Humic substances (HS) are the main pool of total soil organic carbon (TOC). They are partitioned in different fractions according to its solubility in alkali or acid medium, which can be related to soil type and management. This work aimed to compare HS partitioning in Brazilian soils according to soil depth, soil type, biome, and land uses. In a literature review of theses, dissertations, journal articles, and abstracts in proceedings, quantitative data on TOC partitioning in humic acids (HA), fulvic acids (FA), and humin (HU), as well as the HA/FA ratio were compiled. The data were sorted according to comparable depths (0-5, 5-10, 0-20, 20-50, 50-100, and >100 cm), biomes (Atlantic rainforest, Cerrado savanna, Amazon rainforest, southern prairies, and Caatinga scrubland), soil order (Ultisols, Oxisols, Inceptisols, Entisols, Histosols and soils with aquic moisture regimes), and land use systems (native forests, prairies, pastures, eucalypt, and annual or perennial crops). Data were analyzed by descriptive statistics, Spearman correlation, and principal component analysis (PCA). In Ultisols, Oxisols, and Histosols, humin accounted for at least half of TOC and tended to decrease with depth. In Oxisols and Histosols, FA tended to increase in depth. In the Entisols, HA increased with depth, but decreased in Histosols and some Ultisols. Generally, the HA/FA decreased in the deeper layers. Results of PCA indicated no effect of biomes and land use systems on the quantitative partitioning of HS, suggesting its low potential as indicator of management effects or environmental conditions, although useful to discriminate humification processes in some soil taxa.
humin; humic acids; fulvic acids; biome; land use systems; principal component analysis
