Effects of Soil Moisture on Microaggregate Morphology

1

• Determining the Morphology of Soil
  Microaggregates using USAXS
• John F. McCarthy1, Edmund Perfect1, Julie D.
  Jastrow2, and Jan Ilavsky3
• Department of Geological Sciences, University of
  Tennessee, Knoxville, TN (jmccart1_at_utk.edu)
• Environmental Research Division, Argonne National
  Laboratory (3) UNICAT, Advanced Photon Source

Results and Discussion (preliminary analysis of
data collected in January, 2003)

• Goals
• Determine the structural and chemical bases of
  soil microaggregate formation and stability
• Approach
• Multiple state-of-science techniques
• USAXS and SANS, N2 adsorption, SEM, Scanning
  Transmission X-ray Microscopy
• USAXS Objectives
• Examine the scale-independent structure of soil
  microaggregates
• Effects of soil moisture content on surface
  morphology and porosity

• Background
• Global Climate Change and Carbon Sequestration
• Enlarge pools of long-lived organic matter in
  soil to reduce atmospheric CO2
• Evaluate agricultural management strategies and
  land-use options to enhance levels of soil
  organic matter
• Soil Microaggregates
• they protect C against decomposition, resulting
  in much longer residence times for C
• why organic matter (OM) in soil microaggregates
  have such long residence times

• Effects of Soil Moisture on Microaggregate
  Morphology
• USAXS is the best method because N2 adsorption
  requires dry samples
• Wet microaggregates (compared to dry
  microaggregates)
• similar multi-scale structures (Eq. 1)
• Low-Q power law slope is consistently lower for
  all microaggregate samples
• Higher surface fractal dimension
• Higher degree of scale-invariant roughness
• Caused by swelling of soil organic matter?
• Effects on accessibility of pores and extent of
  physical protection of organic matter?

• Microaggregate Structure
• Complex, multiple size-scale structures
• Unified Guinier/Power-Law Approach
• (Eq. 1 Beaucage, G. and D.W. Shaefer .1994. J.
  Non-Crystalline Solids 172-174797-805)
• High-Q Regime
• primary particles are Euclidean solids
• Low-Q Regime
• No evidence of a terminal size
• Power law exponent (P) 3.5
• consistent with a scale-invariant surface fractal
  structure

• Field Sites
• Contrast microaggregate structure under
  experimental manipulations that alter
  accumulation of soil organic matter
• Chronosequence of tallgrass prairie restoration
• Soil disturbance (till vs no-till)
• Contrasting agronomic management systems
  (conventional vs organic)
• Contrasting forage management systems
• A range of soil types with contrasting properties
  that may alter soil microaggregate stability

Data from a prairie restoration
chronosequence Contrast virgin prairie with a
cultivated agricultural soil
Preliminary evaluation of microaggregates from
the virgin prairie