in research about the behavior of heavy metals in soils on abandoned tailing mines in the Swiss Alps (Egli et al., 2017). I also contributed to two publications on paleoenvironmental reconstruction using mineralogy and paleosols as a proxy (Spinola et al., 2017, Spinola et al., 2018).
Pedogenesis across a climatic gradient in tropical high mountains - Peruvian Andes
My Ph.D. research examined soil formation across a climate gradient in the Peruvian Andes. I investigated the interplay among weathering intensity, pedogenic processes, and landscape stability using soil physical, chemical and mineralogical analysis (Portes et al., 2016). The results demonstrated that soils are weakly developed due to landscape instability. Nevertheless, soils at the wetter side are slightly more developed than soils at the drier side, showing higher weathering intensity and mineral transformation.
