Effects of Land Use Change on Tropical Peat Soil Properties: Implications for Peatland Restoration
Creators: name: Kunarso, Adi
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 410601
subject: Land capability and soil productivity
classificationCode: 410101
subject: Carbon sequestration science
classificationCode: 410405
subject: Environmental rehabilitation and restoration

description: Indonesian tropical peatlands play an important role in conserving biodiversity, sequestering carbon and sustaining the livelihoods of local people. Despite these values, Indonesian peatlands have been deforested, drained and converted during the past three decades leading to increased peat decomposition, carbon emissions, fires, and changes in peat soil properties. There is a critical need to reduce the rate of peatland degradation in Indonesia and to restore vast areas of degraded peatlands. Knowledge on physical and chemical peat properties is important to support peatland restoration programs. However, there is limited published data for many peat soil properties which are relevant to peatland restoration efforts. 

To assess the current state of knowledge on peat soil properties in Southeast Asia, a systematic review and meta-analysis study was conducted (Chapter 2). Research on tropical peat properties has increased in the past decades, reflecting growing research interest in this topic. However, research predominantly report on bulk density, pH, carbon (C), and nitrogen (N) content, with other properties remaining under-researched. This meta-analysis revealed significant differences in bulk density and pH between less managed and intensively managed peatlands, indicating that land use changes impact peat soil properties. The review emphasises the need for standardised methodologies to measure peat soil properties and identifies research gaps, including less-explored peat properties and geographic regions.

Bulk measures of C and N, and their ratios, have been most often used to assess the effects of land use change and restoration success of tropical peatlands. However, there is currently only limited information about organic carbon composition of tropical peat soils. 13C NMR spectroscopy was used to evaluate the change in composition of organic carbon under changing land use. This study provides evidence that land use change significantly affected the organic carbon composition of tropical peat soils (Chapter 3). The alkyl C/O-alkyl C ratio is a more informative indicator of the degree of decomposition than the C/N ratio, having a more consistent trend with land use. A ten-year revegetation attempt showed promising results in tree performance but was unlikely to contribute to increasing peat accumulation and soil carbon stocks, indicated by the high dominance of alkyl C, especially in the upper (0–40 cm) soil layers. 

There is a need for rapid and cost-effective methods to monitor the success of restoration efforts. The study explored the use of MIR spectroscopy to characterise organic carbon and predict chemical peat soil properties (Chapter 4). This study demonstrated that MIR spectroscopy combined with Partial Least Squares Regression (PLSR) analysis can be used to develop robust predictive models, with excellent models for N (R2 = 0.90, RMSE = 0.10, LCCC = 0.95, and RPIQ = 0.07) and the C/N ratio (R2 = 0.84, RMSE = 6.23, LCCC = 0.92, RPIQ = 3.64) and a strong model for C content (R2 = 0.78, RMSE = 2.04, LCCC = 0.88, and RPIQ = 1.78). This spectroscopic method could be appropriate for analysing the large numbers of samples that will be required for national-scale monitoring of the success of tropical peatland restoration interventions. 

Finally, to address a key knowledge gap on tropical peat soil physical properties and to understand water movement through peat soil profiles, unsaturated hydraulic conductivity K(ѱ), under five different land uses was measured (Chapter 5). The K(ѱ) varied over two orders of magnitude, i.e. ranging from 0.23 x 10-4 to 3.70 x 10-2 cm s-1 at tension -1 cm and from 0.10 x 10-4 to 0.24 x 10-2 cm s-1 at tension -3 cm. It was significantly different between land uses and between depths at both supply tensions. However, this study did not find a relationship between K(ѱ) and chemical measures of the degree of decomposition (C/N, alkyl C/O-alkyl C, and carbohydrate/aromatic ratio), which may be due to the variability of the botanical origin of the peat materials. The application of MIR-PLSR to predict K(ѱ) was explored in this present study and yielded a moderate prediction model (R2 = 0.55, RMSE = 0.13, LCCC = 0.71, and RPIQ = 2.44) between measured and predicted model of K(ѱ) at tension -1 cm, that could allow the model to be used as initial screening. Direct measurements of K(ѱ) are time-consuming and costly, this prediction model may be useful for field estimation. 

Overall, this thesis provides novel insights into the effects of land use change on tropical peat soil’s physical and chemical properties. The study indicates that the C/N ratio may not be a sufficient measure to monitor the degree of decomposition of tropical peat. Hence, it is essential to incorporate MIR spectroscopy to examine carbon composition and degree of decomposition, as it allows for a rapid process. In addition, the robust MIR-PLSR predictive model developed in this thesis can provide rapid and cost-effective estimates of C, N, and C/N ratios, as well as potentially other properties of tropical peat soil. This comprehensive approach will enhance our understanding of decomposition processes, which will be valuable for developing methods to monitor the success of tropical peatland restoration efforts.