Soil phosphorus fractions in unfertilized fallow-maize systems on two tropical soils

The rotation of crops with either legumes or natural regrowth of native vegetation is known to supply N to crops in tropical land-use systems, but little is known about the effect of such rotations on P supply to crops. An experiment was conducted on two soils in Kenya, an Oxisol (very fine, isohyperthermic Kandiudalfic Eutrudox), and an Alfisol (mixed, isothermic Kandic Paleustalf), to determine the effect of unfertilized, organic-based land-use systems on fractions of soil inorganic P (P-i) and organic P (P-o). The land-use systems involved growth of three maize (Zea mars L,) crops (18 mo total duration) after 17 mo of either (i) Sesbania sesban (L,) Merr, tree growth (sesbania fallow), (ii) natural regrowth of vegetation without cultivation (natural fallow), (iii) three crops of unfertilized maize (maize monoculture), or (iv) bare uncultivated soil (bare fallow), Soil (0-15 cm deep) was collected before post-fallow maize and after the three post-fallow maize crops. Sesbania and natural fallows increased the amount of P in light fraction soil organic matter (OM) (>150 mu m, <1.13 Mg m(-3)) and macroorganic matter (250-2000 mu m) on each soil and increased chloroform extractable P on the Oxisol, Land-use systems had no effect on extractable P-i (bicarbonate, Bray-1, iron-oxide impregnated paper), except for resin P on the Oxisol. Unfertilized sesbania and natural fallows cycled soil P through plant residues, resulting in increased P in soil microbial biomass immediately after the fallows and increased P in light fraction soil OM for three post-fallow cropping seasons.