Organic carbon decomposition rates with depth under an agroforestry system in a calcareous soil

Dr Rémi Cardinael1,2,3, Dr Tiphaine Chevallier2, Dr Bertrand Guenet4, Dr Cyril Girardin5, MSc Thomas Cozzi3, Valérie Pouteau5, Dr Claire Chenu3

1Cirad – UR AIDA, Montpellier, France, 2IRD – UMR Eco&Sols, Montpellier, France, 3AgroParisTech – UMR Ecosys, Thiverval-Grignon, France, 4CNRS – LSCE, Gif-sur-Yvette, France, 5INRA – UMR Ecosys, Thiverval-Grignon, France

Soil inorganic carbon (SIC) in the form of carbonates is found in a large part of soils, especially in arid and semi-arid environments. Despite their important distribution at the global scale, the organic carbon dynamic has been poorly investigated in these soils due to the complexity of measurement and of the processes involved. It requires the removal of carbonates by acid dissolution or the use of natural isotopes to discriminate the carbon originating from the soil organic carbon (SOC) than the one from the carbonates. We incubated soil samples, coming from an 18-year-old agroforestry system (both tree row and alley) and an adjacent agricultural plot established in the South of France, during 44 days. Soil samples were taken at four different depths: 0-10, 10-30, 70-100 and 160-180 cm. Total CO2 emissions, the isotopic composition (δ13C, ‰) of the CO2 and microbial biomass were measured. The contribution of SIC-derived CO2 represented about 20% in the topsoil and 60% in the subsoil of the total soil CO2 emissions. The SOC-derived CO2, or heterotrophic soil respiration, was higher in the topsoil, but the decomposition rates (day-1) remained stable with depth, suggesting that only the size of the labile carbon pool was modified with depth. Subsoil organic carbon seems to be as prone to decomposition as surface organic carbon. No difference in CO2 emissions was found between the agroforestry and the control plot, except in the tree row at 0-10 cm where the carbon content and microbial biomass were higher, but the decomposition remained lower. Our results suggest that the measurement of soil respiration in calcareous soils could be overestimated if the isotopic signature of the CO2 is not taken into account. It also advocates more in-depth studies on dissolution-precipitation processes and their impact on CO2 emissions in these soils.


Professor of Soil Science at AgroParisTech

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