Concentration and distribution of extractable elements in a soil as affected by tillage systems and fertilization

Because of crop production intensification, we hypothesize that concentrations of trace elements will increase and stratify in the soils of the Pampean region. We studied the effects of conventional and zero-tillage and phosphorus fertilization on concentration and stratification of trace elements in a typical soil of the area. The experiment was conducted at the Pergamino Agricultural Experimental Station (33 degrees 56' S; 60 degrees 34' W). There were three treatments: (i) pasture; (ii) conventional tillage (CT); and (iii) zero tillage (ZT). The crop sequence was wheat, soybean and corn. An annual average of 90 kg N/ha and 18 kg P/ha was applied. Two composite samples of each plot were obtained in 5-cm increments until 20 cm depth. The soil was characterized and aluminum (Al), arsenic (As), cadmium (Cd), chromium (Cr), cobalt (Co), iron (Fe), lead (Pb), mercury (Hg), manganese (Mn), molybdenum (Mo), nickel (Ni), selenium (Se) and zinc (Zn), were extracted with DTPA and boron (B) was extracted with hot water. All the element concentrations were determined with ICP-AES. Concentrations of B (av. 0.48 mg/kg), Cd (av. 0.20 mg/kg) Cr (< 0.01 mg/kg), Hg(< 0.005 mg/kg), Mo(< 0.01 mg/kg), and Se (av. 3.33 mg/kg) showed no significant differences between tillage treatments. Copper (max, value 1.46 mg/kg), Fe (max. value 76.27 mg/kg), Mn (max. value 10.48 mg/kg) and Ni (max. value 0.65 mg/kg) concentrations were higher under CT but Zn (max. value 2.03 mg/kg), Co (max. value 0.34 mg/kg) and Pb (max. value 0.94 mg/kg) showed higher concentration under ZT on topsoil. As compared with the tillage treatments, the pasture soil showed lower concentrations of Co (max. value 0.16 mg/kg), Cu (max. value 1.10 mg/kg), Ni (max. value 0.36 mg/kg) and Pb (max. value 0.66 mg/kg) but higher As (max. value 0.09 mg/kg), Fe (max, value 83.55 mg/kg), Mn (max, value 13.78 mg/kg) and Zn concentrations. For some trace elements (Co, Pb and Zn) the extractable concentration and stratification was linked to OM and pH. For other elements the behavior observed in our experiment could not be explained. Surface concentrations of Co, Cu, Ni and Pb was higher in cropped soils and this would be accreditable to fertilizer applications. (C) 1999 Elsevier Science B.V. All rights reserved.