The geochemistry of calcareous forest soils in Sulaimani Governorate, Kurdistan Region, Iraq

Abstract

The properties of and processes in calcareous soils are important in the context of dryland soil management and moderation of atmospheric CO2 by carbonate weathering. This study was conducted to determine the major and trace elemental composition in soils of the Kurdistan region of Iraq, and their geochemical features. Soil samples were taken from seven pedons, five of them from forest soils and two others from non-cultivated soils. Selected physical properties, total concentrations of elements and mineralogical properties were determined on samples from each horizon. Weathering processes in the soils are characterised by loss of Ca, Sr and several trace elements, and accumulation of less mobile elements such as Zr, Ti, and Al. The progress of carbonate weathering was best expressed using Ba/Sr and Rb/Sr ratios. Principal components analysis confirmed that distinctive geochemical differences for two profiles (significantly higher Mg, Co, Cr, Mn, and Ni) were likely to represent control by parent material, a conclusion reinforced by both mineralogical differences and the location of profiles relative to surficial geology. The other main geochemical processes differentiating the pedons were chemical weathering of carbonates and accumulation of residual elements. Calculation of absolute mass flux (τ) values showed that, for some profiles, concentration of mobile elements such as Na and K by evapotranspiration was potentially important, despite evidence for clay eluviation in most profiles. Absolute loss of several elements was also correlated with increased weathering as measured by Ba/Sr ratios. None of the forest soils showed enrichment-depletion (τ) profiles for Fe or Al consistent with podzolisation. This study highlights useful geochemical approaches to understanding chemical weathering and pedogenesis in calcareous soil profiles.