LIGHT FRACTION SOIL ORGANIC-MATTER AND AVAILABLE NITROGEN FOLLOWING TREES AND MAIZE

Trees grown for 2 to 3 yr in rotation with crops (tree fallows) are a potential technology for increasing soil fertility in maize-based crop ping systems in sub-Saharan Africa. The objective of our study was to determine the effect of contrasting fallow options, compared with cont inuous maize (Zea mays L.), on light fraction soil organic matter (SOM ), inorganic N (NO3 and NH4), and N mineralization. Soil was collected 2 and 3 yr after the establishment of fallow and maize systems on a s andy clay loam (Ustic Rhodustalf) in Zambia. Total soil C, N, and P we re not different among the fellow and maize systems. Soil inorganic N and amount of light fraction N (150-2000 mu m, <1.13 Mg m(-3)) were hi gher for the mean of five N-2-fixing trees than the one non-fixing tre e. Inorganic N, anaerobic and aerobic N mineralization, and light frac tion N were (i) higher for the mean of the two trees with lowest (lign in + polyphenol)/N ratios in leaf litter than the two trees with highe st ratios in leaf litter and (ii) higher for Sesbania sesban (L.) Merr . than the mean of other trees. Sesbania and fertilized maize monocult ure resulted in similar soil inorganic N, but N mineralization and lig ht fraction N were greater after sesbania. Comparable effects of the s ystems on light (<1.13 Mg m(-3)) and light + intermediate fraction (<1 .37 Mg m(-3)) SOM suggest that light and intermediate fractions can be combined to simplify the fractionation procedure. Tree species vary g reatly in effect on N availability and hence their suitability for soi l fertility replenishment.