Impact of Long-Term No-Tillage on Soil Mineralogy and Soil Carbon Compared to Native Forest.

The replacement of forest by agricultural systems promotes drastic changes in soil. Studies of cultivated areas for a long time compared with soils under natural forest might contribute to understanding the impacts of agricultural systems on soil attributes. The objective was to evaluate the impact of long-term no-tillage on soil mineralogy and soil carbon compared to native forest. Soil sampling was conducted in March 2011 in a commercial farming area in Maringá, Paraná State, Brazil, in two adjacent areas under no-tillage (NT) and native forest (NF). The soil sampling consisted of randomly opening of 10 mini-trenches which were collected in 40 soil samples, at 0-10 and 10-20 cm depth. In each system of use, it was determining the pedogenic iron oxides (dithionite-citrate-bicarbonate – DCB), poorly crystalline iron oxides (acid ammonium oxalate in darkness – AAOD) and total amount of iron (Sulfuric acid extraction) in clay fraction. The amount of carbon (C) was measured by Walkley-Black method. It was used two more index, the Fe extracted by DCB (Fed) divided by Fe extracted by sulfuric acid (Fet) and Fe extracted by AAOD (Feo) divided by Fed. The clay fraction was treated with hot NaOH solution to obtain concentrated iron oxides. After that, samples were submitted to X-ray diffraction - XRD (20-80^o2θ CuKα). There were no differences between systems of use for Fet and Feo, but it was observed differences for Fed and C levels. It was observed higher values of Fed to the top layers of soil (0-10 cm) in both systems of use. The NT-system showed higher values of Fed than the NF-system. The higher C levels were observed only in top layer of soil (0-10cm) independently of the system of use. The ratio Fed/Fet ranged from 0.60 to 0.90 indicating predominance of iron as being pedogenic iron oxides. The lower values of the ratio Feo/Fed (< 0.05) indicated that these iron oxides are predominantly crystalline, what consists with soil nature (Oxisol). The mainly phase identified in XRD were hematite, maghemite/hematite and goethite, but it was not observed differences between the depth and system of use. So, NT areas present changings in mineralogical attributes, mainly in pedogenic iron oxides and C soil levels compared to NF areas.