Search

Tropical peatlands serve as huge carbon (C) pools in the global C balance, and degradation due to agricultural use is a major concern. The objective of the present study was to examine the relationship between the C loss rate in agriculturally drained tropical peat soils in relation to groundwater level (GWL) and the chemical structure of the resulting soil organic matter (SOM). Soil mesocosms containing three peat soils collected from three forest types in Malaysia, Mixed peat swamp (MPS), Alan Batu (ABt), and Alan Bunga (ABg) forests, which are classified based on dominant plant species, species diversity, and forest structure, were buried in an oil palm plantation for a period of five years. Soil samples packed in the top layer (0–20 cm depth) and the bottom (60–80 cm depth) layer were then analyzed and the results were compared with the initial values for these soils. Weight loss, total C and N contents, ash content, and pyrophosphate solubility index (PSI; an index of the degree of humification) were measured. Changes in the chemical structure of the SOC were investigated using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy with phase-adjusted spinning side bands and thermally assisted hydrolysis and methylation-gas chromatography/mass spectrometry (THM-GC/MS). The C loss in the MPS forest soil, 2–12%, was smaller than that in the ABt and ABg forest soils, 20–37%, especially in the bottom layer. The higher C loss in the ABt and ABg soils, without significant difference between the top and bottom layers, can be attributed to the greater accumulation of less decomposed plant components as evidenced by the larger yields of lignin-derived phenols and polysaccharide-related compounds in THM-GC/MS analysis. The C composition as estimated by 13C NMR did not vary during incubation of the ABt and ABg soils while the yields of lignin-derived and other phenols as well as polysaccharide-related compounds in THM-GC/MS were decreased, especially in the top layer. These findings suggest that plant components with a low degree of decomposition are susceptible to drainage regardless of their structure. The progression of the oxidative degradation of lignin in the top layer of all soils was also suggested by the increases in PSI and the vanillic acid/vanillin ratio. The % O-alkyl C and % alkyl C may control the rate of C loss positively and negatively, respectively. PSI also appears to be useful for roughly estimating the decomposability of tropical peat SOM in various GWLs.

• Field incubation clarified drainage effect on SOM quality in 3 peat forest soils. • C loss after 5 years at depths of 0–20 cm and 60–80 cm was 12–37 % versus 2–31 %. • Decomposition rates of alkyl, O -alkyl, and aromatic C were similar in each soil. • Lignin is also decomposed if groundwater level before drainage is high. • % alkyl C and degree of humification are indicators of the peat SOM decomposability. Tropical peatlands serve as huge carbon (C) pools in the global C balance, and degradation due to agricultural use is a major concern. The objective of the present study was to examine the relationship between the C loss rate in agriculturally drained tropical peat soils in relation to groundwater level (GWL) and the chemical structure of the resulting soil organic matter (SOM). Soil mesocosms containing three peat soils collected from three forest types in Malaysia, Mixed peat swamp (MPS), Alan Batu (ABt), and Alan Bunga (ABg) forests, which are classified based on dominant plant species, species diversity, and forest structure, were buried in an oil palm plantation for a period of five years. Soil samples packed in the top layer (0–20 cm depth) and the bottom (60–80 cm depth) layer were then analyzed and the results were compared with the initial values for these soils. Weight loss, total C and N contents, ash content, and pyrophosphate solubility index (PSI; an index of the degree of humification) were measured. Changes in the chemical structure of the SOC were investigated using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy with phase-adjusted spinning side bands and thermally assisted hydrolysis and methylation-gas chromatography/mass spectrometry (THM-GC/MS). The C loss in the MPS forest soil, 2–12%, was smaller than that in the ABt and ABg forest soils, 20–37%, especially in the bottom layer. The higher C loss in the ABt and ABg soils, without significant difference between the top and bottom layers, can be attributed to the greater accumulation of less decomposed plant components as evidenced by the larger yields of lignin-derived phenols and polysaccharide-related compounds in THM-GC/MS analysis. The C composition as estimated by 13C NMR did not vary during incubation of the ABt and ABg soils while the yields of lignin-derived and other phenols as well as polysaccharide-related compounds in THM-GC/MS were decreased, especially in the top layer. These findings suggest that plant components with a low degree of decomposition are susceptible to drainage regardless of their structure. The progression of the oxidative degradation of lignin in the top layer of all soils was also suggested by the increases in PSI and the vanillic acid/vanillin ratio. The % O -alkyl C and % alkyl C may control the rate of C loss positively and negatively, respectively. PSI also appears to be useful for roughly estimating the decomposability of tropical peat SOM in various GWLs. [ABSTRACT FROM AUTHOR]

Abstract: We test the applicability of two different methods for quantifying carbon (C) stocks in five tropical peat sites in Sumatra, Indonesia; two in an intact peat swamp forest, one in a logged forest and two in an oil palm plantation. Elemental analysis is used to quantify C contents and stocks in all samples, and is the standard to which the two methods are compared. The first method, loss on ignition (LoI), underestimates sample C stocks by 4.9±0.8% compared to results from the elemental analysis across all samples. Loss on ignition is applied to three full peat cores in one of the intact forest sites and one oil palm site, using the standard factor of 1.922 to convert the organic matter (OM) result into C content. Evaluation against independent data suggests that a factor of 1.878 would be more appropriate. The second method, using the equation presented in Warren et al. (2012) for quantifying C density in peat swamp forest soils, shows no significant difference between the equation and elemental analysis derived results for the intact and logged forest sites. We suggest the use of a bulk density (BD) range to further define the limits of the equation. When applied to samples from oil palm sites, there was a significant difference in the results provided by the Warren equation and elemental analysis. Evaluation against independent data suggests that, in this land use, C density (kgCm−3) is more accurately estimated by the equation Cd =(515.44×BD)+3.01 (R2 =0.94). We also present a variable factor for the conversion of the organic matter (OM) to C content by merging the two methods. We recommend the use of the LoI method with the improved factor for conversion of OM to C content, or where bulk density is known the Warren equation for calculating accurate C density values across intact and logged forests on Indonesian peats, and our revised equation for oil palm sites. [Copyright &y& Elsevier]

Abstract: The influence of oil palm development on tropical peat soil decomposition rate was investigated by an incubation experiment. Soil samples from soil surface and around underground water table were taken from forest site, and oil palm site at 1st and 9th year after development. The soil samples were sieved into 0–2mm, 2–8mm and 8–20mm and analyzed for carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) fluxes. The development of oil palm did not change the CO2 emissions and showed inconsistent influence on CH4 flux according to aggregate size, while significantly higher N2O emissions were found for aggregates 0–2mm at high moisture of oil palm plantation soils compared with the original forest. Nitrous oxide fluxes showed significant positive correlation with the CO2 flux, which indicated that soil organic matter decomposition was closely related to the N2O production. On the other hand, CH4 flux showed clear emission for aggregates bigger than 2mm, while aggregates size 0–2mm showed consistent CH4 uptake. These results showed that investigation of greenhouse gas emissions in tropical peat soil must take into account the aggregate characteristics of the soil, which are inhomogeneous and mixed with fresh organic matter. [Copyright &y& Elsevier]