SOIL DEGRADATION DURING THE PERIOD 1967-1995 IN BANGLADESH .1. CARBONAND NITROGEN

Long-term changes in carbon and nitrogen storage in the top 100 cm lay ers of soils from different physiographic units in Bangladesh were eva luated using 460 soil samples from 43 profiles collected in 1967 and 1 994/95. The study area consisted of ten physiographic units, viz.: Old Himalayan Piedmont Plain (OHP), Tista Floodplain (TF), Barind Tract ( BT), Ganges Floodplain (GF), Madhupur Tract (MT), Brahmaputra Floodpla in (BF), Meghna River and Estuarine Floodplain (MF), Surma-Kushiyara F loodplain (SKF), Northern and Eastern Piedmont Plain (NEP), and Chitta gong Coastal Plain (CCP). During the period 1967-1995, mean values of soil carbon and nitrogen declined by 16.2 t C ha(-1) and 1.38 t N ha(- 1) in Bangladesh. The total carbon and nitrogen budgets showed a fall of 42.8X10(6) t C and 3.36X10(6) t N within the 27 y period in Banglad esh. All the physiographic units showed a decline in carbon and nitrog en contents except for BT which showed an increase of 3.76 t C ha(-1) and 0.77 t N ha(-1) for these elements. MT showed the highest decline in carbon (30.5 t C ha(-1)) and nitrogen (3.25 t N ha(-1)). The lowest decline in carbon was observed in SKF (7.18 t C ha(-1)) while the sam e applied to nitrogen in GF (0.50 t N ha(-1)). The other physiographic units showed a moderate decline in the contents of these elements. Ba sed on the land area occupied by the sampled soil series of each physi ographic unit, GF showed the highest decline of 13.7X10(6) t C and a m oderate decline of 0.52X10(6) t N. Decrease in the carbon level for BF was moderate (9.31X10(6) t C) but the decline in the nitrogen level w as the highest (0.95X10(6) t N). Decrease in the amount of carbon and nitrogen stored for the other physiographic units was: OHP (0.67X10(6) t C, 0.03X10(6) t N), TF (2.54X10(6) t C, 0.25X10(6) t N), MT (4.28X1 0(6) t C, 0.45X10(6) t N), MF (6.06X10(6) t C, 0.61X10(6) t N), SKF (2 .87X10(6) t C, 0.24x10(6) t N), NEP (2.68x10(6) t C, 0.22X10(6) t N), and CCP (0.81X10(6) t C, 0.11X10(6) t N). Increase in carbon and nitro gen contents was only observed in BT (0.08X10(6) t C, 0.02X10(6) t N).