How to read elemental soil profiles to investigate the biogeochemical processes in Critical Zone?
Summary
In this exercise, students use elemental chemistry data in a soil profile to explore major biogeochemical processes that dominate in critical zone. Data will be provided, and students calculate and graph the mass transfer coefficients as a function of depth using Excel. Based on these plots, student make generalized statements about how different elements behave in this soil profile and what processes dominate, e.g., depletion by rock-water interaction, addition by dust inputs or elemental loading by human activities etc.
Context
Audience
Undergraduate soil course (Junior/Senior levels)
Skills and concepts that students must have mastered
Students should have basic understanding the mobility of elements during soil genesis processes, and how elemental profiles are different depending on their solubility, affinity to organic matter, and nutrient cycling within a critical zone. Students should be familiar with calculation and graphs in Excel.
How the activity is situated in the course
This exercise should be used after the introduction of major biogeochemical processes in the soils that mobilize and transport different elements and can be a stand-alone exercise.
Goals
Content/concepts goals for this activity
We want students to understand elements behave differently in the soil genesis processes: some are dominated by leaching only through water-rock interaction, some nutrients are reloaded at the surface as leaves fall but lost to biological uptake at depth, some elements have much higher concentrations in the entire soil profile than accounted for by bedrock weathering, indicative of addition at surface, e.g., by dust.
Higher order thinking skills goals for this activity
Students use real data to read elemental profiles and identify the biogeochemical processes within critical zone.
Other skills goals for this activity
Students learn to use Excel, work in groups, and observe trends in the graphs.
Description and Teaching Materials
Soil chemistry data will be provided. Students will pick one sample (the deepest one) and use it as parent materials from what soils are developed. Students will then select an element and use it as an immobile element. Then mass transfer coefficients are calculated for all major and elements of the soils and then plotted a function of depth. Based on shape of these curves, general statements should be made about what processes are dominated in this particular critical zone.
Teaching Notes and Tips
Elemental data for several soil profiles could be provided and soil profiles can be selected along chronosequences or climosequences. In this case, students can evaluate how soil age and climate etc impact mineral weathering rates and elemental depletion rates.
This exercise can also be used in a watershed where human activities have significantly changed the elemental inventory. Students can appreciate the anthropogenic influences in the development of critical zone.
SampleDataset_SolutionSet (Excel 2007 (.xlsx) 66kB Jun6 13)
Assessment
I will work with students for the first soil profile and then students will work in group on their own data. I ask students to turn in their excel worksheet so that I can check for their calculation.
References and Resources
Basic reading of elemental profiles and mass transfer coefficients can be found in the following references:
Brantley, S.B., Godhaber, M.B., and Ragnarsdottir, K.V. (2007) Crossing disciplines and scales to understand the Critical Zone. Elements 3, 307-314.
Brimhall, G.H. and Dietrich, W.E. (1987) Constitutive mass balance relations between chemical composition, volume, density, porosity, and strain in metosomatic hydrochemical systems: results on weathering and pedogenesis. Geochimica et Cosmochimica Acta 51, 567-587.
Data source for Chronosequence: White et al. (2008) Geochimica et Cosmochimica Acta 72, 36-68.
Data source for climosequence: Rasmussen et al. (2011) Earth and Planetary Science Letters 301, 521-530.
Data source for Anthropogenic influcences: Herndon et al. (2011) Environ. Sci. & Tech. 45, 241-247.
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