Soil degradation during the period 1967-1995 in Bangladesh .1. Carbon and nitrogen

Long-term changes in carbon and nitrogen storage in the top 100 cm layers of soils from different physiographic units in Bangladesh were evaluated using 460 soil samples from 43 profiles collected in 1967 and 1994/95. 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 Floodplain (BF), Meghna River and Estuarine Floodplain (MF), Surma-Kushiyara Floodplain (SKF), Northern and Eastern Piedmont Plain (NEP), and Chittagong 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 Bangladesh. All the physiographic units showed a decline in carbon and nitrogen 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 same 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. Based on the land area occupied by the sampled soil series of each physiographic unit, GF showed the highest decline of 13.7X10(6) t C and a moderate 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 was 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 (4.28X10(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 nitrogen contents was only observed in BT (0.08X10(6) t C, 0.02X10(6) t N).