The quantification of carbon in different fractions of soil organic matter (SOM) is necessary because of the interest in understanding the potential to capture and store carbon in various soil use systems. The objective of this study was to quantify the carbon of humic substances in different soil use systems and evaluation seasons and correlate it with some chemical and physical properties of a 6%-clayey soil. The systems selected were: conventional tillage (CT-maize/common beans), no tillage (NT-eggplant/maize), passion fruit/Desmodium sp consortium, fig grove, and agroforest system. Soil samples were taken at two depths (0-5 and 5-10 cm) in two seasons of the year (17/11/2005 - summer and 23/06/2006 - winter). The total organic carbon (TOC) was determinated by wet combustion and SOM chemical fractioning was conducted following IHSS method, with determination of organic carbon in humin fraction (C-HUM), humic acid fraction (C-HAF) and fulvic acid fraction (C-FAF). Most of TOC consisted of C-HUM showing significant correlation with TOC in all evaluated systems and seasons. Values of C-FAH showed soil changes due to the use systems at 0-5cm depth in summer, particularly NT which showed the highest C-FAH values. The C-FAF showed similar pattern, but at 5-10cm depth in winter, especially CT. Except for CT, there were found significant correlations between S value, T value and DMP in all systems. Compared to CT, NT showed increasing levels of C-FAH at both depths and in both seasons.
organic agriculture; crop residues; humin; humic and fulvic acids
