Summary
Mechanisms for C stabilization in soils have received much interest recently due to their relevance in the global C cycle. Here we review the mechanisms that are currently, but often contradictorily or inconsistently, considered to contribute to organic matter (OM) protection against decomposition in temperate soils: (i) selective preservation due to recalcitrance of OM, including plant litter, rhizodeposits, microbial products, humic polymers, and charred OM; (ii) spatial inaccessibility of OM against decomposer organisms due to occlusion, intercalation, hydrophobicity and encapsulation; and (iii) stabilization by interaction with mineral surfaces (Fe-, Al-, Mn-oxides, phyllosilicates) and metal ions. Our goal is to assess the relevance of these mechanisms to the formation of soil OM during different stages of decomposition and under different soil conditions. The view that OM stabilization is dominated by the selective preservation of recalcitrant organic components that accumulate in proportion to their chemical properties can no longer be accepted. In contrast, our analysis of mechanisms shows that: (i) the soil biotic community is able to disintegrate any OM of natural origin; (ii) molecular recalcitrance of OM is relative, rather than absolute; (iii) recalcitrance is only important during early decomposition and in active surface soils; while (iv) during late decomposition and in the subsoil, the relevance of spatial inaccessibility and organo-mineral interactions for SOM stabilization increases. We conclude that major difficulties in the understanding and prediction of SOM dynamics originate from the simultaneous operation of several mechanisms. We discuss knowledge gaps and promising directions of future research.

Stabilization of organic matter in temperate soils: mechanisms and their relevance under different soil conditions – a review

Despite their low carbon (C) content, most subsoil horizons contribute to more than half of the total soil C stocks, and therefore need to be considered in the global C cycle. Until recently, the properties and dynamics of C in deep soils was largely ignored. The aim of this review is to synthesize literature concerning the sources, composition, mechanisms of stabilisation and destabilization of soil organic matter (SOM) stored in subsoil horizons. Organic C input into subsoils occurs in dissolved form (DOC) following preferential flow pathways, as aboveground or root litter and exudates along root channels and/or through bioturbation. The relative importance of these inputs for subsoil C distribution and dynamics still needs to be evaluated. Generally, C in deep soil horizons is characterized by high mean residence times of up to several thousand years. With few exceptions, the carbon-to-nitrogen (C/N) ratio is decreasing with soil depth, while the stable C and N isotope ratios of SOM are increasing, indicating that organic matter (OM) in deep soil horizons is highly processed. Several studies suggest that SOM in subsoils is enriched in microbial-derived C compounds and depleted in energy-rich plant material compared to topsoil SOM. However, the chemical composition of SOM in subsoils is soil-type specific and greatly influenced by pedological processes. Interaction with the mineral phase, in particular amorphous iron (Fe) and aluminum (Al) oxides was reported to be the main stabilization mechanism in acid and near neutral soils. In addition, occlusion within soil aggregates has been identified to account for a great proportion of SOM preserved in subsoils. Laboratory studies have shown that the decomposition of subsoil C with high residence times could be stimulated by addition of labile C. Other mechanisms leading to destabilisation of SOM in subsoils include disruption of the physical structure and nutrient supply to soil microorganisms. One of the most important factors leading to protection of SOM in subsoils may be the spatial separation of SOM, microorganisms and extracellular enzyme activity possibly related to the heterogeneity of C input. As a result of the different processes, stabilized SOM in subsoils is horizontally stratified. In order to better understand deep SOM dynamics and to include them into soil C models, quantitative information about C fluxes resulting from C input, stabilization and destabilization processes at the field scale are necessary.

Deep soil organic matter—a key but poorly understood component of terrestrial C cycle

Natural variations in the carbon isotope values of plants: Implications for archaeology, ecology, and paleoecology

https://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_7/b_fdi_53-54/010016060.pdf

Soil properties under Amazon forest and changes due to pasture installation in Rondônia, Brazil

SUMMARY

Three methods of cultivation, conventional tillage (CT), superficial tillage (ST) and no-tillage (NT), were applied for 17 years to continuous maize. Their effect on soil organic carbon content was investigated through measurements of carbon and 13C/12C ratios, using the natural difference in 13C content between C3 plants and maize, which is a C4 plant. Because the soil had carried C3 plants before the experiment started, the organic carbon remaining from that time (C3,-carbon), was distinguished from the carbon derived from maize. Comparison between continuous wheat and maize plots showed that organic matter from both maize and wheat decomposed without significant 13C enrichment, whereas older C3-carbon was enriched by 1.5% compared to that of fresh wheat material. From the initial 3.6 kg C m−2 in the topsoil (0–30 cm), 0.95 were mineralized in the CT treatment, but only 0.45 in NT. The mineralization was the same in the tilled layer of ST as in CT. The CT treatment accumulated 1.1 kg C m−2 of maize-derived carbon and the NT treatment 0.8. The mineralization of initial C3-carbon was the same at all depths between 0 and 30 cm in the NT treatment; 75% of the carbon derived from maize was found in the 0–5 cm layer.

Effect of tillage on soil organic carbon mineralization estimated from 13C abundance in maize fields

The natural relationship 13C/12C- determined in three soil profiles under grass vegetation indicated a depletion in organic I3C at depth: the 613C was between - 18%0 and - 15%0 in the A horizons and ranged from - 18 to - 22%0 at depth. Previous work showed that in forest soils, where 613C was near - 28%0 in the upper horizon, there was, on the contrary, a relative enrichment of the lower strata. This meant that 6I3C, 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: 613C of grassland and a forest sombric horizon were almost equal at around - 22%0. These results might mean that, in natural ecosystems, the isotopic carbon composition of the soil underlying humus would be independent of the vegetation type. This would have practical implications for the use of I3C as a tracer for soil organic matter studies.

https://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_7/b_fdi_53-54/010016044.pdf

B. Volkoff

Papers

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