Microaggregation and organic matter storage at the silt-size scale

doi:10.1016/j.geoderma.2008.05.021

Geoderma

Volume 146, Issues 1–2, 31 July 2008, Pages 326-335

https://doi.org/10.1016/j.geoderma.2008.05.021 Get rights and content

Abstract

The relationship between soil aggregation at the silt-size scale, and soil organic C dynamics has scarcely been studied. In this work, we developed a fractionation protocol comprising complete dispersion of the fraction > 50 μm of the original soil sample, and subsequent size and density fraction of the < 50 μm fraction to isolate stable aggregates from the silt-size fraction of a silty cultivated temperate soil. This fraction also contained non-aggregated silt-size mineral particles and small amounts of non-occluded light organic particles. Slaking-resistant silt-size aggregates stored almost half of the total soil organic C (TOC) as organic matter bound to silt (8% TOC) and clay minerals (28% TOC) and as occluded organic particles (8% TOC). Clay mineralogy was related to aggregation at this scale, since aggregates were selectively formed from 2:1 clays, while quartz and kaolinite-like clays were preferentially found in the non-aggregated clay fraction. We conclude that stable microaggregates are formed within the silt-size fraction and store organic C both by adsorption and by entrapment of small organic particles. The turnover rate of organic carbon in such microstructures, their sensitivity to land use and cultivation practices, and their importance in different soil types remain to be determined.

Introduction

Soil C storage is an issue of major concern at present. Beyond the potential strategies for atmospheric CO₂ sequestration (Lal, 2004), it is well known that soil quality decreases when soil organic C is lost (Wienhold et al., 2004). Soil organic matter (SOM) can be protected from degradation in the soil through three main mechanisms (von Lützow et al., 2006): selective preservation due to chemical recalcitrance, stabilization by spatial inaccessibility to enzymes and microorganisms, and stabilization by interaction with mineral surfaces and metal ions. The present knowledge on soil organic matter (SOM) dynamics and storage (Balesdent et al., 2000, Six et al., 2002), and the studies on aggregate formation and stabilization (Six et al., 1999a) show that the two latter mechanisms take place at the micrometric scale, and thus protection of SOM occurs preferentially within soil microaggregates rather than in macroaggregates (Six et al., 1999a, Besnard et al., 1996). From the hierarchical theory of aggregates as expressed by Oades (1984), and later demonstrated and completed by many others (Golchin et al., 1994, Six et al., 1999a, Balesdent et al., 2000), we know that in temperate soils microaggregates are formed and preserved within larger aggregates, and that SOM has a major role in this process when other potential binding agents such as Ca, iron oxides and hydroxides or allophanes are not present (Six et al., 2004).

Most studies on aggregation (Six et al., 1999a, Six et al., 2002, Oorts et al., 2006) consider microaggregates to be larger than 50–53 μm and smaller than 250 μm in equivalent diameter, and disregard aggregation at the silt and clay-size fractions. The high contents of organic C in such fractions (20 to 40% of soil C in the silt-size fraction in temperate soils, according to Feller and Beare (1997)), are considered to be due almost exclusively to protection by adsorption to the mineral surfaces and to recalcitrance, rather than to physical entrapment.

However, Christensen (1996) conceptually described the silt-size fraction as composed of (i) “primary organomineral complexes” to which a large proportion of the soil organic matter is associated, (ii) non-occluded or uncomplexed OM (sensu Gregorich et al., 2006), and (iii) aggregates made up of particles belonging to smaller size classes. Studies applying UV photo-oxidation to soil physical fractions < 53 μm (Skjemstad et al., 1993) demonstrated the existence of a SOM pool within this fraction that was resistant to such oxidation, and was thus considered physically protected within microaggregates. By dispersing samples from a silty cultivated soil with increasing dispersing energies, Balabane and Plante (2004) recently demonstrated that the silt-size fraction recovered after weak and moderate dispersion of the soil was in fact composed of an important amount of silt-size aggregates, which would be formed of clay or silt-size organic particles, silt-size mineral particles and clay-size mineral particles. They found that these aggregates can be indirectly quantified using the clay-to-silt ratio found to be constant within aggregates, and the amount of material recovered in the silt-size fraction after dispersion. The amount of C associated with the clay fraction within those aggregates increased with the size of the dispersed aggregates, suggesting that the location of SOM in such aggregates has an indirect effect on its stabilization in the long-term because it offers the time and conditions necessary for occluded OM to become recalcitrant by humification or to adsorb on mineral surfaces.

