DETERMINATION OF TOTAL ORGANIC CARBON AND NITROGEN STOCKS AND THEIR FRACTIONS IN AND ULTISOL UNDER DIFFERENT LAND USES

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-I), 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 C-MICR, 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.