SOIL ORGANIC CARBON DYNAMICS IN SOME SELECTED FOREST AREAS OF BANGLADESH

Abstract

Forests are important ecosystems because of its association with the global energy balance. In this study, soil samples were collected from different locations of some important forest areas of Bangladesh to estimate the relationship between soil organic carbon (SOC) dynamics and some physical and chemical properties of soils. Quantification of SOC stock and measurement of forest biomass carbon density were also done at the same forest areas. The forest areas under study were the hill forest of Chittagong and Sylhet, the sal forest, the Sundarban mangrove forest, and the coastal afforestation areas in Bangladesh. A total of eighteen locations were selected for this study and each location had three plots. The plots were selected based on a simple random sampling method. Each plot was delineated by an 8.92-meter radius circular area. In this plot, all trees having a diameter at breast height (DBH) from >5 cm were measured. Several subplots were established within each main plot for specific purposes. Inside the 8.92-meter radius plot, a subplot with a 5.64-meter radius was established for measuring saplings and seedlings having a DBH from 1‒5 cm. A subplot with a 0.56-meter radius within the main plot was laid out for collecting soil samples from different soil depths, and for collection of leaf litter, herb, and grass. The texture of the soils was found mostly sandy loam to sandy clay loam and occasionally with loamy sand and clay loam in the hill forest of Chittagong and Sylhet, and in the sal forest areas. Sand was found to be the dominant fraction in the soils of these areas and most of the soils developed predominantly from unconsolidated sandy parent materials. On the other hand, the texture of the soils in Sundarban mangrove forest and the coastal afforestation areas was found to be loam. The concentration of SOC was found to increase significantly (p<0.05) with increasing sand fraction in the Chittagong hill forest and coastal afforestation areas and showed a positive correlation whereas Sylhet hill forest showed negative correlation, but the relationship was found highly significant differences (p<0.001) between SOC concentration and sand fraction. On the other hand, the relationship between SOC concentration and silt fraction showed positive correlation and highly significant differences (p<0.001) in the Sylhet hill forest and Sundarban mangrove forest areas whereas Chittagong hill forest and Coastal afforestation areas showed negative correlation and significant differences (p<0.01 and p<0.05). No significant xiii differences were observed between SOC concentration and silt fraction in sal forest areas, but it was found slightly positively correlated with clay fraction. Soil moisture content (MC) was found to vary among different soil depths and the forested site contained higher MC than the adjacent homestead site across the different locations under study. The relationship between SOC concentration and MC showed a positive correlation in Sylhet hill forest, whereas in other areas negative correlation was seen. A significant (p<0.05) relationship was found between SOC concentration and MC in the different forest areas. Soil bulk density (BD) was found relatively higher in homestead sites than in the forested sites and it tended to increase with increasing soil depth in all the studied areas. It could be attributed to the greater compaction in the lower depth of soil related to time. On the other hand, a reverse trend was observed in SOC concentration in the study area. The relationship between SOC concentration and BD showed a negative correlation in all the forest areas except in the Sundarban mangrove forest. Soil pH, total nitrogen (total N) and electrical conductivity (EC) were also observed higher in the forested sites than in the homestead sites and significant differences were found between different soil depths in all the forest areas. The relationship between SOC concentration and total N showed a positive correlation, whereas pH and EC showed a negative correlation with SOC concentration. The mean value of soil organic carbon (SOC) stocks up to 1 m (0−100 cm) soil depth was found to be higher in the forested site of the Sundarban mangrove forest (103 t ha-1, correspond to 379 t ha-1 of soil CO2 mitigation density) followed by the Chittagong hill forest (99 t ha-1, correspond to 363 t ha-1 of soil CO2-mitigation density) and lower in the Sylhet hill forest and sal forest (60 t ha-1, correspond to 220 t ha-1 of soil CO2 mitigation density). On the other hand, the homestead site of the Sylhet hill forest (84 t ha-1, correspond to 307 t ha-1 of soil CO2 mitigation density) and sal forest areas (62 t ha-1, correspond to 229 t ha-1 of soil CO2 mitigation density) stored higher amount of SOC stocks compared to their forested sites. Among the locations, Goneshpara, Thanchi of Chittagong hill district contained higher SOC stocks in both forested (164 t ha-1) and homestead (143 t ha-1) sites followed by the forested site of Bogi Forest Beat (132 t ha-1) in the Sundarban mangrove forest, Sonarchar, Rangabali (96 t ha-1) in the coastal afforestation areas, Lawachara National xiv Park (83 t ha-1) in Sylhet hill forest and Dokhola Forest Range (74 t ha-1) of Madhupur sal forest and lower in Tilagarh Eco Park (23 t ha-1) of Sylhet hill forest. On the contrary, the homestead site of the Tilagarh Eco Park (104 t ha-1) and the Kotbari sal forest (73 t ha-1) areas stored higher amount of SOC stocks compared to their corresponding forested site. Highly significant differences (0<0.001) in SOC stocks were observed in both forested and homestead sites among the locations in all the forest areas. Total biomass density (TBD) was found higher in the Chittagong hill forest (555 t ha 1 ) followed by the Sylhet hill forest (537 t ha-1), whereas the coastal afforestation areas exhibited lower TBD (284 t ha-1). On the other hand, total carbon density (TCD) and total CO2 mitigation density was found higher in the Chittagong hill forest (378 t ha-1 and 1387 t ha-1) followed by the Sylhet hill forest (338 t ha-1 and 1241 t ha-1), while the coastal afforestation areas showed lower TCD (227 t ha-1) and total CO2 mitigation density and (834 t ha-1). Total carbon density (TCD) in Sundarban mangrove forest areas yielded lower amount compared to Bangladesh Forest Inventory (BFI) report-2020 because the study was conducted sporadically in three locations with each having three plots; BFI report included the whole areas of Sundarban. The study results also revealed that tree biomass carbon (CAGB and CBGB) in all the forest areas contributed almost 61−79% of total carbon density (TCD), whereas SOC stocks contributed about 18−35% and the rest of the carbon in leaf litter, herb, and grass (CLHG) and in saplings biomass. As for the individual tree biomass carbon (both AGB and BGB), tree species of Lohakat (Xylia xylocarpa) contained the highest tree biomass carbon (134.12 t ha-1) followed by Pine (Pinus caribea) and the value was 58 t ha-1. In case of the individual sapling biomass carbon, Rong (Morinda angustifolia) and Goran (Ceriops decandra) contained the maximum (0.96 t ha-1) and minimum (0.03 t ha-1) amounts. The overall findings of this research would be useful to policymakers, environmental activists, researchers, and academicians at national and international levels. It is imperative to develop a legacy of sustainable forest and land resources management policy that will protect future generations.