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American Journal of Applied Sciences
Year: 2009  |  Volume: 6  |  Issue: 5  |  Page No.: 824 - 828

Organic Matter, Carbon and Humic Acids in Rehabilitated and Secondary Forest Soils

Lee Yit Leng, Osumanu Haruna Ahmed, Nik Muhamad Ab. Majid and Mohamadu Boyie Jalloh    

Abstract: Problem Statement: Tropical rainforests cover about 19.37 million ha (60%) of Malaysia’s total area and about 8.71 million ha can be found in Sarawak, Malaysia. Excessive logging, mining and shifting cultivation contribute to deforestation in Sarawak. The objectives of this study were to: (i) Quantify soil Organic Matter (SOM), Soil Organic Carbon (SOC) and Humic Acids (HA) in rehabilitated and secondary forest soils and (ii) Compare SOM, SOC and HA sequestrations of both forests. Approach: Soil samples were collected from a 16 year old rehabilitated forest and a secondary forest at Universiti Putra Malaysia, Bintulu Campus. Fifteen samples were taken at random with a soil auger at 0-20 cm and 20-40 cm depths. The bulk densities at these depths were determined by the coring method. The bulk density method was used to quantify the total C (TC), Total Organic Carbon (TOC), Organic Matter (OM), Humic Acids (HA) and total N at the stated sampling depths. Results: Regardless of forest soil type and depth, the amount of SOM of the two forests was similar. Except for 20-40 cm of the secondary forest soil whereby the quantity of total C sequestered was significantly lower than that of the rehabilitated forest soil, C sequestration was similar irrespective of forest type and depth. Nevertheless, stable C (organic carbon) sequestered in HA was generally higher in the rehabilitated forest soil compared with the secondary forest soil. This was attributed to higher yield of HA in the rehabilitated forest soil partly due to better humification at 20-40 cm in the rehabilitated forest soil. Conclusion: Hence, the findings suggest that organic C in HA realistically reflects C sequestration in the soils of the two forests investigated.

Table 1). There was significant difference between the pH (1 M KCl) of the rehabilitated and secondary forest soils at 0-20 and 20-40 cm depths.

The soil texture of the rehabilitated forest at 0-20 and 20-40 cm was clay loam. However, the soil texture of the secondary forest at the aforestated depths was sandy clay loam (Table 2). This suggests that the soils of the two forests are typical of Nyalau Series ((Typic Tualemkuts), a series which is characterized by sandy loam in the top soil and sandy clay loam in the subsoil. The soil bulk densities (Table 2) at the two depths of both forests were found to be within the range reported elsewhere.

Irrespective of forest type and depth, there was no significant difference in the percentages and quantities of SOM of the two forests (Table 3). These values were relatively similar to those reported elsewhere[8].

There was no significant difference in the percentages of total C of both forest soils at 0-20 and 20-40 cm (Table 4). In both forest soils, the TC quantity in the top soil was not significantly different from that of the subsoil. However, the quantity of TC in 20-40 cm depth of the rehabilitated forest was higher that of the secondary forest (Table 4).


Table 1: pH of rehabilitated and secondary forest soils
Note: Means within column with different letter(s) indicate significant difference between soil depths and forest types by independent t-test at p≤0.05

The soil total N of the rehabilitated and secondary forests significantly decreased down the soil profile (Table 5) and this observation was consistent with the general observation that soil N decreases with increasing soil depth because of decrease in organic N.


Table 2: Soil textures and bulk densities of rehabilitated and secondary forest soils
Note: Means within column with different letter(s) indicate significant difference between soil depths and forest types by independent t-test at p≤0.05

Table 3: Soil organic matter (%) and corresponding quantities (Mg ha-1) in rehabilitated and secondary forest soils
Note: Means within column with different letter(s) indicate significant difference between soil depths and forest types by independent t-test at p<0.05

Table 4: Total carbon (%), quantity of carbon (Mg ha-1), carbon (%) in HA and quantity of stable carbon (Mg ha-1), in HA in rehabilitated and secondary forest soils
Note: Means within column with different letter(s) indicate significant difference between soil depths and forest types by independent t-test at p<0.05

Table 5: Total N and C/N ratios of rehabilitated and secondary forest soils
Note: Means within column with different letter(s) indicate significant difference between soil depths and forest types by independent t-test at p< 0.05

