9
Soil carbon storage and stabilisation in andic soils: A review F. Matus a,b, , C. Rumpel d , R. Neculman c,d , M. Panichini c,d , M.L. Mora a,b a Departamento de Ciencias Químicas y Recursos Naturales, Universidad de La Frontera, Temuco, Chile b Scientic and Technological Bioresource Nucleus (BIOREN-UFRO), Universidad de La Frontera, Temuco, Chile c Programa de Doctorado en Ciencias de Recursos Naturales, Universidad de la Frontera, Temuco, Chile d CNRS, BIOEMCO (UMR Université Paris VI et XI-CNRS-IRD-AgroParisTech), Thiverval-Grignon, France abstract article info Article history: Received 30 September 2013 Received in revised form 8 March 2014 Accepted 8 April 2014 Available online xxxx Keywords: Allophane Andic properties Andisol Land-use potential Short-range order clays Variable charge Andic soils contain a large amount of stabilised soil organic matter (SOM). The present study aims to review and integrate the determining factors and mechanisms of SOM stabilisation in andic compared with other (non-andic) soil types. We have reviewed recent literature regarding the nature of SOM and its stabilisation processes in the top- and sub-soil to address and discuss the interaction between the SOM and the mineral phase. The carbon (C) storage capacity by the metal-humus-complex formation of volcanic soils is also evaluated. The most important stabilisation processes are related to the incorporation and decomposition of microbial- derived C along with the changing C storage capacity with increasing soil development. The priming and destabilisation of adsorbed SOM are crucial mechanisms inuencing the soil C sequestration in subsoils. The C storage capacity of andic soils was closely related to the Na-pyrophosphate extractable Al and Fe. The upper boundary for SOM saturation with Al and Fe was a molar metal:C ratio of 0.18. The inuence of climate, miner- alogy and soil disturbances on the SOM storage capacity of andic soils also require further attention. © 2013 Elsevier B.V. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 2. Nature and distribution of organic matter in Andisols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 2.1. Chemical composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 2.2. Distribution across the soil prole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 3. Interactions between the organic matter and the mineral phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 3.1. Chemical stabilisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 3.2. Physical protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 3.3. Stabilisation mechanisms as inuenced by land-use, climate and vegetation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 3.4. SOM destabilisation processes in Andisols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 3.5. SOM storage changes over time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 4. Carbon storage capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 4.1. Evaluation and control of carbon storage capacity in the chemical pool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 6. Future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 1. Introduction Volcanic ash soils contain a disproportionate amount of soil C in soil organic matter (SOM) (Batjes, 1996; Eswaran et al., 1993) because they comprise only 0.84% of the global land area but may contain several times more C than non-volcanic soils (Dahlgren et al., 2004). These soils store approximately 5% of the global soil C (Eswaran et al., 1993). Catena 120 (2014) 102110 Corresponding author at: Departamento de Ciencias Químicas y Recursos Naturales, Universidad de La Frontera, Av. Francisco Salazar 01145, P.O. Box 54-D, Temuco, Chile. Tel.: +56 45 27 44 241. E-mail address: [email protected] (F. Matus). http://dx.doi.org/10.1016/j.catena.2014.04.008 0341-8162/© 2013 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Catena journal homepage: www.elsevier.com/locate/catena

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Page 1: Soil carbon storage and stabilisation in andic soils: A review

Catena 120 (2014) 102–110

Contents lists available at ScienceDirect

Catena

j ourna l homepage: www.e lsev ie r .com/ locate /catena

Soil carbon storage and stabilisation in andic soils: A review

F. Matus a,b,⁎, C. Rumpel d, R. Neculman c,d, M. Panichini c,d, M.L. Mora a,b

a Departamento de Ciencias Químicas y Recursos Naturales, Universidad de La Frontera, Temuco, Chileb Scientific and Technological Bioresource Nucleus (BIOREN-UFRO), Universidad de La Frontera, Temuco, Chilec Programa de Doctorado en Ciencias de Recursos Naturales, Universidad de la Frontera, Temuco, Chiled CNRS, BIOEMCO (UMR Université Paris VI et XI-CNRS-IRD-AgroParisTech), Thiverval-Grignon, France

⁎ Corresponding author at: Departamento de CienciasUniversidad de La Frontera, Av. Francisco Salazar 01145Tel.: +56 45 27 44 241.

