Biochemical dissolution of magmatic apatite in brazilian soils

Carlos Nogueira da Costa Junior¹, Ieda de Carvalho Mendes², José Carlos Gaspar¹ and Arnaldo Alcover Neto³

¹Instituto de Geociências, Universidade de Brasília, Brasília, DF, BRAZIL, 709010-900

E-mail: carlonog@unb.br, gasp@unb.br

²Embrapa Cerrados Caixa Postal 08223 Planaltina, DF, BRAZIL, 73.301-970.

E-mail: mendesi@cpac.embrapa.br

³Centro de Tecnolgia Mineral, CETEM, Rio de Janeiro, RJ, BRAZIL. E-mail: alcover@cetem.gov.br

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The role of biological activity in the dissolution of apatite grains was investigated. The apatites are from a calcite carbonatite from the Jacupiranga Carbonatite Complex, Brazil. Carbonatite is a magmatic rock composed of more than 50% calcite, which is the main Brazilian source of phosphorus. The experiments were conducted under greenhouse and field conditions. For the greenhouse experiments, soil samples were collected in areas under native vegetation and under a no-till corn/soybean rotation. In order to obtain soils with high and low biological activities, in both areas the sampling depths were 0 to 5cm and 50 to 70 cm, respectively. The soil samples were placed in plastic containers and kept at 100% of their field capacity during the whole experiment. To evaluate the biochemical dissolution under field conditions, the apatite grains were buried (5 cm deep) in an area which had been for 8 years under a no till management system with a corn/soybean rotation. In the green house and also in the field experiments, apatite grains measuring about 5 by 1.5 mm were placed inside nylon screens and buried in the soil. After specific time intervals the grains were recovered and analyzed by Scanning Electron Microscopy and EDS. Almost one year since the beginning of the experiment, no evidences of biochemical dissolution were observed on the surface of the grains buried in the soils from the 50 to 70 cm depth. However the apatite grains buried in the field and also in the greenhouse with soil samples collected at the 0 to 5 cm depth presented intense bacterial and fungal activities on their surfaces. It is possible to observe depressions on apatite surfaces caused by the bacterial activity. Further studies are necessary to evaluate the potential of magmatic apatites as a source of slow-release P fertilizers in agriculture.