Carbon turnover and carbon-13 natural abundance in organo-mineral fractions of a tropical dry forest soil under cultivation

Soil organic matter (SOM) turnover is more rapid in tropical than in temper ate soils. One possible reason is a limited ability of tropical soils to st abilize SOM. To test this, we measured C turnover resulting from 12 years' cultivation of a forest soil with sorghum [Sorghum bicolor (L.) Moench]. Ca rbon-id natural abundance (delta C-13) signatures of forest- and sorghum-de rived C were used to quantify C losses and gains in organo-mineral fraction s separated by particle size, and further by density (for sands and silts) and magnetic susceptibility (for clays). Nearly 50% of original C was in th e silt-sized fraction, mostly in microaggregates of intermediate density; 3 0% was held by clays, particularly those of intermediate magnetic susceptib ility; and 20% was of sand-size, low-density, often recognizable plant resi dues. The delta C-13 values in the forest soil showed the more humified SOM to be associated with finer, denser,:md less magnetic fractions. After cul tivation, 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 amou nted to 45%. The sand fraction lost 54% of its forest C, the silts 45% (mos tly from intermediate density fractions), and the clays 23% (mostly from in termediate 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 orga no-mineral fractions, indicating that there were no unique active fractions corresponding with the concept of C pools with defined turnover characteri stics used in models of organic matter turnover.