Changes in delta C-13 composition of soil carbonates driven by organic matter decomposition in a Mediterranean climate: A field incubation experiment

摘要

The current view on the relationship between the delta C-13 of pedogenic carbonates and soil organic matter is based on static studies, in which soil profiles are analysed at a given moment of their development. A dynamic approach to this question should also be possible by studying under field conditions how the delta C-13 of carbonates changes as organic matter decomposes. No such study has been undertaken owing to the slowness of the changes in the delta C-13 of carbonates, since it has been calculated that a detectable change will occur only after millenia. Nevertheless, this may not be true where soil CO2 efflux is intense, as expected in soil zones with high microbial activity. In this paper we test the latter assumption by incubating mixtures of plant material and carbonate-rich red earth in the field at depths of 5, 20 and 40 cm. Four types of plant material were tested: Medicago sativa, Eucalyptus globulus, Quercus ilex and Pinus halepensis. Because the isotopic composition of these plant materials is known, we can determine the isotopic composition of the respired C and study how it relates to the (expected) changes in the delta 13C. After two years of field incubation, the changes in delta C-13 of carbonates were high enough to be reliably detected and quantified, thus showing that the isotopic composition of soil carbonates can change quite rapidly in biologically active soil horizons. The observed changes are possible only if we assume that the increase in delta C-13 in the overall path respired C -> pedogenic carbonate is much higher than the usually applied standard factors (about 15 parts per thousand). These enrichments can be explained by assuming, as does the currently accepted paradigm, that the precipitation of new carbonates occurs in an open system in which the penetration of free-air CO2 plays a major role. On the other hand, these enrichments can also be explained by an alternative interpretation, which assumes that the dissolution-precipitation carbonate cycles occur in systems that can be at least temporarily closed. Thus, we suggest that both possibilities (carbonate dissolution and precipitation in either an open or closed system) can coexist in a given soil, even though one or the other will dominate in any given time period.