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 cropping systems in sub-Saharan Africa. The objective of our study was to determine the effect of contrasting fallow options, compared with continuous 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 sandy clay loam (Ustic Rhodustalf) in Zambia. Total soil C, N, and P were 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 higher for the mean of five N-2-fixing trees than the one non-fixing tree. Inorganic N, anaerobic and aerobic N mineralization, and light fraction N were (i) higher for the mean of the two trees with lowest (lignin + polyphenol)/N ratios in leaf litter than the two trees with highest ratios in leaf litter and (ii) higher for Sesbania sesban (L.) Merr. than the mean of other trees. Sesbania and fertilized maize monoculture resulted in similar soil inorganic N, but N mineralization and light fraction N were greater after sesbania. Comparable effects of the systems 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 greatly in effect on N availability and hence their suitability for soil fertility replenishment.