Subsidence and carbon loss in drained tropical peatlands: reducing uncertainty and implications for CO<sub>2</sub> emission reduction options

Abstract

Conversion of tropical peatlands to agriculture leads to a release of carbon from previously stable, long-term storage, resulting in land subsidence that can be a surrogate measure of CO₂ emissions to the atmosphere. We present an analysis of recent large-scale subsidence monitoring studies in <i>Acacia</i> and oil palm plantations on peatland in SE Asia, and compare the findings with previous studies. Subsidence in the first 5 years after drainage was found to be 142 cm, of which 75 cm occurred in the first year. After 5 years, the subsidence rate in both plantation types, at average water table depths of 0.7 m, remained constant at around 5 cm yr⁻¹. Bulk density profiles indicate that consolidation contributes only 7 % to total subsidence, in the first year after drainage, and that the role of compaction is also reduced quickly and becomes negligible after 5 years. Over 18 years after drainage, 92 % of cumulative subsidence was caused by peat oxidation. The average rate of carbon loss over the first 5 years was 178 t ha⁻¹ yr⁻¹ CO_2eq, which reduced to 73 t ha⁻¹ yr⁻¹ CO_2eq over subsequent years, resulting in an average loss of 100 t ha⁻¹ yr⁻¹ CO_2eq annualized over 25 years. Part of the observed range in subsidence and carbon loss values is explained by differences in water table depth, but vegetation cover and addition of fertilizers also influence peat oxidation. A relationship with groundwater table depth shows that subsidence and carbon loss are still considerable even at the highest water table levels theoretically possible in plantations. This implies that improved water management will reduce these impacts by only 20 % at most, relative to current conditions, and that high rates of carbon loss and land subsidence should be accepted as inevitable consequences of conversion of forested tropical peatlands to other land uses

Carbon dioxide emissions from an &lt;i&gt;Acacia&lt;/i&gt; plantation on peatland in Sumatra, Indonesia

“…The resulting value applies at an average water table depth of 0.8 m, in peatland with a thickness greater than 4 m, for a peat surface covered by vegetation and with limited fertilizer applied only in the first year after planting. This value (80 t CO 2 ha −1 y −1 ) is supported by the results of a parallel study of carbon loss, carried out at the same location at the same time, of peat subsidence and peat characteristics (bulk density and carbon content) (Hooijer et al, 2012).…”

“…This temperature corrected emission value of 80 t CO 2 ha −1 y −1 , at an average water depth of 0.8 m, is close to the value of 76 t CO 2e ha −1 y −1 resulting from subsidence and bulk density measurements conducted partly in the same landscape at the same water table depth (Hooijer et al, 2012). It is also close to the values of 72 to 72.8 t CO 2e ha −1 y −1 suggested by Hooijer et al (2010) and Couwenberg et al (2010) in their meta-analyses, for a plantation water depth of 0.8 m. In two other studies, Murdiyarso et al (2010) and Koh et al (2011) apply a much lower estimate (34.1 t ha −1 y −1 at 0.5 m water table depth) for heterotrophic respiration emissions under plantation conditions.…”

“…Considering that oil palms are generally planted further apart than Acacia trees (4-6 m compared to 3.5 m between tree rows), there will be relatively more peat surface where autotrophic respiration emission is negligible. Moreover, mature oil palm stands have heterotrophic CO 2 emissions that are in the same range as those from mature Acacia tree stands, and they have similar water table depth and limited canopy cover (compared to natural conditions) (Hooijer et al, 2012). We therefore propose that the autotrophic respiration contribution to total soil CO 2 emissions in Acacia plantations are also applicable to other peatlands that have been drained and converted to other types of peatland cultivation.…”

“…Peat samples for determination of bulk density and ash content were collected following the method explained in Hooijer et al (2012).…”

“…meta-analyses by Couwenberg et al, 2010;Hooijer et al, 2010). Two principal methods have been employed, namely closed chamber monitoring (real time) of gaseous CO 2 emissions from the peat surface, and measurement of the lowering of the peat surface as a result of subsidence over time combined with information on peat carbon concentration and bulk density (Couwenberg et al, 2010;Hooijer et al, 2012). Carbon loss estimates based on the latter approach have been hampered by small numbers of measurements, incomplete field information (inadequacy and inconsistency of monitoring) and a lack of reference data on peat characteristics that would enable accurate calculation of carbon loss rates from subsidence.…”