Continuing population growth, deterioration of existing land especially due to erosion and increasing competition for land from industry and urbanisation are setting continuous pressure on peatland development. Between 1990 and 2015, 50% of the total 15.7 million hectares (Mha) of peat swamp forestland in Peninsular Malaysia and the islands of Borneo and Sumatra was converted to managed land uses, while only 6% showed no clear signs of human influence (Miettinen et al. 2016). The change is fast, by 2008 about 10% of the peatlands of Peninsular Malaysia, Borneo and Sumatra remained in an intact or only slightly degraded condition (Miettinen and Liew, 2010). By 2015, about 20% (3.2 Mha) of the peatlands in the region was characterized as ‘open undeveloped’ and ‘secondary regrowth’ (Miettinen et al. 2016).
Implications of high rate of land use change, and the increasing frequency and extent of fires, for losses of peat carbon receive attention increasingly (e.g. Hooijer, 2010; Couwenberg et al., 2010; Miettinen and Liew 2010; Murdiyarso et al., 2010). Peat has specific characteristics that differ from mineral soils in agricultural use requiring drained conditions – peat subsides, i.e. soil surface level lowers continuously due to enhanced loss of organic organic substrate in oxic conditions created in the topmost peat, and this phenomeon is clearly related to the applied peat water table (Hooijer et al. 2012, Hooijer and Couwenberg 2013).
Impact of permanetly lowered water tables on peat
Permanently lowered water tables lead to peat subsidence, which is sign of carbon loss due to increased organic matter decomposition (Jauhiainen et al 2012; Hooijer et al. 2010; 2012; Drösler et al. 2014). Also drainage canals in reclaimed peatlands can form potentially marked GHG source (Jauhiainen & Silvennoinen, 2012). Agriculture and plantation development on permanently clear-felled peat can not be regarded as sustainable land uses as all land uses in permanently drained peat result in progressive peat subsidence (Lahiru et al., (2016).
Impacts of fertilization on peat should be studied
Impacts of heavy fertilization on nutrient poor (including available nitrogen) peat decomposition is one of the subjects that should receive more attention. Peat is typically rich in carbon but poor in available (labile) nitrogen, and productivity of certain crops species requires quite heavy and regular fertilization for periods lasting decades. For example, typical reported NPK-N fertilisation rate for oil palm grown on deep peat is reported to be 50–100 kg ha-1 y-1 during immature stage and 120–160 kg ha-1 y-1 during mature stage (Ng et al . 1990) last of which covers more than half of the ca. 25 years long production cycle. Work aiming to find ways to reduce GHG emissions and peat loss in actively managed tropical peatlands should be supported.
Impacts of high tempertures on exposed peat start gradually open
Because of intense solar radiation in the equatorial region, notable long-term differences in tropical peat surface temperature conditions can form between areas due to differing shading intensities provided by vegetation, and a short-term diurnal temperature fluctuation in the peat surface may exist between day and night. Rotation-based production on reclaimed peatlands includes repeated temperature changes as the soil surface can be exposed for prolonged periods after harvesting operations remove the cropped biomass. In recent experimental study providing several shading conditions (unshaded, 28%, 51% and 90% shading) in bare tropical peat surface, the largest peat temperature difference in the topmost 50-cm peat profile was between the unshaded and 90% shaded surface, where the average temperatures at 5 cm depth differed up to 3.7 ºC, and diurnal temperatures at 5 cm depth varied up to 4.2 °C in the unshaded and 0.4 °C in the 90% shaded conditions (Jauhiainen et al. 2014). In experiments conducted on bare agricultural land, where 90% shading from the full exposure resulted in a 33% lower CO2 emission average on the unfertilised plots and a 66% lower CO2 emission average on the fertilised (NPK-fertilisation 50 kg ha-1) plots. Correlation between peat temperature and CO2 flux suggested an approximately 8% (unfertilised) and 25% (fertilised) emissions change for each 1 °C temperature change at 5 cm depth on the agricultural land. Even the highest vegetation biomass representing plantations have long periods with exposed soils when enhanced peat decomposition may be experienced due to combined effects of elevated temperatures and fertilization in exposed peat. The rotation cycle of Acacia lasts up to 6–7 years and the canopy closure takes place about two years after planting. The rotation cycle of crops, e.g. oil palm, lasts at least 20 years, canopy closure takes place between 4–6 years and canopy volume stabilises by the age of 10 years. As a comparison, diurnal peat temperature differences remain spatially small in steady-state peat swamp forests, where crown cover consists of multiple crown layers and is nearly continuous, and only relatively small patches of peat surface near recently fallen trees are exposed at a time.
Until recently, problems connected to tropical peat reclamation were not taken into account with adequate measures – as can now be seen, for example, in large areas of the Mega Rice project area. There the land reclamation activities did not match with the targeted production aims but created numerous problems – poverty, unused land, disturbed hydrology, biodiversity losses, and fires causing repeatedly haze and vast amounts of carbon released to the atmosphere and thus enhancing the climate change. Optimized peat water table for crops production with minimized peat subsidence is a challenge to be resolved. Tropical peatlands are fragile ecosystems that are often underlained by potentially acidifying pyrite sediments (PAS) or nutrient deficient quartz sand. If substrate is developed for agriculture,peat substrated oxidation of the will dominate in carbon dynamics, and the process will lead to peat disappearance or result in condition when drainage is no longer possible and the land can no longer be cultivated due to risk of flooding.
Key words in finnish: troopiset suot, suo, maankäyttö, muutos, eroosio, kasvihuonekaasut, hiili, hiilidioksidi, päästöt, vesitase, pohjavesi, ilmastomuutos, ilmastonmuutos, savusumu, plantaasi, akaasia, öljypalmu, viljely