Soil acidification under long-term tobacco plantation results in alterations of mineralogical properties in an Alisol

In the present study, soil samples collected from 20-year (1993‒2013) long-term tobacco plantation and perennial fallow were analyzed to study the influence of soil acidification on soil physicochemical and mineralogical properties. Results showed a significant decrease of soil pH, Ca2+, Mg2+, Na+ and total exchangeable cations, except K+, under tobacco plantation than under fallow land. Further X-ray diffraction (XRD) analysis for 1 mm soil size demonstrated an increase of vermiculite and kaolinite, but an obvious decrease of original potassium-bearing mica and potassium feldspar, under tobacco plantation than under fallow. For the <2 μm soil clay fraction, the XRD patterns showed an obvious desilication and accumulation of Al and Fe under tobacco plantation than under fallow. Other potassium-bearing minerals, such as illite and chlorite, had also visibly weathered and slaked under tobacco plantation than under fallow, resulting in an increase of vermiculite and hydroxy-Al-vermiculite, and an accelerated decomposition of 2:1 minerals and formation of 1:1 minerals. These results demonstrated that long-term continuous tobacco plantation has exerted negative effects on soil acidification and maintenance of soil potassium-bearing minerals.