How to read elemental soil profiles to investigate the biogeochemical processes in Critical Zone?

1. How to read elemental soil profiles to investigate the biogeochemical processes in Critical Zone? Lixin Jin University of Texas at El Paso

2. Audience: • seniors in a soil class (but can be slightly modified for MS levels) • Skills and concepts that students must have mastered • Basic understanding the mobility of elements during soil genesis processes; • How elemental profiles are different depending on their solubility, affinity to organic matter, and nutrient cycling within a critical zone; • Calculation and graphs in Excel. • How the activity is situated in the course • After the introduction of major biogeochemical processes in the soils that mobilize and transport different elements; • Can be a stand-alone exercise or case studies

3. Goals • Content/concepts goals for this activity • 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.

4. Soil formation • Soil formation is defined as the physical, chemical and biological changes in this weathered material. • These include: • Additions of organic matter, water, energy, pr sediments • Losses of materials due to leaching, including gases, solutes or sediments • Translocation of clays and dissolved materials (Al, Fe) from one horizon to another • Transformations within horizons (dissolution of primary minerals to precipitate secondary clays, decay and stabilization of organic matter.

5. Elemental concentrations in soils:  values C C j , w i , p 1 t = - > 0, addition = 0, parent composition < 0, depletion i , j C C j , p i , w i, immobile element j, element of interest p, parent w, weathered soil parent Brimhall and Dietrich, 1987

6. Selection of parent materials and immobile element • Choices of parent materials • The deepest soil sample • Rock fragment from the soil core • Nearby outcrop • Choices of immobile elements • Ti • Zr • Y

7. Immobileprofile:  versus depth Brantley et al., 2007

8. Depletiononly profile: Cu Brantley et al., 2007

9. Addition only profile Brantley et al., 2007

10. Addition-depletion profile Brantley et al., 2007

11. Biogenic profile Brantley et al., 2007

12. Teaching Notes and Tips: • Chronosequences or climosequences can evaluate how soil age and climate etc impact mineral weathering kinetics and elemental depletion rates; • Additional profiles could show anthropogenic influences in the development of critical zones; • Discussion of paleosols and climate reconstruction; • Unit conversion needed? • Excel tricks.

13. Assessment • I will work with students for the first soil profile and then students will work in group on their own data • I will ask students to submit their Excel worksheets so that I check their equations and know where they might do wrong

14. References and Data sources • Basic reading of elemental profiles and mass transfer coefficients: • Brantley et al. (2007) Elements 3, 307-314. • Brimhall, G.H. and Dietrich, W.E. (1987) Geochimica et Cosmochimica Acta 51, 567-587. • Chronosequence • White et al. (2008) Geochimica et Cosmochimica Acta 72, 36-68. • Climosequence • Rasmussen et al. (2011) Earth and Planetary Science Letters 301, 521-530. • Anthropogenic • Herndon et al. (2011) Environmental Science & Technology 45, 241-247.