Inorganic nitrogen dynamics in fallows and maize on an Oxisol and Alfisol in the highlands of Kenya

Fallows with naturally regenerated or planted vegetation are important in m any subsistence agricultural systems of tropical regions, but the underlyin g soil processes in fallows are not properly understood. We investigated N dynamics under different fallow vegetation on a Kandiudalfic Eutrudox (2372 -mm rain in 16 months) and a Kandic Paleustalf (1266-mm rain in 15 months) in the Kenyan highlands. The treatments, which extended for three cropping seasons (15-16 months), were Zea mays (maize), natural regrowth of vegetati on (natural fallow), planted Sesbania sesban (sesbania fallow) and uncultiv ated soil without vegetation (bare fallow), Inorganic N (nitrate + ammonium -N) to 2-m depth under bare fallow increased by 242 kg N ha(-1) year(-1) on the Oxisol and 54 kg N ha(-1) year(-1) on the Alfisol, indicating that N m ineralization exceeded N losses. Subsoil inorganic N (0.5-2.0 m) remained r elatively unchanged after three crops of unfertilized maize, which produced limited total biomass because of P deficiency. Inorganic N decreased durin g natural and sesbania fallows, and both fallows similarly depleted subsoil inorganic N. The fallows depleted inorganic N at 0.5-2.0 m by 75-125 kg N ha(-1) year(-1) down to a minimum N content between 40 and 80 kg N ha(-1). After slashing sesbania and incorporating the above-ground biomass with 154 -164 kg N ha(-1), soil inorganic N increased within 2 months by 136 kg N ha (-1) on the Oxisol and 148 kg N ha(-1) on the Alfisol. Inorganic N decrease d after cropping the bare fallow on the Oxisol with maize, indicating that inorganic N was prone to leaching during heavy rains when the maize was sma ll. A considerable part of the N in biomass of the natural fallow was recyc led. Much of the total N accumulated by the sesbania fallow was removed wit h the wood and the amount of N recycled was similar an the Oxisol and Alfis ol. We conclude that sesbania fallows can retrieve considerable subsoil ino rganic N on deep soils with high subsoil N and effectively cycle this N thr ough its rapidly decomposable biomass to subsequent crops. (C) 2000 Elsevie r Science B.V. All rights reserved.