Variation in Soil Properties Regulate Greenhouse Gas Fluxes and Global Warming Potential in Three Land Use Types on Tropical Peat

This study investigated spatial factors controlling CO2, CH4, and N2O fluxes and compared global warming potential (GWP) among undrained forest (UDF), drained forest (DF), and drained burned land (DBL) on tropical peatland in Central Kalimantan, Indonesia. Sampling was performed once within two weeks in the beginning of dry season. CO2 flux was significantly promoted by lowering soil moisture and pH. The result suggests that oxidative peat decomposition was enhanced in drier position, and the decomposition acidify the peat soils. CH4 flux was significantly promoted by a rise in groundwater level, suggesting that methanogenesis was enhanced under anaerobic condition. N2O flux was promoted by increasing soil nitrate content in DF, suggesting that denitrification was promoted by substrate availability. On the other hand, N2O flux was promoted by lower soil C:N ratio and higher soil pH in DBL and UDF. CO2 flux was the highest in DF (241 mg C m(-2) h(-1)) and was the lowest in DBL (94 mg C m(-2) h(-1)), whereas CH4 flux was the highest in DBL (0.91 mg C m(-2) h(-1)) and was the lowest in DF (0.01 mg C m(-2) h(-1)), respectively. N2O flux was not significantly different among land uses. CO2 flux relatively contributed to 91-100% of GWP. In conclusion, it is necessary to decrease CO2 flux to mitigate GWP through a rise in groundwater level and soil moisture in the region

Soil N₂O Emissions under Different N Rates in an Oil Palm Plantation on Tropical Peatland

(1) Background: Nitrogen (N) fertilization on drained tropical peatland will likely stimulate peat decomposition and mineralization, enhancing N₂O emission from the peat soil. (2) Methods: A field experiment was conducted to quantify the N₂O emissions from soil in an oil palm plantation (Elaeis guineensis Jacq.) located in a tropical peatland in Sarawak, Malaysia, under different rates of N fertilizers. The study was conducted from January 2010 to December 2013 and resumed from January 2016 to December 2017. Nitrous oxide (N₂O) flux was measured every month using a closed chamber method for four different N rates; control-without N(T1), 31.1 kg N ha⁻¹ yr⁻¹(T2), 62.2 kg N ha⁻¹ yr⁻¹(T3), and 124.3 kg N ha⁻¹ yr⁻¹ (T4); (3) Results: Application of the N fertilizer significantly increased annual cumulative N₂O emissions for T4 only in the years 2010 (p = 0.017), 2011 (p = 0.012), 2012 (p = 0.007), and 2016 (p = 0.048). The highest average annual cumulative N2O emissions were recorded for T4 (41.5 ± 28.7kg N ha⁻¹yr⁻¹), followed by T3 (35.1 ± 25.7kg N ha⁻¹yr⁻¹), T1 (25.2 ± 17.8kg N ha⁻¹yr⁻¹), and T2 (25.1 ± 15.4kg N ha⁻¹yr⁻¹), indicating that the N rates of 62.2kg N ha⁻¹yr⁻¹ and 124.3kg N ha⁻¹yr⁻¹ increased the average annual cumulative N₂O emissions by 39% and 65%, respectively, as compared to the control. The N fertilization had no significant effect on annual oil palm yield (p = 0.994). Alternating between low (deeper than -60cm) and high groundwater level (GWL) (shallower than -60cm) enhanced nitrification during low GWL, further supplying NO₃- for denitrification in the high GWL, and contributing to higher N₂O emissions in high GWL. The emissions of N₂O ranged from 17µg N m⁻² hr⁻¹ to 2447 µg N m⁻² hr⁻¹ and decreased when the water-filled pore space (WFPS) was between 70% and 96%, suggesting the occurrence of complete denitrification. A positive correlation between N₂O emissions and NO₃- at 70-96% WFPS indicated that denitrification increased with increased NO₃- availability. Based on their standardized regression coefficients, the effect of GWL on N₂O emissions increased with increased N rate (p < 0.001). Furthermore, it was found that annual oil palm yields negatively correlated with annual N₂O emission and NO₃- for all treatments. Both nitrification and denitrification increased with increased N availability, making both processes important sources of N₂O in oil palm cultivation on tropical peatland.; and (4) Conclusions: To improve understanding of N₂O mitigation strategies, further studies should consider plant N uptake on N₂O emissions, at least until the completion of the planting

Effect of groundwater level fluctuation on soil respiration rate of tropical peatland in Central Kalimantan, Indonesia

