Aggregation levels and correlation with humic substances in Oxisols and Ultisols of coastal tablelands, Campos dos Goytacazes-RJ, Brazil

Abstract

The objective of this study was to evaluate soil aggregation, the amounts of organic carbon of the aggregates and the relationships between aggregation and humic substances in coastal tablelands soils, under different vegetation covers in the north of Rio de Janeiro state, Brazil. Undisturbed samples were collected at 0-5, 5 10, 10-20 and 20-40 cm in depth in areas of Xanthic Oxisol and Xanthic Ultisol, under three plant covers: pasture, sugar cane crop and secondary forest. The mean weight diameter of aggregates (MWD) and the aggregate organic carbon were determined in each aggregate class. The sensitivity index (SI) and humic substances content (fulvic acids, humic acids and humin) were also quantified. MWD highest levels were observed in the
forest area and the lowest in the sugar cane crop area. Regarding the soils, the Ultisol showed the highest values of MWD. The highest SI value was found in the pasture area. The carbon amounts of aggregates decreased in relation to smaller diameter class, with the lowest values observed in pasture and sugar cane areas. The humin showed the highest correlation values with soil aggregation in the Ultisol; however, humic acids presented the best correlation in the Oxisols, mainly in the forest area.