STABILITY OF ORGANIC CARBON IN DEEP SOIL LAYERS

STABILITY OF ORGANIC CARBON IN DEEP SOIL LAYERS CONTROLLED BY FRESH CARBON SUPPLY Fontaine S. , Barot S. , Barré P. , Bdioui N. , Mary B. , Rumpel C. Nature (2007), Vol. 450, 277 -281

Introduction Soil reservoir of organic C: more than biomass and atmospheric CO 2 combined Mean residence time of SOC increases with depth Which factors control the stability of carbon in deep soil layers? - Hypotheses: 1. Chemical structure Fixation on minerals Conditions for microbes 2. 3.

Introduction New theory of SOC dynamics: Slow SOC turnover at depth results from scarcity of fresh C Soil humus → result of long-term accumulation of biochemically recalcitrant compounds with low energy content Near soil surface: fresh C available → energy → decomposition Deep soil layers: fresh C input extremely low → decomposition strongly reduced

Material and methods Study site: grassland in France (for >50 yr, 2000 yr ago: chestnut/hornbearn forest) Characterization of the soil profile: 3 independent soil samples in layers of 0. 2 m down to depth of unweathered parent material Organic C and bulk density measured to determine SOC content and storage Studies of stability of SOC: focused on 2 layers: 0 -0. 2 m, 0. 6 -0. 8 m

Stability-chemical structure Methods: 14 C content was measured to date SOC MRT of SOC was determined with a flux model Chemical composition was analyzed by 13 C CPMS, NMR and FTIR spectroscopy Results: 14 C content of SOC declined with depth Surface layer: young fast-cycling C, subsoil: ancient slow-cycling C Differences in MRT were not mirrored by changes in the chemical composition

Fixation on minerals Methods: Amount of C bound to soil minerals was estimated by the demineralization technique Clay mineralogy was determined by X-ray diffraction of oriented samples Iron and aluminium oxides and oxyhydroxides were estimated by the dithionite-citrate-bicarbonate method Results: Proportion of SOC bound to minerals increased slightly with depth (surface: 50± 0. 5%, subsoil: 58± 1%) Organo-mineral complexes at depth must be ten times more stable than in surface → not supported by results

Delivery of fresh C Methods: Soils were incubated at 20°C, at a water potential of -100 k. Pa for 161 days Dual-labelled cellulose was mixed with half of the incubated soils (1 g cellulose C/kg soil). Control soils were also mixed CO 2 evolved was trapped in Na. OH and measured by continuous flow colorimetry 13 C and 14 C analysis of carbonates was carried out by IRMS and AMS Results: Addition of cellulose stimulated microbial respiration and growth Stimulation of decomposers induced significant increase in production of unlabelled soil-originated CO 2 (Priming) Total microbial biomass and priming effect significantly decreased with exhaustion of cellulose

Conclusion: Although microbes decompose ancient C, acquisition of this energy is not sufficient to sustain long-term biological activity → energy required to break down recalcitrant SOC is higher than energy supplied by catabolism of such substrate → long-term activity of decomposer populations depends on permanent supply of fresh C

Implications Biological and physical processes that bury recalcitrant SOC below the deposits of fresh C protect it from decomposition and allow C storage over millennia Even under favorable conditions for microbial activities, SOC from deep soil does not provide enough energy to sustain active microbial populations/production of enzymes Deep SOC decomposition may be reactivated (changes in land use/agriculture practices)

Thank you for your attention!