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American Journal of Agricultural and Biological Sciences
Year: 2009  |  Volume: 4  |  Issue: 2  |  Page No.: 123 - 130

Effects of Converting Secondary Forest on Tropical Peat Soil to Oil Palm Plantation on Carbon Storage

Ch`ng Huck Ywih, Osumanu Haruna Ahmed, Nik Muhamad Ab. Majid and Mohamadu Boyie Jalloh    

Abstract: Problem statement: Peat has been identified as one of the major groups of soils found in Malaysia. Sarawak as the largest state in Malaysia has the biggest reserve of peat-land. There are about 1.5 million ha of peat-land in Sarawak, which are relatively under developed. As is the case with any plant, oil palm trees do sequester carbon as they grow. Nevertheless, the process of clearing forest in order to establish a plantation may release carbon. Little studies have been done on the comparison of soil organic matter, soil organic carbon and yield of humic acids when secondary forest on peat soil is converted to oil palm plantation. The objective of this study was to compare carbon storage of secondary forest and early stages of oil palm plantations on a tropical peat soil.
Approach:
Soil samples were collected from the secondary forest, 1, 3, 4 and 5 year old oil palm plantations in Tatau district, Sarawak. Ten samples were taken at random with a peat auger at 0-25 and 25-50 cm depths. The bulk densities at these depths were determined by the coring method. The bulk density method was used to quantify the total carbon, total organic matter, total nitrogen, humic acids and stable carbon at the stated sampling depths on per hectare basis.
Results:
There were no significant differences in the amounts of stable C of both secondary forest and different ages of the oil palm plantations at 0-25 and 25-50 cm soil depth. The amounts of stable C in the secondary forest, 1, 3, 4 and 5 year old oil palm plantations at 0-25 cm depth were generally higher than those in the 25-50 cm depth. This was attributed to higher yield of HA in the secondary forest, 1, 3, 4 and 5 year old oil palm plantations soil partly due to better humification at the 0-25 cm soil depth.
Conclusion:
Conversion of secondary forest on peat to initial stages of oil palm plantation seems to not exert any significant difference on carbon storage in tropical peat soil.

Table 1). The pH of water and 1 M KCl at the depth of 0-25 cm of secondary forest, 1 and 3 year old oil palm plantations were lower than those at the depth of 25-50 cm except for 4 and 5 year old oil palm plantations which showed opposite effect (Table 2).

The soil bulk densities (Table 3 and 4) at the two depths of both secondary forest and oil palm plantations were found to be within the range reported by Andriesse[4]. The bulk densities of 1, 3, 4 and 5 year old oil palm plantations showed no significant difference at 0-25 and 25-50 cm depths except for the secondary forest.

Table 1: pH of secondary forest and oil palm plantations (different ages)
Note: Means within column with different letters indicate significant difference between soil depths by independent t-test p≤0.05

Irrespective of secondary forest, 1, 3 and 5 year old oil palm plantations and soil depths, there were no significant differences in the percentages and quantities of SOM (Table 5 and 6). The percentages and quantities of SOM at 0-25 cm of 3 and 5 year old oil palm plantations were higher than those at 25-50 cm depth.

Table 2: Comparison of pH between secondary forest and oil palm plantations (different ages)
Note: Means within column with different letters indicate significant difference between locations by Tukey test at p≤0.05

Table 3: Bulk density of secondary forest and oil palm plantations (different ages)
Note: Means within column with different letters indicate significant difference between soil depths by independent t-test at p≤0.05

Table 4: Comparison of bulk density between secondary forest and oil palm plantations (different ages)
Note: Means within column with different letters indicate significant difference between locations by Tukey test at p≤0.05

Table 5: Soil organic matter (%) and corresponding quantities (Mg ha-1) of secondary forest and oil palm plantations (different ages)
Note: Means within column with different letters indicate significant difference between soil depths by independent t-test at p≤0.05

Table 6: Comparison of soil organic matter (%) and corresponding quantities (Mg ha-1) between secondary forest and oil palm plantations (different ages)
Note: Means within column with different letters indicate significant difference between locations by Tukey test at p≤0.05

On the other hand, the percentages and quantities of SOM of the secondary forest, 1 and 4 year old oil palm plantations at 0-25 cm were lower than at 25-50 cm depth. These values were typical of Saprists of Sarawak, Malaysia[4].

There were no significant differences in the percentages and quantities of total C of secondary forest, 1, 3 and 5 year old oil palm plantations at 0-25 and 25-50 cm depth (Table 7). The total C at 0-25 cm of 3 and 5 year old oil palm plantations were higher than those of 25-50 cm depth. However, the total C in secondary forest, 1 and 4 year old oil palm plantations at 0-25 cm were lower than that of 25-50 cm depth (Table 8).

