Abstract
With the advancement of time, the land use as well as soil properties in Bangladesh has been undergone some changes, which the present study addressed. This has been assessed through semi-detailed soil survey, GIS techniques and laboratory analysis of soil samples of six Upazilas (sub unit of District) under Barind Tract, Brahmaputra Floodplain and Ganges River Floodplain.
In 1970, Level Barind Tract under Shajahanpur Upazila the dominant cropping pattern was Fallow - T. Aman rice (59%) which changed to Boro rice - Fallow - T. Aman in 1990 covering 61%
area that further increased to 71% in 2015 followed by T. Aman rice (69%). For High Barind Tract
under Chapai Nawabganj Sadar Upazila, the area was greatly covered with grasses or barren accounting 54% in 1970 which little changed in 1990 due to inclusion of Boro rice (8%), that was
doubled in 2015. In Brahmaputra Floodplain, the pattern was mostly Fallow - Aus rice or Jute - т.
Aman rice in 1970 comprising 51% area, later in 2015 cropping pattern was diversified dominating Boro rice - Fallow -T. Aman rice (51%). In 1970, Ganges River Floodplain was largely
used for rabi crop/Fallow - Mixed Aus and B. Aman rice accounting 44% area coverage, then in
1990, cropping sequence became as Boro rice - Fallow - T. Aman rice (21%). In 2015, Boro rice
coverage increased to 37% area. Top soil properties were critically evaluated between 1990 and
2015 in three physiographic units. Acidification increased with decline of average pH from 6.20
to 5.69 (0.51) in Barind Tract, 5.78 to 5.33 (0.45) in Brahmaputra Floodplain whereas alkalinity
decreased with decline of average pH from 8.08 to 7.44 (0.64) in Ganges River Floodplain during
the period 1990 to 2015. Soil organic matter concentration slightly increased with an average
value of 1.65 to 1.90% in Barind Tract, 1.35 to 1.90% in Brahmaputra Floodplain and 1.54 to 1.98%
in Ganges River Floodplain from 1990 to 2015. Available P concentration of soil in Brahmaputra Floodplain greatly went down to 12.7 from 21.5 mg kg-1 and almost remains stable in Barind
Tract (13.8 to 12.7 mg kg-¹) and Ganges River Floodplain (10.1 to 9.4 mg kg-¹). Available K
concentration of soil declined by 9%, 40% and 29% in Barind Tract, Ganges River Floodplain and Brahmaputra Floodplain respectively, within last 25 years (1990-2015). Available S concentration of soil in Barind Tract, Brahmaputra Floodplain and Ganges River Floodplain decreased from 18.4 to 15.1, 15.3 to 9.8 and 17.6 to 13.8 mg kg-1¹ soil, respectively, between 1990 and 2015. Exchangeable Ca concentration decreased in Barind Tract while it slightly increased both in Brahmaputra Floodplain and Ganges River Floodplain. However, uneven changes were observed in exchangeable Mg concentration in studied three physiographic units. Available Zn concentration in Barind Tract, Brahmaputra Floodplain and Ganges River Floodplain gradually decreased from 2.38 to 1.38, 1.73 to 1.50 and 1.19 to 0.39 mg kg-¹ soil, respectively. Soil profiles
were studied for seven selected soil series (at same spot of RSS, 1970) in the studied area for determining changes (1970 to 2015) at profile level. Among the studied soil profiles, the major changes were observed in very young soil of which human intervention with vegetation over the time enhanced the soil formation and changed Brahmaputra Sandy Alluvium soil to Melandaha
series. A rapid soil formation was observed in calcareous Ganges River Floodplain with an enlargement of B horizon of Ishurdi series from 21 to 41 cm (20 cm). Contrary, B horizon of Dhamrai series under non-calcareous Brahmaputra Floodplain was enlarged to 16 cm (28 to 44 cm) between 1970 and 2015. Most of the profile became greyer (2015) from brown to olive grey (1970) due to remain prolonged reduced condition for Boro rice cultivation. Clay eluviations was observed in the surface horizons of all the soils with the accumulation in subsoil. There was an accumulation of SOC in surface horizon (A) of most of the soil profiles. Depletion of total nitrogen (TN) was observed in most of the soil series. Soil pH of A horizon was observed to decrease. The exchangeable Ca also decreased however exchangeable Mg concentration and Cation Exchange Capacity (CEC) were mostly increased in all the horizons of studied profiles from 1970 to 2015. Intensification with irrigated rice based cropping system with high yielding crop varieties over time promotes nutrient removal from top soil and prolonged submergence with soil and crop management issues including soil forming factors also enhances clay eluviation, accumulation of SOC and decrease of TN, pH and exchangeable Ca in the surface horizon. These changes of paddy soil biogeochemistry claims further attention to maintain sustainable soil fertility and crop production.