Net greenhouse gas balance of fibre wood plantation on peat in Indonesia

Abstract

Tropical peatlands cycle and store large amounts of carbon in their soil and biomass^1-5. Climate and land-use change alters greenhouse gas (GHG) fluxes of tropical peatlands, but the magnitude of these changes remains highly uncertain^6-19. Here we measure net ecosystem exchanges of carbon dioxide, methane and soil nitrous oxide fluxes between October 2016 and May 2022 from Acacia crassicarpa plantation, degraded forest and intact forest within the same peat landscape, representing land-cover-change trajectories in Sumatra, Indonesia. This allows us to present a full plantation rotation GHG flux balance in a fibre wood plantation on peatland. We find that the Acacia plantation has lower GHG emissions than the degraded site with a similar average groundwater level (GWL), despite more intensive land use. The GHG emissions from the Acacia plantation over a full plantation rotation (35.2 ± 4.7 tCO₂-eq ha^-1 year^-1, average ± standard deviation) were around two times higher than those from the intact forest (20.3 ± 3.7 tCO₂-eq ha^-1 year^-1), but only half of the current Intergovernmental Panel on Climate Change (IPCC) Tier 1 emission factor (EF)²⁰ for this land use. Our results can help to reduce the uncertainty in GHG emissions estimates, provide an estimate of the impact of land-use change on tropical peat and develop science-based peatland management practices as nature-based climate solutions.

Fig. 1. Location of the study area, Kampar Peninsula in Sumatra, Indonesia.

a, Location of research sites with satellite images from Landsat 8 (source: https://earthexplorer.usgs.gov/). Photographs of the eddy covariance instruments installed at the top of the tower at Acacia plantation (b), degraded site (c) and intact site (d). For detailed site information, see Methods. Integrated eddy covariance footprint contour lines from 10% to 80% in 10% intervals over Acacia plantation for October 2016–May 2021 (e), degraded site for October 2016–May 2022 (f) and intact site for June 2017–May 2022 (g). GWL, peat subsidence, oxidative peat decomposition, soil N₂O flux and soil-sampling locations at Acacia plantation (h), intact site (i) and degraded site (j). An integrated climatologic footprint analysis indicated that approximately 80% of fluxes originated within 1,000 m in the upwind direction of each tower. Esri, HERE, Garmin, (c) OpenStreetMap contributors and the GIS user community.

Fig. 2. GHG balance of Acacia plantation, degraded and intact peat swamp forest in Sumatra, Indonesia.

To quantify total GHG balance in carbon dioxide equivalent (CO₂-eq), we used a sustained-flux global-warming potential (SGWP) of 1, 45 and 270 for CO₂, CH₄ and N₂O, respectively, over a 100-year time period. Total GHG balance = (net ecosystem CO₂ exchange + net ecosystem CH₄-C exchange + fluvial C export + C export in harvested wood, where applicable) + (net ecosystem CH₄ exchange × SGWP) + (soil N₂O flux × SGWP). We assumed that all fluvial C export is ultimately converted to CO₂ (ref. ). Avoided emissions from bioenergy production are calculated by assuming that 54% of harvested wood is used for bioenergy production (details in Supplementary Methods). The bold numbers indicate net impact of land-use change. Positive values indicate emission to the atmosphere and negative values indicate avoided emission.

Fig. 3. GWL controls carbon dioxide and methane fluxes in tropical peatlands.

a, Carbon dioxide (CO₂). b, Methane (CH₄). Relationship between net CO₂ and CH₄ fluxes and GWL were derived from published eddy covariance flux studies in tropical peatlands. The solid lines show the best-fit models and the dashed lines show 95% confidence intervals. The statistical test used a significance level of 5%. Positive values indicate net emission to the atmosphere, negative values indicates net uptake by the ecosystem. CO₂ results are compared with a previous relationship between CO₂ fluxes and GWL derived from subsidence and soil flux chamber measurements and a relationship for peatlands in the United Kingdom and Ireland is based on eddy covariance measurements. Positive and negative GWL values indicate the water level above and below the peat hollow surface, respectively.

Extended Data Fig. 1. Acacia plantation, degraded and intact peat swamp forest in Sumatra, Indonesia, are emitting CO₂ and CH₄ to the atmosphere.

Cumulative measured net CO₂ (a) and net CH₄ (b) fluxes with cumulative flux uncertainty (random error, friction velocity threshold and gap-filling approach) at the Acacia plantation (blue), degraded site (red) and intact site (green). Carbon export in harvested wood at the Acacia plantation is added in the end of plantation rotation, conservatively assuming that all harvested C would be returned to the atmosphere as CO₂. Intact and degraded sites were considered to have had no biomass C export during the study period. Positive values indicate emission to the atmosphere.

Extended Data Fig. 2. GWL is controlled by the balance between rainfall and evapotranspiration at intact peat swamp forest in Sumatra, Indonesia.

a, Time series of average GWL from three piezometers spanning 12 km with difference between 90 days moving average of rainfall and evapotranspiration. Negative difference indicates rainfall deficit. Positive and negative GWL values indicate the water level above and below the peat hollow surface, respectively. Diel pattern of evapotranspiration during dry season (February and July) (b) and wet season (April and November) (c) over the measurement periods show negligible evapotranspiration in the nighttime. The boxes show the median value and the interquartile range and whiskers denote the full range.

Extended Data Fig. 3. Soil N₂O emissions increase as GWLs decrease in tropical peatlands.

Temporal variation (a) and spatial variation (b) in soil N₂O fluxes from Acacia plantation (blue), degraded site (red) and intact site (green). The boxes show the median value and the interquartile range and whiskers denote the full range of all chambers. The plus signs (+) in the boxes of panel b show the average values. The n values represent the total number of soil N₂O flux measurements. c, Relationship between N₂O fluxes and GWL were derived from soil flux chamber measurements on various land uses across tropical peatlands. Positive and negative GWL values indicate the water level above and below the peat surface, respectively. Positive flux value indicates emission to the atmosphere and negative value indicates uptake by the soil.