Table 7: Comparison of total carbon (%) and corresponding quantities (Mg ha-1) between secondary forest and oil palm plantations (different ages)
Note: Means within column with different letters indicate significant difference between locations by Tukey test at p≤0.05

Table 8: Total carbon (%) and corresponding quantities (Mg ha-1) of secondary forest and oil palm plantations (different ages)
Note: Means within column with different letters indicate significant difference between soil depths by independent t-test at p≤0.05

Table 9: Total N and C/N ratios of secondary forest and oil palm plantations (different ages)
Note: Means within column with different letters indicate significant difference between soil depths by independent t-test at p≤0.05

The soil total N of 1 and 3 year old oil palm plantations significantly decreased down the soil profile (Table 9) On the other hand, there were no significant differences in the total N between the depths of 0-25 and 25-50 cm of secondary forest, 4 and 5 years old oil palm plantations. The percentages of N obtained for the different ages of oil palm plantations were in the range reported elsewhere. There was significant difference in the C/N ratios of the secondary forest and different ages of oil palm plantations at the depths of 0-25 and 25-50 cm (Table 10).

The percentages of HA yields and the corresponding quantities in Mg ha-1 of the secondary forest at 0-25 and 25-50 cm depths were not statistically different. Similar observation was made for the different ages of the oil palm plantations (Table 11). However, the percentage yield of HA and the quantity of HA in Mg ha-1 at 0-25 and 25-50 cm depth of the 3, 4 and 5 year old oil palm plantation were significantly greater than those of secondary forest and the 1 year old oil palm plantation (Table 12).

There were no significant differences in the quantities of stable C of both secondary forest and different ages of oil palm plantations at 0-25 and 25-50 cm (Table 13). The quantities of stable C of the secondary forest, 1, 3, 4 and 5 year old oil palm plantations at the depth of 0-25 cm were generally higher than those in the 25-50 cm although there was no significant difference between the depths (Table 14).

Table 10: Comparison of total N and C/N ratios between secondary forest and oil palm plantations (different ages)
Note: Means within column with different letters indicate significant difference between locations by Tukey test at p≤0.05

Table 11: Humic acids yield (%) and corresponding quantities (Mg ha-1) in secondary forest and oil palm plantations (different ages)
Note: Means within column with different letters indicate significant difference between soil depths by independent t-test at p≤0.05

Table 12: Comparison of humic acids yield (%) and corresponding quantities (Mg ha-1) between secondary forest and oil palm plantations (different ages)
Note: Means within column with different letters indicate significant difference between locations by Tukey test at p≤0.05

Table 13: Carbon in HA (%) and quantity of stable C (Mg ha-1) in secondary forest and oil palm plantations (different ages)
Note: Means within column with different letters indicate significant difference between soil depths by independent t-test at p≤0.05.

Table 14: Comparison of carbon in HA (%) and quantity of stable C (Mg ha-1) between secondary forest and oil palm plantations (different ages)
Note: Means within column with different letters indicate significant difference between locations by Tukey test at p≤0.05.

DISCUSSION

The significantly higher pH (1 M KCl) values at 25-50 cm of the secondary forest and the 1 and 3 year old oil palm plantations compared to those at 0-25 cm soil depth (Table 1) could be attributed to the leaching of basic cations from 0-25 to 25-50 cm. However, no such observation was made for pH (water) whereby the pH ranged from 3.10-3.74 and were in the range reported by Murtedza et al.[16]. This may be because the KCl used was more effective in displacing the hydrogen ions. The significant differences between the soil pH of the secondary forest and the different ages of oil palm plantations regardless of soil depths suggest that different soil management has significant effect on the soil pH. The variations within this range of pH were due to specific locations of peat swamp[4]. According to Andriesse[8], these variations occur in different sections of the peat where the surface layer of the thickest section are lower in pH compared to the shallow organic soils near the edge.

The values of the bulk density of the secondary forest and different ages of oil palm plantations were below 0.5 g cm-3 (Table 3) suggesting that the peats were well decomposed sapric materials[8]. The general absence of significant difference between the bulk densities of the 1, 3, 4 and 5 year old oil palm plantations regardless of depth was because before planting, the soil is usually compacted using machinery. The soil bulk density of the secondary forest was significantly higher at 0-25 than 25-50 cm depth probably because of machinery and other traffic. The absence of significant difference in the soil bulk densities of the different ages of oil palm plantation irrespective of depth could be partly associated with no significant difference in SOM (Table 3 and 4).

