DEGRADATION IN SOIL CHEMICAL PROPERTIES OF SIX SOIL SERIES OF CHAKARIA SUNDARBANS MANGROVE FOREST IN BANGLADESH

A comparative semi-detailed soil survey was made during January to July 2012 to assess the impact of deforestation on changing soil chemical properties in six soil series of Chakaria Sundarbans mangrove forest since 1973. Soil profiles are described in the field following USDA Soil Taxonomy. Soil physical and chemical analyses have been made following conventional soil analysis methods and as followed during Reconnaissance Soil Survey (RSS) carried out in 1973. Six identified soil series viz. Badarkhali, Barabakia, Chakaria, Cheringa, Kutubdia & Pahartali have been considered for the study. Changes in soil pH decreased during the period 1973-2012 in the study area. The increases in exchangeable acidity (acid sulphate soil) and the depletion of exchangeable Ca may account for this general decline in soil pH. Salinity increased in the study area tremendously and EC increased by 33.32, 15.51 and 24.09 dS/cm in the top soil, sub soil and substratum respectively over the years. The highest depletion of TC of 21.16 Mt/ha in top soil of Badarkhali soil series was observed and lowest in Chakaria soil series and was 1.04 Mt/ha. Average depletion of TN was observed as 0.19 Mt/ha from top soils and 1.05 Mt/ha from whole profile (0-100 cm) during the periods. The highest depletion of TN occurred in Badarkhali soil series (1.04 Mt/ha). The lowest TN was in Barabakia and Kutubdia soil series. C/N ratio was lower in top soils of all the soil series. The C/N ratio of the top soil (0-15) ranged from 8.85 to 12.42 in 1973 which ranged from 7.69 to 10.64 in 2012. The highest reduction of C/N ratio occurred in Badarkhali soil series and the lowest in Cheringa soil series. The lowest fall in the amounts of total carbon and nitrogen stored was observed in the acid basin clay soils of Chakaria soil series. This area also showed the highest total carbon contents.  The annual depletion of Average exchangeable calcium (Ca) in the top soil (0-15 cm depth), subsoil (15-30 cm depth), substratum (30-100 cm depth) and the whole profile (0-100 cm depth) were as 27.76 kg, 53.61 kg, 59.51 kg and 145 kg/ha/year respectively. Unlike other elements exchangeable magnesium (Mg) content in all the soil series under study area increased significantly. The average increase in the top layer (0-15 cm depth, i.e., cultivable layer) is about 87 kg/ha/year. The highest accumulation of 144.75 kg/ha/year was observed in the top soil of Chakaria soil series and the lowest of 21.71 kg/ha/year was in Barabakia soil series. In the study area the average CEC in 1973 was 15.0 meq/100g soils which were 18.17 meq/100g soils in 2012 in the top soil. In the subsoil it was 16.67 meq/100 g soils in 1973 and 19.0 meq/100 g soils in 2012 and in the substratum (30-100 cm depth) it was 17.00 meq/100 g soils in 1973 and 20.17 meq/100 g soils in 2012.  The findings of this study reveal that the degradation process of land and soils is in progress over time and indicates a need to reverse this trend.