E-mail address: [email protected] (F. Matu

http://dx.doi.org/10.1016/j.catena.2014.04.0080341-8162/© 2013 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 September 2013Received in revised form 8 March 2014Accepted 8 April 2014Available online xxxx

Keywords:AllophaneAndic propertiesAndisolLand-use potentialShort-range order claysVariable charge

Andic soils contain a large amount of stabilised soil organic matter (SOM). The present study aims to reviewand integrate the determining factors and mechanisms of SOM stabilisation in andic compared with other(non-andic) soil types. We have reviewed recent literature regarding the nature of SOM and its stabilisationprocesses in the top- and sub-soil to address and discuss the interaction between the SOM and the mineralphase. The carbon (C) storage capacity by themetal-humus-complex formation of volcanic soils is also evaluated.The most important stabilisation processes are related to the incorporation and decomposition of microbial-derived C along with the changing C storage capacity with increasing soil development. The priming anddestabilisation of adsorbed SOM are crucial mechanisms influencing the soil C sequestration in subsoils. The Cstorage capacity of andic soils was closely related to the Na-pyrophosphate extractable Al and Fe. The upperboundary for SOM saturation with Al and Fe was a molar metal:C ratio of 0.18. The influence of climate, miner-alogy and soil disturbances on the SOM storage capacity of andic soils also require further attention.

© 2013 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1022. Nature and distribution of organic matter in Andisols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

2.1. Chemical composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1032.2. Distribution across the soil profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

3. Interactions between the organic matter and the mineral phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1043.1. Chemical stabilisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1043.2. Physical protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1053.3. Stabilisation mechanisms as influenced by land-use, climate and vegetation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1053.4. SOM destabilisation processes in Andisols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1063.5. SOM storage changes over time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

4. Carbon storage capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1074.1. Evaluation and control of carbon storage capacity in the chemical pool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1076. Future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

Químicas y Recursos Naturales,, P.O. Box 54-D, Temuco, Chile.

s).

1. Introduction

Volcanic ash soils contain a disproportionate amount of soil C in soilorganic matter (SOM) (Batjes, 1996; Eswaran et al., 1993) because theycomprise only 0.84% of the global land area but may contain severaltimes more C than non-volcanic soils (Dahlgren et al., 2004). Thesesoils store approximately 5% of the global soil C (Eswaran et al., 1993).

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The hypotheses that explain SOM accumulations in andic soil arethe following: 1) phosphorus is a rate-limiting factor for organicmatter mineralisation (Munevar and Wollum, 1977), and 2) miner-al associations reduce SOM mineralisation (Zunino et al., 1982).Moreover, enzymes and microbial by-products may be deactivatedby adsorption on short-range order (SRO) mineral surfaces ofandic soils (Saggar et al., 1994). In volcanic soils, the microbialdecomposition of organic matter may be limited by Al toxicity(Illmer et al., 2003; Tate and Theng, 1980) and low pH values. Thisis particularly important in alu-andic soils (dominated by Al-humus-complexes), where the pH is low, versus sil-andic soils(dominated by allophane and allophane-like minerals), or andicsoils, where the pH is higher (Dahlgren et al., 2004). Another impor-tant factor in the preservation and accumulation of SOM in volcanicsoils is burial by repeated additions of volcanic deposits. Thisrejuvenates landscapes and creates new ecosystems that maysequester organic C (Imaya et al., 2010).

