Carbon Turnover and Carbon‐13 Natural Abundance in Organo‐Mineral Fractions of a Tropical Dry Forest Soil under Cultivation

Abstract

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 (δ13C) 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 δ13C 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.