Another study on a different silty soil (Chenu and Plante, 2006) demonstrated that the clay-size fraction after so-called “complete dispersion” of a soil sample consists indeed mainly of nano- to microaggregates, which are suspected to be sites of preferential stabilization of soil OM.

In this study we hypothesized that stable microaggregates (secondary organomineral complexes according to Christensen (1996)) exist in the silt-size fraction of the soil (i.e., 2–50 μm), and that they represent important sites of C storage and stabilization, as C adsorbed to the surfaces of clay and silt-size mineral particles or as small occluded organic particles. The objective of our work was to test the existence of water-stable aggregates in the silt-size fraction of a silty cultivated temperate soil and to determine their characteristics by (i) developing a methodology based on size and density fractionation which allows for their isolation, (ii) quantifying their abundance and associated organic matter, and (iii) comparing their constituents with their non-aggregated counterparts, i.e. occluded vs. non-occluded organic matter and clay minerals.

Section snippets

Soil and sampling

We studied soil from the current Closeaux Field Experiment, at the INRA Experimental Station in the park of the Château de Versailles (France). The field has been under conventional cereal monoculture since about 1930. The soil is classified as a Eutric Cambisol in the FAO reference base, which corresponds to a Eutrochrept in Soil Taxonomy (Soil Survey Staff (S.S.S.), 2003). The upper horizon (Ap) contains 1.3% organic C, and has a pH of 6.8. Soil texture consists of 32% sand, 51% silts and 17%

Evaluation of the separation method

After dispersing the samples with the size-separation device (Table 1), we obtained the same mass of sand-size particles (> 50 μm) as that given by the classical method using 16 h of agitation with glass beads (Balabane and Plante, 2004). Hence, the objective of dispersing all aggregates > 50 µm was attained. Furthermore, the results of the laser analysis demonstrated that the particle-size distribution (as % volume) of the silt-size separates after the dispersion and sieving of the samples was

Methodology

The study of stable silt-size aggregates requires their proper isolation. The proposed methodology allowed the separation of true slaking-resistant aggregates in the size fraction combining both size and density separations. Density separations implied a loss of soluble C. Soil C losses while using SMT solutions for density fractionation have been reported by several authors (Chenu and Plante, 2006, Crow et al., 2007). Chenu and Plante (2006) reported recoveries between 81 and 88% while

Conclusions

Studies on SOM dynamics are increasingly based on physical fractionations and whereas a large fraction of SOM is usually recovered in the silt + clay fraction, this fraction has been little considered in the studies on aggregation. We used a combination of size and density fractionations to separate different fractions within the silt-size fraction of the soil. These fractions corresponded to the ones we had conceptually described before the fractionation, i.e., uncomplexed non-occluded light

Acknowledgements

The authors thank Victor Lopez for preliminary experiments on the fractionation method, Gérard Bardoux and Nicolas Péchot for elemental analysis, Nelly Wolff for assistance with the microscope, and Jeanne-Chantal Dur for laser granulometry assistance. Christophe Moni and Cornelia Rumpel are thanked for revising an early version of the manuscript. Also the Dirección de Política Científica (Basque Government) is acknowledged for funding the fellowship of I.Virto.

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For example, the organic carbon localized in microaggregates within stable macroaggregates represented in average 31 % of total SOC for three soils under different tillage regimes (Denef et al., 2004). The organic carbon localized within stable silt-size microaggregates represented 44 % of total SOC (Virto et al., 2008). The amount of protected SOC estimated by the crushing test is hence much less than the amount of SOC measured inside the aggregates by aggregate fractionation methods.
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Microaggregation and organic matter storage at the silt-size scale

Abstract

Introduction

Section snippets

Soil and sampling

Evaluation of the separation method

Methodology

Conclusions

Acknowledgements

Soil and Tillage Research

Organic Geochemistry

Geoderma

Geoderma

Geochimica et Cosmochimica Acta

Geoderma

Geoderma

Soil Biology and Biochemistry

Soil & Tillage Research

Soil Biology & Biochemistry

Aggregation and carbon storage in silty soil using physical fractionation techniques

European Journal of Soil Science

Fate of particulate organic matter in soil aggregates during cultivation

European Journal of Soil Science

Particulate soil organic-matter changes across a grassland cultivation sequence

Soil Science Society of America Journal

Clay-sized organo-mineral complexes in a cultivation chronosequence: revisiting the concept of the ‘primary organo-mineral complex’

European Journal of Soil Science

Carbon in primary and secondary organomineral complexes

Density fractionation of forest soils: methodological questions and interpretation of incubation results and turnover time in an ecosystem context

Biogeochemistry