Soil respiration (SR) rate was measured at the burned land (BL), the cropland (CL), the forest land (FL) and the grassland (GL) of a tropical peatland in Central Kalimantan, Indonesia from 2002 to 2011 for the purpose of analysis with a relation to the drying and rewetting. The SR rate was fitted with groundwater level (GWL) to the equation of log(SR) = alpha -beta x GWL using hierarchical Bayesian analysis where alpha and beta were regression coefficients classified by GWL changing directions (drying, rewetting and fluctuating), water-filled pore space (WFPS) ranges in topsoil (low 0-0.54, intermediate 0.54-0.75 and high 0.75-1 m(3) m(-3)), and land uses (BL, CL, FL and GL). SR rate (Mean +/- SD, mg C m(-2) h(-1)) was the significantly largest in the CL (333 +/- 178) followed by GL (259 +/- 151), FL (127 +/- 69) and lastly BL (100 +/- 90). In the CL, the significantly larger SR rate was found in the rewetting period than in the drying period in the high WFPS range. Also, the significantly steeper slope (beta) in the rewetting period was obtained in the high WFPS range than in the drying period. These results suggested that the rewetting of peatland enhanced the SR rate rapidly in the CL, and that the further rise of GWL decreased the SR rate. In contrast, the SR rate in the rewetting period was significantly smaller than in the drying period in the BL in the high WFPS range, because the BL in the high WFPS range was flooded in most cases. The SR rate in the rewetting period was not significantly different from the drying period in the FL and GL. All of beta were significant in the high WFPS range in all land uses, but not in the low-intermediate WFPS ranges, suggesting that GWL was not controlling factor of the SR rate when the GWL was deep due to the disconnection of capillary force under dry conditions. According to the results of correlation analysis of the alpha and beta, the a was significantly correlated with relative humidity, soil temperature and soil pH, suggesting that the a was enhanced by dry condition, high soil temperature and neutralization of soil acidity, respectively. The beta was significantly correlated with exchangeable Na+ and Mg2+ in the soil, but the reason was not clear. In conclusion, SR rate was enhanced by rising GWL with rewetting in the CL in the high WFPS ranges as well as by deepening GWL

Soil carbon dioxide emissions due to oxidative peat decomposition in an oil palm plantation on tropical peat

Soil carbon dioxide (CO₂) efflux was measured continuously for two years using an automated chamber system in an oil palm plantation on tropical peat. This study investigated the factors controlling the CO₂ efflux and quantified the annual cumulative CO₂ emissions through soil respiration and heterotrophic respiration, which is equivalent to oxidative peat decomposition. Soil respiration was measured in close-to-tree ( 3 m, FT) plots, and heterotrophic respiration was measured in root-cut (RC) plots by a trenching method. The daily mean CO2 efflux values (mean ±1 standard deviation) were 2.80±2.18, 1.59±1.18, and 1.94±1.581 μmol m⁻² s⁻¹ in the CT, FT, and RC plots, respectively. Daily mean CO₂ efflux increased exponentially as the groundwater level or water-filled pore space decreased, indicating that oxidative peat decomposition and gas diffusion in the soil increased due to enhanced aeration resulting from lower groundwater levels. Mean annual gap-filled CO₂ emissions were 1.03 ± 0.53, 0.59 ± 0.26, and 0.69 ± 0.21 kg C m⁻² yr⁻¹ in the CT, FT, and RC plots, respectively. Soil CO₂ emissions were significantly higher in the CT plots (P < 0.05), but did not differ significantly between the FT and RC plots. This implies that root respiration was negligible in the FT plots. Heterotrophic respiration accounted for 66% of soil respiration. Annual CO₂ emissions through both soil and heterotrophic respiration were smaller than those of other oil palm plantations on tropical peat, possibly due to the higher groundwater levels, land compaction, and continuous measurement of soil CO₂ efflux in this study. Mean annual total subsidence was 1.55 to 1.62 cm yr⁻¹, of which oxidative peat decomposition accounted for 72 to 74%. In conclusion, water management to raise groundwater levels would mitigate soil CO₂ emissions from oil palm plantations on tropical peatland

Carbon loss from aboveground woody debris generated through land conversion from a secondary peat swamp forest to an oil palm plantation

Palm oil accounts for about 40% of the global demand of vegetable oil. To meet the demand, oil palm plantations have expanded in the humid tropics at the expense of tropical forests. Land conversion begins with clear cutting and generates much woody debris, which was stacked in rows. Woody debris decomposes and emits carbon dioxide (CO2), but the time course of the decomposition is not well understood, especially at the early stage. Thus, we measured carbon (C) stock in woody debris in a newly established plantation after clear cutting of a secondary peat swamp forest in Sarawak, Malaysia. A litter bag method was applied to examine the decomposition of woody debris scattered on the ground. Also, we periodically measured apparent cross-sectional area (ACSA) of a stacking row (about 5 m wide and 90 m long) assuming that the cross-sectional form was triangular. The C stock of the stacking row was estimated from ACSA and measured C fractions using a significant sigmoidal relationship. The decomposition rate constants (k) for C content were determined to be 0.231-0.313 yr-1 for ground woody debris and 0.459 yr-1 for stacked woody debris. In addition, the total decomposition of the aboveground woody debris proceeded according to another k of 0.440 yr-1 during the experimental period of 740 days. The total C stock of aboveground woody debris was 48.4 Mg C ha-1 at the beginning of the field experiment, about 16 months after clear cutting. The C stock accounted for 63% of the C of forest aboveground biomass. Despite the uncertainty in the spatial representativeness, we think that simply measurable ACSA is useful to quantify the C stock of stacked woody debris. The technique could be applicable to large-area estimation using drone technology