This document summarizes a study that used the DNDC process-based model to simulate greenhouse gas emissions from oil palm plantations on tropical peatland in Indonesia. The model was calibrated using field measurements of biomass, water tables, soil properties, and gas fluxes from three oil palm plots. The model reasonably simulated water tables and total soil respiration but overestimated heterotrophic respiration. Model runs indicated that default IPCC estimates overestimate long-term net CO2 emissions from peat decomposition in older oil palm plantations. Improving the soil temperature and carbon sub-pool simulations were noted as areas for further refinement of the model.

1. Process-based modelling of greenhouse
gas emissions from oil palm plantations
in an Indonesian peatland
Erin Swails, Kristell Hergoualc’h, Jia Deng, Steve Frolking
Annual Meeting of the Society of Wetland Scientists, 1 – 10 June 2021

2. Tropical peatlands
• Extremely high soil C stocks –
total 350 Gt over 110 Mha
(Gumbricht et al 2017)
• Poor drainage → soil
waterlogging and SOC
accumulation
• Global extent - Southeast
Asia, Africa, South and
Central America
Planas Clarke et al. 2020

3. Land-use conversion in Southeast Asian peatlands
Miettinen et al. 2016

4. GHG emissions from Indonesian peatlands
Deforestation Burning Oil Palm Plantation
Drainage
Peat Forest
b
• Land-use conversion increases GHG emissions from peat
soils (Leifeld et al 2019, Hergoualc’h & Verchot 2014)

5. Peat GHG emissions
• OP peat emissions primarily released as CO2 (Swails et al 2021)
(Drösler et al 2014)

6. Data for estimation of peat GHG emissions
• 2013 IPCC supplement on wetlands: 1st
detailed GHG emission factors for tropical
peatlands
• EF based on data from Southeast Asia
(Hergoualc’h & Verchot 2014)
• Limited data for OP EF primarily based on
young plantations
• Process-based models can extrapolate point-
based measurements to larger regions over
extended time periods

7. Can DNDC simulate tropical peat GHG emissions?
• DeNitrification DeComposition (DNDC): 1-D process-based
model of C and N biogeochemistry
• Adapted for agricultural ecosystems, forests, and wetlands

8. Test site: Field measurements
• Permanent plots in smallholder oil palm plantations
(n=3) on peat
• Three years monthly measurements
o Peat GHG flux (total soil respiration, CH4, N2O)
o Controlling factors (water table, soil moisture + temp)
• Heterotrophic respiration estimated w/ site-specific
partitioning ratios

9. Modelling GHG fluxes and biogeophysical drivers
• Initialization – on-site measurements and literature-
based values
• Forcing
o Daily weather data from airport
o Management: planting, fertilization, harvest
• Calibration – vegetation growth, hydrology, SOC sub-
pools
• Testing – annual average peat GHG fluxes (total and
heterotrophic soil respiration, N2O, CH4)
• 30-year model runs – CP/FP simulations in each plot
• Up-scale outputs to plot level w/ spatial ratios

10. Calibration results: OP standing biomass,
litterfall, root mortality
• DNDC vegetation outputs fell within range of values
reported in the literature

11. Calibration results: Water table level
• DNDC adequately simulated WT fluctuations
OP1 OP2 OP3

12. Calibration results: Soil water-filled pore space
• DNDC adequately simulated fluctuations and micro-
spatial variation in soil WFPS
OP1 OP2 OP3

13. Calibration results: Soil temperature
• DNDC did not simulate the influence of vegetation
shading on soil temperature
OP1 OP2 OP3
closed canopy open canopy
intermediate

14. Calibration results: SOC sub-pools
• DNDC simulated the magnitude of total soil respiration
well overall
OP1 OP2 OP3

15. Model test results: Total and heterotrophic soil
respiration
RMSD = 3.0 Mg CO2-C ha-1 yr-1 RMSD = 4.0 Mg CO2-C ha-1 yr-1
Observed mean annual SR kg CO2-C ha-1 d-1
Predicted
mean
annual
SR
kg
CO
2
-C
ha
-1
d
-1
Observed mean annual SRh kg CO2-C ha-1 d-1
Predicted
mean
annual
SRh
kg
CO
2
-C
ha
-1
d
-1

16. What are long-term net CO2 emissions from peat
decomposition in OP plantations?
• Modelled net peat CO2 emissions declined
over time
IPCC
DNDC: yr 0 - 10
DNDC: yr 10 - 20
DNDC: yr 20 - 30
heterotrophic respiration
root mortality
litterfall

17. Takeaways and next steps
• Total soil respiration and biogeophysical
drivers were simulated reasonably well in
model runs, but..
o Heterotrophic respiration in our young OP
plantations was overestimated
• Model simulations indicate IPCC default
overestimates net CO2 emissions from peat
decomposition in older OP plantations

18. Takeaways and next steps
• Improve simulation of soil temperature
• Refine SOC sub-pool partitioning and spatial
representation of near/far conditions in OP
model simulations
• Use DNDC to investigate relationships between
GHG fluxes and potential proxies
• Model peat GHG emissions in forest

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