Long-term effect of rice-wheat cropping system on cobalt in relation to manganese and iron content in coarse textured calcareous alluvial soils

Abstract

Deficiency of Cobalt (Co) in coarse textured soils leads to its deficiency in plants and animals affecting their productivity. As large area of coarse textured calcareous alluvial soils in the Indian Punjab has been brought under irrigated transplanted rice cultivation in rotation with wheat, submerged/reduced soil conditions tend to favor leaching of Co, leading to its deficiency. Objective of this study is to understand the changes with time, in transformation and leaching of Co in relation to that of Mn and Fe, and also the possibility of Co deficiency when these fragile leaky soils are used to grow rice and wheat in an annual rotation on a long-term basis. Available (AA) (2.5% acetic acid, pH 2.5 extractable) and oxide (OX) (0.2 M ammonium oxalate + 0.2 M oxalic acid, pH 3.0 extractable) fractions of Co, Mn and Fe were estimated in 102 surface (0-15 cm) soils and 34 soil profiles under rice-wheat cultivation for 5 to 20 years in contiguous area of coarse textured calcareous alluvial soils in Ludhiana, Ferozepur, Faridkot and Kotkapura blocks in the Punjab. Available Co content ranged from deficient level of 0.01mgkg-1 to adequate level of 0.76mg kg-1 with mean content of 0.26, 0.28, 0.32 and 0.30 mg kg-1 and a corresponding 25, 42, 47 and 42% Co deficient soils in Ludhiana, Ferozepur, Faridkot-Kotkapura and all the blocks, respectively. It followed the pattern of decrease in the content of OX-Co, -Mn and -Fe and increase in CaCO3content and electrical conductivity in these districts. In these soils, OX-Co and OX-Mn coexist (r = 0.38L and r = 0.628F) in the oxide components but not with the OX-Fe (r = -0.437L and r = 0.001F) fraction. The OX-Mn also regulates both available Co (AA-Co, r = 0.697L) and AA-Mn (r = 0.705L). The CaCO3 content in the soil controlled both OX-Mn and OX-Co (r = 0.282L and r = 0.661L) as well as, of AA-Mn and AA-Co (r = 0.58L and 0.52L, 0.43F) fractions. Thus, Co fractions are largely associated both with the soil Mn-oxide and CaCO3 constituents. But in the soils of Ferozepur block, organic carbon negatively regulates OX-Mn (r= -0.43F), OX-Co (r = -0.42F) and positively both OX-Fe (r =0.35F) and AA-Fe (r = 0.59F) fractions. Long term effects of 5-20 years pertaining to alternate reduced and oxidized soil conditions under irrigated transplanted rice-wheat cultivation caused leaching losses of the oxides of Co, Mn and Fe, as well as that of AA-Co, -Mn and -Fe fractions from the surface soil horizons to sub-surface horizons.