Table 6: Humic acids yields (%) and corresponding quantities (Mg ha-1) in rehabilitated and secondary forest soils
Note: Means within column with different letter(s) indicate significant difference between soil depths and forest types by independent t-test at p< 0.05

The percentages of HA yields and corresponding quantities in Mg ha-1 of the rehabilitated at 0-20 and 20-40 cm depth were not statistically different. Similar observation was made for the secondary forest (Table 6). However, the percent yield of HA and the quantity of HA in Mg ha-1 at 0-20 and 20-40 cm of the rehabilitated forest soil were significantly greater than those of the secondary forest soil (Table 6). This finding was probably because of lack of N for efficient conversion of biomass C into humus C in the secondary forest soils which is much required by the humification of biomass returned to soil (through litter and roots).

There was significant difference in the quantities of stable C of both forest soils at 0-20 and 20-40 cm, except for secondary forest at the two depths. The quantity of stable C depends on amount of HA. Since the C in HA are more stable, it is more realistic to quantify the amount of C sequestered in forest soils.

The E4/E6 ratios at 0-20 and 20-40 cm of the rehabilitated forest soil were 6.382 and 6.599 respectively, while those of the secondary forest soil at the stated depths were 6.144 and 6.747, respectively (Table 7). This indicates prominence of aliphatic compounds of HA or relatively low molecular weight[9].

Except for the total acidity of the secondary forest, the E4/E6, carboxylic-COOH, phenolic-OH and total acidity of rehabilitated and secondary forest soils (Table 7) were found to be consistent with the ranges reported elsewhere[10]. Higher carboxylic group in HA of the secondary forest soils contributed to higher acidity, probably due to inclusion of amides and esters in the analysis by spectroscopy.

DISCUSSION

The higher pH (1 M KCl) values at 20-40 cm than those of 0-20 cm of the two forests (Table 1) could be attributed to leaching of basic cations from 0-20 to 20-40 cm. However, no such observation was made for pH (water). This may be because the KCl used was more effective in displacing hydrogen ions. The general absence of significant difference between the soil pH of the rehabilitated and secondary forests regardless of soil depth suggests that forest type had no significant effect on the soil pH.

Even though the soil textures of both forests were different, the soil bulk densities of these forests significantly increased down the soil profile. This observation also suggests that regardless of forest type, the soils get compacted down their profiles. Perhaps some of clay in the top soil may have been eluviated vertically and deposited in the subsoil. The absence of significant difference in the soil bulk densities of the rehabilitated and secondary forests irrespective of depth could be partly associated with no significant difference in the SOM of the two forests at both 0-20 and 20-40 cm (Table 3). The similar quantities of SOM irrespective of forest type and depth, suggests that SOM in the rehabilitated forest might have reached equilibrium.

The soil total N regardless of depth and type of forest were typical of Ultisol. The significant accumulation of N at 20-40 cm in the rehabilitated forest soil compared to that of the secondary forest could be attributed to the difference in soil texture. It was possible that the N leached from 0-20 cm got accumulated in 20-40 cm of the rehabilitated forest (clay loam) while in the case of secondary forest (sandy clay loam), it may have been leached out of the soil profile.


Table 7: E4/E6 ratios, carboxylic-COOH, phenolic-OH and total acidity of rehabilitated and secondary forest soils
Tan[9], Schnitzer[10]

The increase in C/N ratio with increasing soil depth in both forests suggests that there was more humification at 0-20 cm than in 20-40 cm. Although the degree of humification at 0-20 cm was observed to be statistically similar for both forest soils, the significant difference observed in the C/N ratios of the of the rehabi rehabilitated and secondary forest soils at 20-40 cm may not necessarily suggest differences in humification levels. The lower C/N ratio of the rehabilitated forest compared with that of the secondary forest could be due to the significant accumulation of N at 20-40 cm as discussed previously.

The SOM and TC sequestered in the rehabilitated and secondary forest soils were similar but the TC sequestered by HA was significantly higher in the rehabilitated forest soil compared to the secondary forest soil irrespective of depth. Hence, the finding suggest that the stability of C in HA realistically reflects C sequestration in this study.

CONCLUSION

The SOM and TC sequestered in the rehabilitated and secondary forest soils were similar but the TC sequestered by HA was significantly higher in the rehabilitated forest soil compared to the secondary forest soil irrespective of depth. Hence, the finding suggest that the stability of C in HA realistically reflects C sequestration in this study. This is partly because the quantity of stable C depends on the amount of HA.

ACKNOWLEDGMENT

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

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