Soil management system effects on size fractionated humic substances

Abstract The humic substances (HS) of two Brazilian Oxisols, one under native grassland (NG) and the other submitted to a long-term (19-year) tillage systems experiment including no-tillage (NT), minimal tillage (MT) and an adjacent eucalyptus-planted forest (F), were extracted with sodium pyrophosphate, analysed and fractionated by exclusion chromatography on controlled pore glass (CPG). Chromatographic analysis of HS was performed at 400 nm (visible region), the resulting chromatograms deconvoluted and statistical analysis of data made using multivariate methods (factor and Pearson correlation analysis). Increase of tillage intensity, estimated by factor analysis, resulted in a general HS content decrease, not affecting soil C content. Tillage increased the relative proportion of small molecular size HS, determined directly by CPG and indirectly by the relative increase of fulvic acids (FA) content (increase of fulvic acids/humic acids ratio). This suggests that tillage caused preferential degradation of large molecular size HS and/or a neo-synthesis of small HS due to increased fresh-carbon input partially induced by liming and fertilisation. The HS fractionation method on CPG permitted gray humic acids (GHA) separation into two distinct fractions. The larger molecular size fraction showed an unusual high E 4 /E 6 ratio, possibly due to a small conjugation of aromatic rings.