Abstract: Iron-manganese nodules are widely distributed in karst soil with high geological background, the content of which is much higher than that in nonkarst soils. To better understand the formation mechanism of iron-manganese nodules in karst soil, the nodules in the soil (0-20 cm) of central area of Guangxi were collected. The formation process of iron-manganese nodules in the soil of high geological background area was examined via the optical microscope (OP), field emission scanning electron microscope (FESEM), and energy spectrum analysis (EDS), combined with the complex soil forming process in karst area. The results showed that there were two kinds of nodules in the karst soil, i.e., iron nodules and manganese nodules. Generally, the nodules were sub-circular to elliptical in shape, and the particle size ranged from 0.5 to 4.0 mm. There were two kinds of iron-manganese nodules, with or without banded structure. Manganese nodules were mainly composed of iron oxides and manganese oxides, while iron oxides were mainly distributed in iron nodules. Based on those findings and the process of karst soil formation, the formation process of iron and manganese nodules in high geological background soil is summarized: (1) in the process of soil formation in karst area, iron and manganese were aggregated in different levels, which provides rich material sources for the formation of nodules; (2) the seasonal dry-wet cycling climate conditions led to the alternation precipitation of iron and manganese, which enhanced the formation of nodules; (3) the micropore structure in the nodules can provide space for microbial activities and channels for material flow, the special climate and environmental conditions rich in organic matters and microorganisms accelerate the formation of iron-manganese nodules, and eventually form iron-manganese nodules that are widely deve loped in karst soils.

Key words: iron nodule, manganese nodule, soil forming process, alternation precipitation, banded structure, concentrating.