Rice systems in Asia have intensified rapidly in the past 30 years, and significant areas of irrigated lowland rice are now supporting two or three rice crops per year. Our objective was to compare the chemical composition of soil organic matter (SOM) from four fields with different histories of rice cropping intensity and soil submergence: (i) a single-crop rainfed, dryland rice system without soil submergence, (ii) an irrigated rice and soybean rotation, and irrigated (iii) double-or (iv) triplecrop 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 modem age by I4C-dating, and represented about 20% of total N and organic C. The MHA was enriched 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 that slower lignin decomposition caused by deficiency of 0 2 in submerged soil leads to incorporation of phenolic moieties into young SOM fractions. The increased phenolic character of these fractions may influence N cycling and the N supplying capacity of lowland soils supporting two or three annual crops of irrigated rice.
Comparisons were made between the phenolic and carbohydrate signatures of soil profiles developed under grass, spruce and ash stands. Samples were collected from a brown earth soil which was originally under the same land use, but over the past 43 years has supported different monocultures. Distinct signatures associated with each litter type were recorded in individual profiles. A relatively undecomposed phenolic fraction from lignin and hydrolysable carbohydrate fraction from plants had accumulated in the soils under spruce and ash. This largely reflected the quantity and quality of the litter inputs from the spruce and ash compared with the grass. The phenolic and hydrolysable carbohydrate fractions accounted for as much as 60% of the total organic carbon concentration in the deep horizons. In the grassland profile both fractions were more decomposed than under ash and spruce suggesting that the forest profiles had rapidly accumulated a carbon pool with a comparatively slow rate of decomposition. This was most apparent from the spruce profile (which contained 398 mg g-' C carbohydrate hydrolysed using trifluoracetic acid (TFA) in the C horizon compared with 165 and 45 mg g-' C under ash and grass respectively). We conclude that the decay rate of these fractions is a function of the vegetation type.
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