Soil organic matter pools and carbon fractions in soil under different land uses

► Converting native forest into agricultural area reduced labile C pool. ► Integrated management increased soil carbon content as compared to the native forest. ► Carbon fractions were more effective as indicators of soil quality than total carbon. Changes in tropical land use have profound effects on soil organic matter (SOM) status. It has been suggested that alterations in the different fractions of SOM are more effective in indicating changes in soil use than total soil organic matter content. The main objective of this study was to investigate changes in the content of humic substances in an Ultisol under different land uses, in the northeast region of Brazil. Soil samples were collected from the 0–10 and 10–30cm layers, in three agricultural areas (conventional coconut orchard, integrated coconut orchard, and citrus orchard). A native forest soil was used as reference. Organic C and total N were determined to characterize the SOM. Humic substances were chemically fractionated into fulvic acid, humic acid, and humin, based on solubility in acid and alkali. Significant loss (47.5%) of soil organic matter was observed in the surface layers of the conventional coconut and citrus orchards, compared to the native forest. There was increased SOM content in the integrated coconut orchard soil, due to the presence of cover crops as well as management of crop residues. However, in the subsurface soil of the integrated coconut orchard, cultivation modified the distribution of the more labile fractions of the soil organic matter, as measured by the ratio between humic and fulvic acids (>1.0), indicating a substantial loss of fulvic acids. The degree of humification was in the range 40–97%. The distributions of the soil organic matter fractions varied in the ranges 12–32.5% (fulvic acids), 12–34.5% (humic acids), and 40–69.5% (humin).