Abstract
The present study was made to determine the kind and degree of degradation and its impact on land, soils, socio-economic and environmental conditions in the deforested mangrove of Chakaria Sundarbans over time. Primary data have been generated on land and soil of the area through semi-detailed soil survey, GIS techniques and soil chemical analysis. Review of secondary data on land and soil has been done and tertiary data have been generated. The primary, secondary and tertiary data have been carefully interpreted to determine the states of degradation and its impact. Time series land use maps, starting from the base year of 1974 to 2008 have been generated through field survey and interpretation of aerial photographs of 1974, 1984, 1994 and land sat image of 2008. 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. Some physical parameters and nutrient elements were analyzed in the study area for assessing long term changes (1973 to 2008) in physical, chemical and physico-chemical properties of soil. Analysis of nutrients has been made for determining soil. fertility changes during 1990 to 2008. The most prominent and significant changes have been observed in land use. In 1974 the forest was almost virgin and there was no land under shrimp culture. It increased to about 26% of the study area in 1994 and slightly declined to 21% in 2008. Agricultural land shrunk from 36% in 1974 to 7% in 2008. (Salt farming increased from 2% to 23% in 2008. Forest cover was almost cleared in 1994. Major changes in land use occurred during 1984 to 1994. Change in land capability class occurred in Chakaria and Kutubdia soil series due to increase in soil salinity and exposure of jarosites, which comprises about 30% of the study area. No change was observed in land capability class in the rest four soil series, i.e. Badarkhali, Barabakia, Cheringa and Pahartali. Marked decrease in clay content was observed in top soil of Badarkhali, Chakaria and Pahartali soil series. Over three and half decades, pH decreased in top soils, subsoils and substratum by 0.8, 1.35 and 0.44 unit, respectively. Salinity increased by 33.3, 15.5 and 24.1 dScm1 in the top soil, subsoil and substratum respectively. Average depletion of total carbon (TC) from the root zone (0-15 cm depth) was observed to be 5.68 MTha1 and from total depth of soil (0-100 cm depth) 21.67 MTha1. Average depletion of total nitrogen (TN) was 0.19 MTha1 from top soils and 1.05 MTha1 from the whole profile (0- 100 cm). The C/N ratio of the top soil (0-15) ranged from 8.85 to 12.42 in 1973 which came down to 7.69 to 10.64 in 2008. The annual depletion of calcium in the top soil, subsoil, substratum and the whole profile were 27.76 kg, 53.61 kg, 59.51 kg and 145 kg ha''yr'1 respectively. The average annual increase in magnesium in the top layer was 87 kg ha'. CEC increased from 15.0 me100g1 soils in 1973 to 18.17 me100g1 soils in 2008 in the top soil.) Average annual depletion of organic matter was 236 kg ha'yr'. Annual losses of nitrogen and phosphorus were 9.65 and 1.25 kg ha'yr'' respectively. Unlike the nitrogen and phosphorus, sulphur and boron increased by many folds. Average increment of sulphur and boron was about 99 and 0.35 kg ha 'yr' respectively. Annual depletion of zinc and potassium was 0.64 and 41.44 kg hayr1 respectively. Socio-economic observation shows that shrimp culture and salt cultivation are more profitable than agriculture. On the other hand, local people, specially the traditional fishermen have lost their income sources. In general, depletion of soil fertility is mainly due to intensive exploitation of land for salt production, shrimp culture and agriculture without proper replenishment. 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.