Soil sampling for C:

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Soil sampling for C Influence of landscape,
temporal variability, and physical properties
Richard T. Conant Natural Resource Ecology
Laboratory Colorado State University
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Talk Outline

• Carbon sequestration basics
• What are the major challenges to quantifying soil
  C sequestration?
• How can we overcome those challenges?

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Carbon sequestration basics
Atmospheric CO2
Photosynthesis
Soil respiration
Root respiration
Plant respiration
Soil C
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Carbon sequestration basics
Soil C stocks
Initial cultivation
Management change
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What are the major challenges? Measurement vs.
sampling
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What are the major challenges? Measurement vs.
sampling
Soil sampling
Soil sampling
Repeatable with minimal error for forest and
cultivated fields
Analytical precision 5
How much do different steps contribute to ?2 ?
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What are the major challenges? Measurement vs.
sampling

• Distribution of soil organic matter in a
  cultivated field
• Spatially variable in visually uniform field
• Large soil C range (0.85-1.93)
• Significant spatial structure
• Substantial fine-scale variability (2-5m)

Data from Robertson et al. (1997)
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What are the major challenges? Scale
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What are the major challenges? Scale
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What are the major challenges? Changes over time
Experimental Treatments
Fenceline comparisons
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What are the major challenges? Changes over time
Average Cultivated soil C 35 Mg C ha-1
Accumulation rate 0.5 Mg C ha-1 yr-1
Soil C pool
2 years 2.8
25 years 36
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What are the major challenges? Depth
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What are the major challenges? Depth
Average Cultivated soil C (top 20cm) 14.5 Mg C
ha-1
Accumulation rate (top 20cm) 0.27 Mg C ha-1 yr-1
20cm
2 years change 3.7
25 years change 46.6
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What are the major challenges? Bulk density
Management
Rocks
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What are the major challenges? Cost vs. precision
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What are the major challenges? Summary

• Must sample to maximize sensitivity to changes
  over time
• Must deal with plot- or field-scale variability
• Must measure soil C per unit area
• Have to account for changes in bulk density
• Small signal to noise ratio
• Background amounts are large relative to rate of
  change.
• Cost vs. accuracy tradeoff
• Many controlling factors (e.g. climate,
  vegetation, topography, soil properties,
  drainage, management and land use history).

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How can we overcome challenges? Scale
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How can we overcome challenges? statistics
Diff? f(xs1, ?2s1, xs2, ?2s2)
Has unique history led to differences?
Soil 1
Soil 2
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How can we overcome challenges? statistics
Implication Large covt1 t2 small ?2
(t1-t2) Small ?2 (t1-t2) likelihood of
difference covt1 t2 can be maximized
by ensuring uniform treatments, texture, slope,
aspect, etc. re-sampling same location
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How can we overcome challenges? Statistics
Has history led to changes over time?
Time 2
Time 1
Decreasing statistical confidence (i.e.,
increasing acceptable risk of Type II error)
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How can we overcome challenges? Benchmarks
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Conclusions

• Quantifying soil C presents unique
• challenges, but
• we have an extensive knowledge base on soil
  carbon dynamics and good resource databases
• efficient sampling designs can be developed to
  verify soil C changes at various scales
• integrated modeling and measurement approaches
  offer the best alternative for accurate,
  cost-effective quantification as well as decision
  support capabilities.