Short-term effects of soil amendment with tree legume biomass on carbon and nitrogen in particle size separates in Central Togo

We examined the effects of adding different plant materials on short-term organic matter dynamics of a West African savanna soil. Leaves and roots from three alley-cropped tree legumes were added to soil and exposed in the field for 120 d using PVC cylinders. Particle size fractionation of the exposed soil separating particulate organic matter (POM, 20–2000 μm), silt (2–20 μm) and clay (<2 μm) demonstrated that most of the C was stored in the silt and most of the N in the clay size separates. Adding Senna leaves to the soil led to an increase of the organic C content in the silt and clay fractions, and to an increase of the N content in the silt fraction. In Gliricida amended soil, the N content increased in silt and clay size separates. Roots of all three species had no effect on C or N contents in particle size fractions. The lignin content and the polyphenol-to-nitrogen ratio (PP-to-N ratio) decreased from coarser to finer particle size separates, while the acid-to-aldehyde ratio (Ac-to-Al ratio) increased. The addition of Senna leaves decreased the polyphenol content and the PP-to-N ratio in all particle size fractions. The lignin content was not affected. In the silt fraction, the alkyl-C content was decreased, the carboxyl- and amide-C content increased by the addition of Senna leaves indicating that it was not unaltered plant material but microbial metabolites which were responsible for the increase of C. The PP-to-N ratio of the added plant material seems to control the effects of soil amendment with tree legume biomass through its influence on decomposition processes and on the formation of organo–mineral complexes for both C and N.