The humic substances (HS) of two Brazilian Oxisols, one under native grassl
and (NG) and the other submitted to a long-term (19-year) tillage systems e
xperiment including no-tillage (NT), minimal tillage (MT) and an adjacent e
ucalyptus-planted forest (F), were extracted with sodium pyrophosphate, ana
lysed and fractionated by exclusion chromatography on controlled pore glass
(CPG). Chromatographic analysis of HS was performed at 400 nm (visible reg
ion), the resulting chromatograms deconvoluted and statistical analysis of
data made using multivariate methods (factor and Pearson correlation analys
is). Increase of tillage intensity, estimated by factor analysis, resulted
in a general HS content decrease, not affecting soil C content. Tillage inc
reased the relative proportion of small molecular size HS, determined direc
tly by CPG and indirectly by the relative increase of fulvic acids (FA) con
tent (increase of fulvic acids/humic acids ratio). This suggests that tilla
ge 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 m
olecular size fraction showed an unusual high E-4/E-6 ratio, possibly due t
o a small conjugation of aromatic rings. (C) 1999 Elsevier Science B.V. All
rights reserved.