Dc. Olk et al., CONSERVED CHEMICAL-PROPERTIES OF YOUNG HUMIC-ACID FRACTIONS IN TROPICAL LOWLAND SOIL UNDER INTENSIVE IRRIGATED RICE CROPPING, European journal of soil science, 49(2), 1998, pp. 337-349
Increasing cropping intensity (i.e. number of crops per year) of irrig
ated rice seems to cause an accumulation of phenolic compounds in the
soil organic matter (SOM). We have studied the chemical nature of SOM
in a broad range of soil types at different sites with long-term doubl
e-and triple-crop irrigated rice trials. Accumulation of phenols, as m
easured by C-13 nuclear magnetic resonance spectroscopy, was found in
both the mobile humic acid (MHA) and calcium humate (CaHA) fractions a
t all sites, regardless of soil type, hydrology during the fallow, and
with and without inorganic fertilizer or green manures. Although phen
ols accumulated consistently in MHA and CaHA, the C, N and hydrolysabl
e amino acid concentrations, degree of humification and amounts of MHA
and CaHA were significantly altered by crop management, and they vari
ed from site to site. Our results are consistent with the hypothesis t
hat the accumulation of phenols is a characteristic of the anaerobic,
or nearly anaerobic, soil conditions that exist at the initial stages
of SOM formation in submerged irrigated rice soils. By contrast, other
SOM properties are additionally influenced by soil conditions that go
vern the degradation and turnover of existing SOM. The chemical proper
ties of MHA and CaHA indicated that they are labile, and the quantitie
s of these HA fractions were more sensitive to recent management than
were total soil C or N.