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

The rotation of crops with either legumes or natural regrowth of native veg etation is known to supply N to crops in tropical land-use systems, but lit tle is known about the effect of such rotations on P supply to crops. An ex periment was conducted on two soils in Kenya, an Oxisol (very fine, isohype rthermic Kandiudalfic Eutrudox), and an Alfisol (mixed, isothermic Kandic P aleustalf), 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 la nd-use systems involved growth of three maize (Zea mars L,) crops (18 mo to tal duration) after 17 mo of either (i) Sesbania sesban (L,) Merr, tree gro wth (sesbania fallow), (ii) natural regrowth of vegetation without cultivat ion (natural fallow), (iii) three crops of unfertilized maize (maize monocu lture), or (iv) bare uncultivated soil (bare fallow), Soil (0-15 cm deep) w as collected before post-fallow maize and after the three post-fallow maize crops. Sesbania and natural fallows increased the amount of P in light fra ction 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 (bicarbon ate, Bray-1, iron-oxide impregnated paper), except for resin P on the Oxiso l. Unfertilized sesbania and natural fallows cycled soil P through plant re sidues, resulting in increased P in soil microbial biomass immediately afte r the fallows and increased P in light fraction soil OM for three post-fall ow cropping seasons.