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a Dep. of Chemistry and Biochemistry, South Dakota State Univ., Brookings, SD 57007 USA
b Dep. of Soil Science, Univ. of Saskatchewan, 51 Campus Drive, Saskatoon, SK, Canada S7N 5A8
shangchao{at}hotmail.com
Soil organic matter (SOM) turnover is more rapid in tropical than in temperate soils. One possible reason is a limited ability of tropical soils to stabilize SOM. To test this, we measured C turnover resulting from 12 years' cultivation of a forest soil with sorghum [Sorghum bicolor (L.) Moench]. Carbon-13 natural abundance (
13C) signatures of forest- and sorghum-derived C were used to quantify C losses and gains in organo-mineral fractions separated by particle size, and further by density (for sands and silts) and magnetic susceptibility (for clays). Nearly 50% of original C was in the silt-sized fraction, mostly in microaggregates of intermediate density; 30% was held by clays, particularly those of intermediate magnetic susceptibility; and 20% was of sand-size, low-density, often recognizable plant residues. The 13C values in the forest soil showed the more humified SOM to be associated with finer, denser, and less magnetic fractions. After cultivation, total C content was 28% lower, with 59% of this reduction in the silts, 28% in the sand, and 19% in the clays. Loss of forest-derived C amounted to 45%. The sand fraction lost 54% of its forest C, the silts 45% (mostly from intermediate density fractions), and the clays 23% (mostly from intermediate magnetic fractions). Gains in sorghum-derived C amounted to 32% of C in the sand fraction, 12% in the silts (relatively evenly distributed among densities) and 13% in the clays (mostly in the nonmagnetic fraction). Thus, losses of forest C and gains of sorghum C occurred in different organo-mineral fractions, indicating that there were no unique active fractions corresponding with the concept of C pools with defined turnover characteristics used in models of organic matter turnover.
Abbreviations: SOM, soil organic matter • 13C, carbon-13 natural abundance
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