Soil carbon stocks are the result of interaction of factors that determine their formation and decomposition. This study aimed to quantify the total organic recovery in various C pools under the following different land uses of an Ultisol: secondary forest, rubber tree plantation, pasture, and citrus plantation. The soil was sampled at depths of 0 to 10 cm and 10 to 20 cm and the following characteristics were determined: total organic carbon (TOC), total nitrogen (TN), fulvic acid fraction-C (FA), humic acid fraction-C (HA), humin-C (HN), labile organic matter-C (C L), microbial biomass-C (C-MICR), light organic matter (LOM), C and N of LOM (C LOM and N LOM), water-soluble C (WSC), and Carbon Management Index (CMI). The TOC values varied from 20.9 to 13.3 g kg-1 in the 0-10 cm layer for forest and pasture, respectively, and from 13.5 to 9.8 g kg-1 in 10-20 cm, respectively. The CMI was high in 0-10 cm under rubber cultivation (69) and citrus plantation (70) soils and low in the pasture soil (54). These results were similar in the 10-20 cm layer. In the studied layers, the capacity of preserving and/or recovering TOC and TN concentrations as well as the labile compartments such as CMICR, CSA, MOL, C MOL and N MOL was greater in the rubber and citrus systems. In these plantations the quality of humified fractions increased in parallel to an increment in the condensed alkaline-soluble humic substances. On the other hand, the potential for preservation of soil C under pasture was limited. Citrus and particularly rubber plantations are therefore important land uses to maintain the soil quality. Levels of C MICR, WSC, LOM varied more intensively among the land uses than TOC and can therefore be considered more sensitive indicators of alterations in soil organic matter.
land use; soil organic matter; carbon pools
