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 low fertility can increase the soil organic matter (SOM) content and soil fertility. Our objective was to follow the quantitative and qualitative changes of SOM associated with sand-, silt-and clay-sized separates after establishment of a grass-alone (Brachiaria decumbens) and a grass-legume (Brachiaria decumbens + Pueraria phaseoloides) pasture on isohyperthermic savanna soils of Colombia. Additionally, earthworm casts of anecic and endo-anecic species derived from the grass-legume pasture were investigated. Whole soil and size separates were analyzed for C, N, lignin-derived phenols (alkaline CuO-oxidation) and carbohydrates (4 m trifluoroacetic acid hydrolysis). For native savanna, grass-alone and grass-legume pasture the C content increased in the order sand (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 years 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. Decreasing yields of phenolic CuO-oxidation products and increasing side-chain oxidation of the remnant lignin indicated progressive lignin alteration with decreasing particle size. Soil under savanna was substantially lower in intact lignin structures than soil under grass-alone and grass-legume pastures. Enrichment factors, E = (mg kg−1 soil under pasture/mg kg−1 soil under savanna), for lignin-derived phenolics after establishment of B. decumbens and B. decumbens + P. phaseoloides were 3.9 and 2.7, 4.3 and 2.4, and 2.6 and 2.6 in sand-, silt-and clay-sized separates, respectively. Clay-bound SOM was rich in microbially-derived sugars, whereas the SOM of the sand-sized separates contained large amounts of plant-derived sugars. Silt-associated SOM had 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 increasing amount of unprotected macro-OM after establishment of grass-alone and grass-legume pastures. Concurrently, lignin and carbohydrate signature indicated higher contents of relatively undergraded plant-derived SOM, notably in the sand-sized separates. It was concluded that the increase 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, intact lignin and plant-derived sugars in the sand-sized separates, compared with the surrounding soil. This suggested a pronounced incorporation of fresh litter into the mineral soil (anecic effect), that decomposition processes were dramatically slowed in the casts, and a pool of relatively labile, but physically-protected SOM was built up.