Dc. Olk et al., CHANGES IN CHEMICAL-PROPERTIES OF ORGANIC-MATTER WITH INTENSIFIED RICE CROPPING IN TROPICAL LOWLAND SOIL, European journal of soil science, 47(3), 1996, pp. 293-303
Rice systems in Asia have intensified rapidly in the past 30 years, an
d significant areas of irrigated lowland rice are now supporting two o
r three rice crops per year. Our objective was to compare the chemical
composition of soil organic matter (SOM) from four fields with differ
ent histories of rice cropping intensity and soil submergence: (i) a s
ingle-crop rainfed, dryland rice system without soil submergence, (ii)
an irrigated rice and soybean rotation, and irrigated (iii) double- o
r (iv) triple-crop rice systems in which soil remains submerged during
much of the year. In all four soils, extracted mobile humic acid (MHA
) and calcium humate (CaHA) fractions were of modern age by C-14-datin
g, and represented about 20% of total N and organic C. The MHA was enr
iched in N and hydrolysable amino acids (AA) compared with CaHA in all
soils. With increased frequency of irrigated rice cropping, however,
there was a large increase in phenolic content of SOM. We speculate th
at slower lignin decomposition caused by deficiency of O-2 in submerge
d soil leads to incorporation of phenolic moieties into young SOM frac
tions. The increased phenolic character of these fractions may influen
ce N cycling and the N supplying capacity of lowland soils supporting
two or three annual crops of irrigated rice.