Drainage: a key concern for tropical peatlands

Peatlands are globally important carbon (C) stores, containing more C than the world's vegetation, whilst covering only a small portion of the Earth’s surface (~3%). When drained for agriculture or peat harvesting, stored C is released to the atmosphere as carbon dioxide (CO2 - a greenhouse gas, GHG). Globally, peatland drainage is responsible for 5% of human-derived CO2 emissions; drainage also increases the risk of fire, with associated environmental and socio-economic costs, and of flooding, through the process of (peat)land subsidence. Large areas of peatland are currently drained for agriculture (with extensive ditch / canal (Fig. 1) networks developed), forestry, and peat extraction across the world.

Ditch channel in Boreno

Fig. 1: Ditch channel in Borneo. Image: Sarah Cook, University of Leicester.

In Southeast Asia, peatlands have been drained for conversion to plantations (for palm oil and paper pulp); in Europe and North America, lowland peatlands have been drained for vegetables, cereals and livestock pastures (Fig. 2). Unless responsibly managed, drainage, conversion to agricultural land and fertilisation of peat soils can result in negative environmental impacts within decades due to soil subsidence and fires (nutrient depletion, soil erosion, destabilisation of soil structure). Agriculture, forestry and mining have so far impacted about 25% of the peatlands on Earth. While large parts of the enormous peatlands of North America and Russia are still relatively intact, in many parts of Europe, Central and Southeast Asia, Argentina and Chile, peatlands have been significantly degraded. There is increasing interest in protecting and restoring peatlands in order to conserve existing C stocks, help mitigate climate change, and preserve ecosystem benefits (i.e., water quality, flood prevention and biodiversity). In the tropics, it is deforestation and drainage that causes the initial destalibilsation of peatlands and makes them increasingly vulnerable to on-going degradation.

Cultivation on a tropical peatland

Fig. 2: A drained tropical peatland, which has been converted for agricultural use. Image: Sue Page, University of Leicester.

As a result of peat drainage (Fig. 3), the organic carbon—which has built up in the peat deposit over thousands of years and is normally under water—is suddenly exposed to oxic (oxygenated) conditions. It decomposes (aerobically) and turns into CO2, which is released into the atmosphere. Undamaged peatlands are usually net accumulators of carbon; i.e., more carbon is taken up via photosynthesis than is lost from decomposition and subsequent release to the atmosphere or from export in flowing water (Fig. 3). In damaged peatlands, this role may be reversed (Fig. 3). Peatland plant productivity may decline, so that less atmospheric CO2 is sequestered, and rates of litter and peat decomposition may increase because of a thickening or deepening of the oxic zone caused by lowering of the water table. Oxic decomposition rates are many times – up to orders of magnitude – greater than those in the absence of oxygen (anoxic conditions). Therefore, a deepening of the oxic zone can cause increases in the rates of CO2 emission from a peatland and a loss of carbon sink function.

Peatland progression after drainage

 Fig. 3: Schematic of changing carbon dynamics of peatlands throughout stages of draining. Image: Sue Page, University of Leicester.

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