2018 SWCS International Annual Conference July 29-August 1, 2018 Albuquerque Convention Center

1. Presenter: Wayne Roper
Co-authors: Wayne Robarge, Deanna Osmond, Joshua Heitman
Department of Crop and Soil Sciences, NC State University
Using Low Organic Matter Soils to Compare
Conventional Soil Organic Carbon Measurements

2. Soil health and soil organic carbon
Soil health is “the capacity for soil to function as a ecosystem that sustains life”
Soil organic carbon (SOC) is a critical component of soil health
Stimulates microbial activity

3. SOC is derived from plant, animal, and microbial residues that are incorporated into soil
Animal manure Microbial biomass
CO2
& byproducts
Oxidant
Organic Carbon reactivity
As carbon compounds are degraded by biological or
chemical processes, CO2 is released
Glucose
Plant litter
Chemistry of soil organic carbon

4. Mass loss on ignition (LOI)
Automated dry combustion (ADC)
Walkley-Black (WB)
360°C 𝑔 𝑆𝑂𝑀 × 0.58
𝑔 𝑠𝑜𝑖𝑙
SOC
Calculation
Thermal digestion
SOC oxidation
925°C
Thermal
detection
Methods to measure soil organic carbon
Ceramic
crucible
Mass soil
with SOM
Muffle
furnace
SOC
oxidation
Mass soil
without SOM
Gas
chromatography
𝑔 𝐶
𝑔 𝑠𝑜𝑖𝑙
SOC
Calculation
(𝑚𝑒𝑞 𝐾2 𝐶𝑟2 𝑂7 − 𝑚𝑒𝑞 𝐹𝑒𝑆𝑂4)(0.003)
𝑔 𝑠𝑜𝑖𝑙
SOC
Calculation
~ 10 g
soil
~ 0.03 g
soil
~ 1 g
soil
Oxidant (K2Cr2O7)
Heat (H2SO4)
SOC
oxidation
Filter
solution
Titration
(FeSO4)

5. Do the methods lead to similar interpretations of SOC?
Assess the ability for different methods to differentiate among soils
Objective 1
Compare correlations among measurements of SOC from different methods
Objective 2
Study objectives

6. Experiment locations
Mountain
Mills River, NC
Began: 1990
Soil:
Delanco silt loam
Typical SOM (0-15 cm):
0.5-3% (5-30 g kg-1)
Treatment factors:
Organic management
Conventional management
No-till
Conventional tillage
Piedmont 1
Reidsville, NC
Began: 1984
Soil:
Toast coarse sandy loam
Typical SOM (0-15 cm):
0.5-3% (5-30 g kg-1)
Treatment factors:
No-till, chisel plow, disc plow,
moldboard plow
Piedmont 2
Reidsville, NC
Began: 1984
Soil:
Pacolet sandy loam
Typical SOM (0-15 cm):
0.5-2% (5-20 g kg-1)
Treatment factors:
No-till, disc plow
Coastal Plain
Goldsboro, NC
Began: 1999
Soil:
Wickham sandy loam
Typical SOM (0-15 cm):
0.5-2% (5-20 g kg-1)
Treatment factors:
Organic management
conventional management
No-till
Four long-term agronomic trials in North Carolina

7. Soils collected from
top 15 cm...
…ground up and forced
through a 2-mm mesh…
Automated dry
combustion
Mass loss
on ignition
Walkley-Black
then processed using different methods
Soil preparation

8. ResultsResults

9. Method N Mean SD Minimum Maximum Range
–––––––––––––––––––– g C kg-1 –––––––––––––––––––––
WB 84 12.99 a 2.05 9.7 18.8 9.1
LOI 84 13.47 a 3.30 8.4 22.5 14.1
ADC 84 7.70 b 2.05 3.7 13.3 9.6
Letters after means indicate statistical grouping
• SOC mostly fit ranges expected for these soils (< 2% or < 20 g kg-1)
• ADC measured less SOC than LOI and WB
• LOI had the largest range and variability
Results – summary statistics
Summary statistics for soil organic carbon measurements

10. 0
6
12
18
24
0 6 12 18 24
LOISOC(gkg-1)
ADC SOC (g kg-1)
r2 = 0.40
n = 84 Trial r2
ADC vs LOI Overall 0.40
Coastal Plain 0.62
Piedmont 1 0.33
Piedmont 2 0.81
Mountain 0.71
Results – correlations
Correlation between SOC measured by ADC and LOI

