Changes in forms and distribution pattern of soil iron oxides due to long-term cropping in the Northwest of Iran

To assess effects of long-term agricultural practices on iron (Fe) oxide forms, 12 soil pedons belonging to six soil subgroups from the cultivated soils and the adjoining uncultivated soils were described and sampled. Soil samples were analyzed for physicochemical properties, mineralogical compounds, and different status of Fe oxides including free Fe oxides (Fe_d), crystalline Fe oxides (Fe_cry), poor crystalline Fe oxides (Fe_o), and organic complex Fe (Fe_p). For most of the examined soils, long-term cropping caused a considerable drop in organic carbon (12–64 %), electrical conductivity (3–18 %), exchangeable and soluble cations (2–96 %), and calcium carbonates (20–52 %) along with a noticeable rise in the values of clay and cation exchange capacity (3–23 %) as a result of accelerated alteration by farming activities and interaction between the used irrigation water and its receiving soils. X-ray diffraction pattern revealed that the soils were similar in mineralogical compositions, consisting of Fe-bearing minerals (such as mica, pyroxene, amphibole, and feldspars), goethite, hematite, illite, chlorite, smectite, kaolinite, and mixed minerals for both cultivated and uncultivated soils but some changes were observed in peak intensity, position, and figure mainly due to increasing the number of wetting–drying periods induced by irrigation practices as well as cropping and the dynamic of Fe and K. A relative enrichment (RE) in Fe oxide forms was highlighted with cultivation which could be attributed to the variation of weathering rate, pedogenic accumulation, and daily and seasonal fluctuations in temperature and moisture. For instance, long-term continuous cultivation led to an improvement in Fe_d from 2.1 % (RE of 1.02) to 181 % (RE of 2.81), Fe_cry from 52 % (RE of 1.53) to 667 % (RE of 7.7), crystalline index from 14 % (RE of 1.14) to 134 % (RE of 2.34), Fe_o from 2 % (RE of 1.02) to 18 % (RE of 1.18), and Fe_p from 23 % (RE of 1.23) to 500 % (RE of 6) as compared to the adjacent uncultivated soils. Soil pH, organic matter, active and equivalent of calcium carbonates were found to be the major agents in controlling the dynamic, nature, and distribution of Fe oxide status as appeared in significant correlation among the physicochemical properties and the evaluated Fe oxide forms.