Irrespective of secondary forest, 1, 3 and 5 year old oil palm plantations and soil depth, there were no significant differences in the percentages and quantities of SOM within the same depth (Table 5 and 6). This suggests that SOM in the secondary forest, 1, 3 and 5 year old of oil palm plantations have reached equilibrium. The variations of the amount of SOM between the two different depths of the different ages of oil palm plantation could be due to mixing and compaction process usually carried out by the management of oil palm plantation during forest clearance for planting of the oil palm plants. This may have led to the uneven decomposition rate of organic materials between the two depths.

There were no significant differences in the percentage and quantities of total C of secondary forest 1, 3 and 5 year old oil palm plantations within 0-25 and 25-50 cm depths (Table 7 and 8). This observation could be ascribed to the absence of significant differences in the percentage and quantities of SOM within the 0-25 and 25-50 cm depths of the forest, 1, 3 and 5 year old oil palm plantations soils. This finding is partly consistent with the observation that SOM is a major source and sink of atmospheric C in the global C cycle[17]. The TC in the secondary forest, 3 and 5 years old oil palm plantations at of 0-25 cm depth was higher than at 25-50 cm depth.

This pattern is associated with deep organic soils due to large content of ligneous materials in oligotrophic Histosols[16]. However, the quantity of TC in 25-50 cm depth of the 1 and 4 year old oil palm plantations was higher than at 0-25 cm depth.

The soil total N of the 1 and 3 year old oil palm plantations significantly decreased down the soil profile. On the other hand, there was no significant difference in the total N between the 0-25 and 25-50 cm depths (secondary forest, 4 and 5 year old oil palm plantations). However, the soil total N of all the different ages of oil palm plantations at the 25-50 cm depth was generally lower than at 0-25 cm depth. This observation was consistent with the general observation that soil N decreases with decreasing soil depth because of decrease in organic N. The soil total N of the secondary forest at 25-50 cm depth was higher than at 0-25 cm depth which could be due to the leaching of N from 0-25 cm and accumulation in 25-50 cm depth (Tables 9 and 10).

The increase in C/N ratio with increasing soil depth in the secondary forest and different ages of oil palm plantations suggests that there was more humification at 0-25 cm than in 25-50 cm depth. The lower C/N ratio of the secondary forest compared to the different ages of the oil palm plantation could be due to the significant accumulation of N at 25-50 cm depth as discussed previously.

The percentages of HA yield and corresponding quantities in Mg ha-1 of the secondary forest at 0-25 and 25-50 cm depths were not statistically different. Similar observations were made for the different ages of oil palm plantations (Table 11). However, the percentages of yield HA and the quantity of HA in Mg ha-1 at 0-25 and 25-50 cm of 3, 4 and 5 year old oil palm plantations were significantly greater than those of secondary forest and the 1 year old oil palm plantation (Table 12). This finding was probably because of low N for efficient conversion of biomass C into humus C in the secondary forest and the 1 year old oil palm plantation, a process required for humification of biomass.

Table 15: Comparison of ranges of phenolic-OH, carboxylic, total acidity and E4/E6 ratio of HA of secondary forest and different ages of oil palm plantations with related reports
Tan[18], Schnitzer[19]

There was no significant difference in the quantity of stable C for both the secondary forest and different ages of oil palm plantation at 0-25 and 25-50 cm soil depth (Table 13 and 14). This shows that conversion of secondary forest to oil palm plantations at initial stages (till 5 years old) does not exert any difference in the amount of C sequestered in the peat soil. Since the C in HA is more stable[20], it is more realistic to quantify the amount of C sequestered upon the conversion of secondary forest on peat to oil palm plantations at initial stages.

The relatively high E4/E6 values in the secondary forest and different ages of oil palm plantations indicate prominence of aliphatic components or the HA in this study were of low molecular weights[18,21]. The effectiveness of washing the HA with distilled water is to indicate its purity without altering its chemical characteristics. The total acidity, carboxylic-COOH and phenolic-OH of the secondary forest and different ages of oil palm plantations (Table 15) were found to be consistent with the ranges reported by other researchers[21].

CONCLUSION

Conversion of secondary forest on peat to initial stages of oil palm plantation seems to not exert any significant difference on carbon storage in tropical peat soil.

ACKNOWLEDGEMENT

The authors acknowledge the financial support (Fundamental Research Scheme) of this research received from the Ministry of Higher Education, Malaysia via University Putra Malaysia.

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