Carbon isotopic fractionation in subtropical Brazilian grassland soils. Comparison with tropical forest soils

The natural relationship ¹³C/¹²C determined in three soil profiles under grass vegetation indicated a depletion in organic ¹³C at depth: the δ¹³C was between — 18‰ and — 15‰ in the A horizons and ranged from — 18 to — 22‰ at depth. Previous work showed that in forest soils, where δ¹³C was near — 28‰ in the upper horizon, there was, on the contrary, a relative enrichment of the lower strata. This meant δ¹³C that, initially different in the various topsoils, became more equal at depth. Comparison between dark, deep horizons (sombric horizons), which are certainly of illuvial origine, would confirm this: δ¹³C of grassland and a forest sombric horizon were almost equal at around — 22‰. These results might mean that, in natural ecosystems, the isotopie carbon composition of the soil underlying humus would be independent of the vegetation type. This would have practical implications for the use of ¹³C as a tracer for soil organic matter studies.