LIGNIN AND CARBOHYDRATE ALTERATION IN PARTICLE-SIZE SEPARATES OF AN OXISOL UNDER TROPICAL PASTURES FOLLOWING NATIVE SAVANNA

Establishment of improved tropical pastures on acid savanna soils of l ow fertility can increase the soil organic matter (SOM) content and so il fertility. Our objective was to follow the quantitative and qualita tive changes of SOM associated with sand-, silt-and clay-sized separat es after establishment of a grass-alone (Brachiaria decumbens) and a g rass-legume (Brachiaria decumbens + Pueraria phaseoloides) pasture on isohyperthermic savanna soils of Colombia. Additionally, earthworm cas ts of anecic and endo-anecic species derived from the grass-legume pas ture were investigated. Whole soil and size separates were analyzed fo r C, N, lignin-derived phenols (alkaline CuO-oxidation) and carbohydra tes (4 M trifluoroacetic acid hydrolysis). For native savanna, grass-a lone and grass-legume pasture the C content increased in the order san d (1.6-3.3 g C kg(-1)) < silt (25.8-32.1 g C kg(-1)) < clay(38.8-43.0 g C kg(-1)); the C-to-N ratio decreased in the same order. Fifteen yea rs of pasture establishment resulted in an increase of C by factors of ca. 2 (sand), 1.3 (silt) and 1.1 (clay) compared to native savanna. D ecreasing yields of phenolic CuO-oxidation products and increasing sid e-chain oxidation of the remnant lignin indicated progressive lignin a lteration with decreasing particle size. Soil under savanna was substa ntially lower in intact lignin structures than soil under grass-alone and grass-legume pastures. Enrichment factors, E = (mg kg(-1) soil und er pasture/mg kg(-1) soil under savanna), for lignin-derived phenolics after establishment of B. decumbens and B. decumbens + P. phaseoloide s were 3.9 and 2.7, 4.3 and 2.4, and 2.6 and 2.6 in sand-, sill-and cl ay-sized separates, respectively. Clay-bound SOM was rich in microbial ly-derived sugars, whereas the SOM of the sand-sized separates contain ed large amounts of plant-derived sugars. Silt-associated SOM bad low concentrations of both. Establishment of grass-alone and grass-legume pasture increased the amount of easily-degradable plant-derived sugars by factors of 3.0 and 3.5 (sand), 1.9 and 1.3 (silt), and 1.1 and 1.1 (clay). SOM distribution across size separates suggested an increasin g amount of unprotected macro-OM after establishment of grass-alone an d grass-legume pastures. Concurrently, lignin and carbohydrate signatu re indicated higher contents of relatively undegraded plant-derived SO M, notably in the sand-sized separates. It was concluded that the incr ease of this SOM fraction has beneficial effects on the soil internal N cycle as far as N mineralization and N immobilization is concerned. Earthworm casts were characterized by a tremendous enrichment of C, in tact lignin and plant-derived sugars in the sand-sized separates, comp ared with the surrounding soil. This suggested a pronounced incorporat ion of fresh litter into the mineral soil (anecic effect), that decomp osition processes were dramatically slowed in the casts, and a pool of relatively labile, but physically-protected SOM was built up.