Assessing the role of artificial compaction on physicochemical properties and carbon emissions of tropical peat soils

Samuel, Marshall Kana (2020) Assessing the role of artificial compaction on physicochemical properties and carbon emissions of tropical peat soils. PhD thesis, University of Nottingham.

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Compaction is undertaken because of claims that confined and restricted peat pores may enhance soil moisture due to the effect of capillary rise. The increase in soil moisture is said to lead to additional benefits of reduced C emissions and reduced risk and extent of peat fires. Nevertheless, information pertaining to induce compaction on peat physicochemical properties and carbon emission seems to be in scarcity. This thesis explored the notion of a misconception in definition of peat compaction in tropical peatland landscape due to confounding factors between (passive) natural compaction by oxidation, shrinkage and compression and (active) artificial compaction- by heavy machinery. This study suggest that this could be a reason for wide interpretation on peat compaction processes. Therefore, the aim of this research project was to assess the impact of artificial compaction on the tropical peat physicochemical properties and carbon emissions from peat surface as well through peat fire.

To make such a contribution, a series of experiments was conducted at matured oil palm plantation (OP) and secondary peat swamp forest (SF) located in Selangor, Peninsular Malaysia. This study first to explore the source of compaction efforts in OP. Holding an assumption that microsites are different in term of bulk density value (compaction) due to agri-management, the findings are inconclusive. Despite fruitless experiment, this study continues to assess the potential and effects of artificial compaction by profile compaction magnitude (induce compaction efforts) on physicochemical properties and carbon emission by implementing in-situ and ex-situ approaches. The mechanisms of artificial compaction are explored further by measured and determined physicochemical properties and carbon emission from peat surface and fire. Subsequently, correlations between physicochemical properties and carbon emission were established.

To simplify, this study yielded a number of main key findings: (1) Tropical peat compaction as marked by bulk density (BD) value in matured oil palm plantation relies on the degree of peat oxidation by periodical drainage. (2) It is challenging to achieve artificial compaction on tropical peatland due to labile soil organic matter that leads to self-regulatory mechanism through water table fluctuation. (3) Extremely low water table level and high labile carbon proportion may aid artificial compaction by consolidating varied decomposition degrees between peat horizons from surface and deeper peat. This resulted in average temporary inverse CO2 and CH4 emissions by the percentage ratio of ca. 1:2, and later, taken over by water table level control. (4) Oxidative or passive compaction leads to higher emission factor (EF) from fire path, owing to peat in advanced decomposition state.

In other words, this study does not recommend applying artificial compaction to either land use type. Primarily this is because: (1) there is no effect from the practice on already drained peatlands; (2) undertaking this in degraded sites as a mechanism to reduce emissions as part of rehabilitation in sites marked for reforestation and rewetting would likely fail due to the fact that compressed peat will return to its original structure due to the increased water table fluctuation or even via rainfall, or any interaction with water sources such that the efforts to induce artificial compaction on tropical peatland are rendered useless; (3) when considered in relation to global warming potential, the results from compacting treatments within the forest soils indicated negligible CH4 emission in relation to CO2-eq though the enhancement is 2-fold greater than the reduction of CO2. As such the net effect is 41.1 t CO2-eq ha-1 yr-1 . However, even if net emissions are reduced by 33%, emissions are still not countered by organic matter inputs so are still net emissions to atmosphere. Furthermore, soils would still be are vulnerable to the “birch effect” when seasonal rewetting does occur. (4) To minimize fires moving forward as well the emissions from future fires, raising water tables will both reduce fire occurrence but will also contribute to decreasing BD value where fires do occur. This result also highlights that any efforts to compact peat as a mitigation measure against fire is in fact more likely to increase EFs overall. Lastly, it is suggested that the terminology of compaction should be differentiated according to the nature of compaction: (1) oxidative or shrinkage compaction or passive compaction —oxidation of peat due to lowered water table, causing peat to collapse and become compressed under its own weight; and (2) artificial or active compaction—induced by horizontal load compression that causes admixture (consolidation) between distinct degrees of decomposition in peat.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Evers, Stephanie
Ashfold, Matthew
Smith, Thomas
Sjögersten, Sofie
Keywords: bulk density, carbon emission, decomposition degree, peat compaction definition, peat surface and peat fire, physicochemical alteration; tropical peatland, water table level
Subjects: S Agriculture > SB Plant culture
Faculties/Schools: University of Nottingham, Malaysia > Faculty of Science and Engineering — Science > School of Biosciences
Item ID: 60776
Date Deposited: 27 Jul 2020 08:57
Last Modified: 27 Jul 2020 08:57

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