Changes in soil properties during long-term fallow and continuous cultivation after forest clearing in Nigeria

Changes in soil chemical properties under natural bush regrowth, planted fallows (i.e. Guinea grass (Panicum maximum), Leucaena leucocephala, and pigeon pea (Cajanus cajan)) and under continuous cropping with minimum tillage (i.e. maize (Zea mays L.)/cassava (Manihot esculenta Crantz) intercrop and maize monoculture with residue returned or removed), were monitored for 13 years after clearing of secondary forest on a kaolinitic Alfisol in Nigeria, West Africa. Under bush fallow, soil organic C (SOC) decreased during the first 7 years, then increased to the original level of approximately 20 g kg(-1) at 12 years, whereas soil pH, exchangeable Ca and Mg and ECEC remained relatively constant. Chemical soil properties under Guinea grass and leucaena fallows compared favorably to those under bush fallow, whereas chemical properties of the surface soil under pigeon pea fallow deteriorated. Under continuous maize cropping, soil organic C decreased during the first 7 years, then reached a steady-state at about 65% of the level maintained by bush fallow. The surface soil under continuous maize also became acidified. Maize grain yields of the first season declined from 6.0 t ha(-1) to 2.5 t ha(-1) during 7 years of cropping. Relative yield sustainability of the three cropping systems was maize/cassava intercrop > maize monoculture with residue returned > maize monoculture with residue removed.