Soil carbon pools and dynamics in the Sal forests of Bangladesh

Abstract

Carbon (C) concentrations in different Sal forests of Bangladesh have not yet been estimated to explore their C storage potential. The present study compared the C pools among three selected Sal forests located in Madhupur, Lalmai and Singra to examine the effect of management intensity on C stocks. A total of 19 plots (each 10 m × 10 m for tree, 5 m × 5 m for shrubs and 0.5 m × 0.5m for herbs in size), 9 from Madhupur forest and each 5 from the other two forests, were established to collect data on plant composition, plant DBH and organic C concentrations. Allometric method was used to estimate C concentrations for both aboveground and belowground parts of juvenile and adult tree plants of the selected plots. Wood biomass was converted to C content by multiplying the value with the van Bemelen factor (1.724). Litter biomass C, soil organic C and fine root C were determined during winter (December) and summer (May) of 2016. Seasonal variation in culturable soil bacterial colony counts was also studied in three replicated plots selected from Madhupur Sal forest. The results showed that woody C concentration was significantly (p = 0.0156) higher in Lalmai (552.46±64.09 t/ha) and Singra forests (547.13±62.43 t/ha) than in Madhupur Sal forest (354.03±38.43 t/ha). The mean DBH of Sal tree at breast height was significantly (p = 0.022) higher in Lalmai (38.84±2.12 cm) and Singra (37.36±1.97 cm) forests than in Madhupur forest (30.72±1.93 cm). On the other hand, fine root C and soil C were determined at three different depths: 0-10 cm, 10-20 cm and 20-30 cm. The litter and fine root dry biomass were converted to C content and expressed as tonnes C per hectare area. Soil C content was calculated using the bulk density method. The decomposition of Sal leaf litter and seasonal changes in bacterial communities were studied to explain the observed temporal changes in biomass C content in the study area. The decomposition rate (mass loss and N mineralization rate) of Sal leaf litter was studied by incubating leaf litter with soil collected from three selected Sal forests following a reciprocal experimental design where each Sal leaf litter was incubated with soil collected from three Sal forests. The number of bacterial colonies was compared in summer, monsoon, late autumn and winter seasons in a plot of Madhupur Sal forest. Litter biomass C was significantly higher in May compared to December, although the effect of forest was not significant. Fine root biomass C was significantly affected by forest type (p < 0.0001) and depth (p < 0.0001) where the maximum amount was found in Madhupur sal forest and the lowest in Lalmai Sal forest and 0-10 cm depth showed the highest value among the three selected Sal forests. Soil C content was significantly affected by both forest type (p < 0.0001) and depth (p < 0.05) where the value was higher in Singra National Park, followed by Madhupur Sal forest and Lalmai Sal forest, however, the highest value was recorded at 10 cm depth, followed by 20 cm and 30 cm depth. Nitrogen mineralization rate was significantly affected by soil type and the mean value was higher in Lalmai Sal forest. For fast-growing bacteria, the number of small- sized colonies was higher in late autumn but lower in winter, while the number of large-sized colonies was higher in winter but lower in late autumn. For slow-growing bacteria, the number of smallsized colonies was higher in late autumn but lower in winter, while the number of large-sized colonies was higher in winter but lower in late autumn. This study suggests that long-term forest conservation may be an effective method for increasing C stocks in forest ecosystems. Result also suggest to consider assessment of C stocks of temporal scale for more accurate estimation of C dynamics.