11. 0
6
12
18
24
0 6 12 18 24
WBSOC(gkg-1)
LOI SOC (g kg-1)
r2 = 0.31
n = 84 Trial r2
LOI vs WB Overall 0.31
Coastal Plain 0.08
Piedmont 1 0.15
Piedmont 2 0.40
Mountain 0.33
Results – correlations
Correlation between SOC measured by LOI and WB

12. 0
6
12
18
24
0 6 12 18 24
WBSOC(gkg-1)
ADC SOC (g kg-1)
r2 = 0.30
n = 84 Trial r2
ADC vs WB Overall 0.30
Coastal Plain 0.05
Piedmont 1 0.18
Piedmont 2 0.28
Mountain 0.59
Results – correlations
Correlation between SOC measured by ADC and WB

13. 1. ADC reports less SOC than LOI and WB
2. Separating the data by location did not consistently improve correlations
3. The amount of SOC measured by one method is not predictive of SOC that
would be measured by another method
Objective 1
Compare correlations among measurements of SOC from different methods
RESULTS
Results – correlations
Automated dry
combustion
Mass loss
on ignition
Walkley-Black

14. Trial Treatment ADC LOI WB
–––––––– g C kg-1 ––––––––
Coastal
Plain
No-till Conventional 8.3 9.9 13.2 a
Conv. Tillage 9.0 9.9 10.7 b
Conv. Tillage organic 1 8.5 10.3 13.3 a
Conv. Tillage organic 2 9.0 11.9 11.9 ab
ADC: No differentiation
LOI: No differentiation
WB: + Organic with tillage
+ No-till
Results – agronomic management
Comparison of coastal plain treatments

15. Trial Treatment ADC LOI WB
–––––––– g C kg-1 ––––––––
Piedmont 1 No-till chemical 7.5 ab 12.6 bc 11.9
In-row subsoiling 9.5 a 16.9 a 14.0
Disk, spring 5.3 b 13.8 abc 14.3
Chisel, fall 7.9 ab 13.6 abc 12.4
Chisel, spring 8.1 ab 14.9 ab 13.7
Chisel, disk, fall 7.1 ab 12.3 bc 12.2
Chisel, disk, spring 6.5 ab 13.4 abc 12.1
Moldboard, fall 5.3 b 12.7 bc 10.9
Moldboard, spring 5.5 b 10.2 c 11.6
Piedmont 2 No-till chemical 7.1 11.6 11.6
Alternate year tillage 7.1 12.0 13.6
Disk, spring 6.7 11.6 12.7
Twice disking, spring 5.9 10.2 12.4
ADC: subsoiling > moldboard, disc
LOI: subsoiling > moldboard, chisel + disc
WB: no differentiation
ADC: no differentiation
LOI: no differentiation
WB: no differentiation
Results – agronomic management
Comparison of piedmont treatments

16. Trial Treatment ADC LOI WB
–––––––– g C kg-1 ––––––––
Mountain No-till Organic 12.3a 20.1 a 16.4 a
No-till Conv. 8.8b 16.3 b 14.6 ab
Conv. Tillage Organic 9.3b 17.3 ab 13.9 ab
Conv. Tillage 7.3b 15.3 b 12.7 b
Conv. Tillage Fallow 8.8b 16.4 b 15.0 ab ADC: + no-till organic
LOI: + no-till organic
WB: + no-till organic > conventional
Results – agronomic management
Comparison of mountain treatments

17. • Statistical differentiation of the effects of agronomic management practices
on SOC was not consistent among the methods when comparing results
within and across agronomic trials
Objective 2
Assess the ability for different methods to differentiate among soils
RESULTS
Results – correlations

18. • Interpretations of SOC content in these soils should be relative to a specific method and
should not be compared to different procedures.
Conclusions and implications
• Correlations between methods for measuring SOC are not predictive of each other
(for the low SOC soils used in the experiment)
• Differences in how agronomic management affects SOC content are not consistent among
conventional methods

19. Acknowlegements
Ph.D. committee:
Deanna Osmond & Joshua Heitman
Michael Wagger
Chris Reberg-Horton
Co-author:
Wayne Robarge
Soil analyses:
Environmental and Agricultural Testing Service
Dean Hesterberg soil chemistry laboratory
Assistance:
Adam Howard
Wesley Childres
Melissa Bell & Tomas Moreno
All research station personnel
Funding

20. Thank you
Questions?