Carbon dioxide emissions from an <i>Acacia</i> plantation on peatland in Sumatra, Indonesia

Abstract

<jats:p>Abstract. Peat surface CO2 emission, groundwater table depth and peat temperature were monitored for two years along transects in an Acacia plantation on thick tropical peat (&gt;4 m) in Sumatra, Indonesia. A total of 2300 emission measurements were taken at 144 locations, over a 2 year period. The autotrophic root respiration component of CO2 emission was separated from heterotrophic emission caused by peat oxidation in three ways: (i) by comparing CO2 emissions within and beyond the tree rooting zone, (ii) by comparing CO2 emissions with and without peat trenching (i.e. cutting any roots remaining in the peat beyond the tree rooting zone), and (iii) by comparing CO2 emissions before and after Acacia tree harvesting. On average, the contribution of autotrophic respiration to daytime CO2 emission was 21% along transects in mature tree stands. At locations 0.5 m from trees this was up to 80% of the total emissions, but it was negligible at locations more than 1.3 m away. This means that CO2 emission measurements well away from trees were free of any autotrophic respiration contribution and thus represent only heterotrophic emissions. We found daytime mean annual CO2 emission from peat oxidation alone of 94 t ha−1 y−1 at a mean water table depth of 0.8 m, and a minimum emission value of 80 t ha−1 y−1 after correction for the effect of diurnal temperature fluctuations, which may result in a 14.5% reduction of the daytime emission. There is a positive correlation between mean long-term water table depth and peat oxidation CO2 emission. However, no such relation is found for instantaneous emission/water table depth within transects and it is clear that factors other than water table depth also affect peat oxidation and total CO2 emissions. The increase in the temperature of the surface peat due to plantation establishment may explain over 50% of peat oxidation emissions. Our study sets a standard for greenhouse gas flux studies from tropical peatlands under different forms of agricultural land management. It is the first to purposefully quantify heterotrophic CO2 emissions resulting from tropical peat decomposition by separating these from autotrophic emissions. It also provides the most scientifically- and statistically-rigorous study to date of CO2 emissions resulting from anthropogenic modification of this globally significant carbon rich ecosystem. Our findings indicate that past studies have underestimated emissions from peatland plantations, with important implications for the scale of greenhouse gas emissions arising from land use change, particularly in the light of current, rapid agricultural conversion of peatlands in the Southeast Asian region.</jats:p>

Subsidence and carbon loss in drained tropical peatlands

“…Assuming an average peat surface temperature of 10 • C in temperate peatlands and 30 • C in the tropics, the oxidation rate would be expected to be four times higher in the latter and make up a far larger proportion of subsidence. Peat temperature and its potential impact on tropical peat oxidation is discussed in more detail in Jauhiainen et al (2012). While the oxidation contribution to peat subsidence increases over the first few years after drainage as primary consolidation and compaction diminish, the net carbon loss in fact decreases over this period, before stabilizing.…”

Congo Basin peatlands: threats and conservation priorities

“…The more oxygenated conditions in the surface peat allow rapid decomposition of the organic matter (Hooijer et al 2010). Even when there is no active drainage, the removal of trees from forested peatlands could lead to a reduction in organic matter inputs (Könönen et al 2016) and the loss of canopy cover can expose the peat surface to higher temperatures, contributing towards drying (Jauhiainen et al 2012) or can reduce evapotranspiration leading to an increase in waterlogging . Furthermore, drainage of the peatland increases the risk of subsidence (sometimes eventually flooding the land again) and fire occurrence (Page et al 2002;Jauhiainen et al 2012).…”

“…Even when there is no active drainage, the removal of trees from forested peatlands could lead to a reduction in organic matter inputs (Könönen et al 2016) and the loss of canopy cover can expose the peat surface to higher temperatures, contributing towards drying (Jauhiainen et al 2012) or can reduce evapotranspiration leading to an increase in waterlogging . Furthermore, drainage of the peatland increases the risk of subsidence (sometimes eventually flooding the land again) and fire occurrence (Page et al 2002;Jauhiainen et al 2012). As peatlands are often nutrient poor, acidic environments, conversion to agricultural land often requires the application of fertilisers, which can also enhance decomposition (Takakai et al 2006).…”

Greenhouse gas emissions along a peat swamp forest degradation gradient in the Peruvian Amazon: soil moisture and palm roots effects

“…In addition, tropical peatlands are among the most efficient terrestrial ecosystems for carbon sequestration (Dommain et al 2011;Page et al 2004), as continuous input of organic material from lowland tropical evergreen vegetation combined with anaerobic soil conditions lead to a build-up of soil organic matter (SOM) over time (Jauhiainen et al 2012). In natural conditions, the portion of SOM that is decomposed and emitted as carbon dioxide (CO 2 ) or methane (CH 4 ) is usually outweighed by the continuous input of fresh litter and roots (Jauhiainen et al 2005;Hergoualc'h and Verchot 2011;Hoyos-Santillan et al 2015).…”