Andic soils, including Andisols, as described by the USDA classifica-tion (Soil Survey Staff, 2008), or Andosols in theWRB system classifica-tion (FAO-ISRIC-ISSS, 1998), cover more than 120 million hectaresworldwide (Dahlgren et al., 2004) and display unique morphological,physical and chemical properties attributed to the composition oftheir mineral phase. These soils' minerals consist of SRO materialssuch as allophane, imogolite, ferrihydrite and Al- and Fe-humus com-plexes lacking a long-range crystal atomic order (Harsh et al., 2002).Based on the mineralogical composition of the A horizons, two groupsof Andisols are included: non-allophanic and allophanic Andisols. Thelatter are dominated by allophane and imogolite-type materials andthe former by Fe- and Al-organic complexes (humus-complexes). At apH N5, carbonic acid weathering causes the soil to produce allophaneformations, allowing the Al to polymerise and co-precipitate withSi, whereas at a pH b5, the formation of metal (Al and Fe)-humuscomplexeswill be favoured. Allophanes are weatheredmetastable non-crystalline materials in both temperate and tropical humid environ-ments. As weathering proceeds, the SRO minerals evolve to morestable crystalline minerals (e.g., halloysite, kaolinite, gibbsite) leadingto other soil orders (e.g., Ultisols, Inceptisols or Alfisols) (Dahlgrenet al., 2004). In perhumid tropical environments, Oxisols occur. Undercool–cold-humid climates, Podzols are often the dominant soil devel-oped under acidic conditions (Ugolini et al., 1977). The high SOMstorage capacity of Andisols is a function of the high surface areas ofnoncrystalline constituents that are available for the sorption of organicmatter (Baldock and Nelson, 2000; Saggar et al., 1994). Therefore,Andisols would be expected to have a higher potential for SOM seques-tration than non-andic soils.

The present review aims specifically to update and integrate thefundamental aspects determining SOMcomposition and its stabilisationand destabilisation processes that occur in andic soils (hereafter re-ferred to as Andisols). Stabilisation is defined as the absence of SOMbio-degradation (Sollins et al., 1996), where the humification dominatesover mineralisation. In particular, we want to note the differences andsimilarities between Andisols and other soil types to answer the ques-tion of why Andisols are able to store more organic matter than anyother soil type. We have reviewed recent literature concerning the na-ture of SOM and its relationship to the stabilisation processes operatingin the top- and subsoil. As a result, this review has three sections. First,we focus on the nature and origin of SOM in Andisols and its importanceon the stabilisation processes. In the second section, we discuss the in-teractions between the SOM and the mineral phase and its implicationsfor C-stabilisation. We will also consider the destabilisation processesrelated to the priming action, dissolved organic matter (DOM) andwater extractable organic matter (WEOM). The latter two have beenproposed to serve as a crucial mechanism of soil C transport into thesubsoil. Lastly, in the third section, the C storage capacity change overtime is discussed and the metal-humus-complex formation for C se-questration of Andisols is evaluated.

2. Nature and distribution of organic matter in Andisols

2.1. Chemical composition

The chemical composition of SOM may provide valuable informa-tion regarding SOM precursors and the mechanisms of stabilisation(Derenne and Largeau, 2001). Most studies on the chemical composi-tion of SOM in Andisols were carried out after alkaline extraction ofhumic and fulvic acids using NaOH and Na4P2O7 (Nierop et al., 2005).The chemical properties of humic substances were determined to bedifferent for Andisols and adjacent non-andic soils. The Andisols accu-mulate more unsaturated C than the non-andic soils. This indicatesmore carboxyl and methoxyl functional groups from poorly degradedlignin, which may be involved in stable Al complex formation fromamorphous materials (Conte et al., 2003). Humic materials extractedfrom Andisols in Japan have been reported to exhibit a higher degreeof condensation compared with those of non-andic soils (Kuwatsukaet al., 1978; Yonebayash and Hattori, 1988). The high aromaticity ofthese fractions could be related to the presence of charred plants fromthe regular burning of vegetation and melanic epipedon characteristics(Golchin et al., 1997; Shindo et al., 2004). However, Nierop et al. (2005)indicated that the NaOH extractable SOM of volcanic soils is unaffectedby burning; these soils were found to be dominated by polysaccharide-derived compounds. These studies contrast with others noting that therecalcitrant plant-derived compounds are scarcely preserved and thatmost of the SOM in Andisols is composed of easily degradablemicrobial-derived material (Buurman and Nierop, 2007; Buurmanet al., 2007; González-Pérez et al., 2007; Naafs et al., 2004; Nieropet al., 2005; Suárez-Abelenda et al., 2011). This finding is in accordancewith the general literature regarding SOM stabilisation in non-volcanicsoils, where the chemical recalcitrance of plant litter compounds is nolonger regarded as an SOM stabilisation mechanism (Dungait et al.,2012; Kleber et al., 2011; Marschner et al., 2008). Even black C, a recal-citrant SOM component that is generally preserved in other soil typesover centuries (Hammes et al., 2008; Rumpel et al., 2008), does notseem to accumulate in Andisols (Cusack et al., 2013). This may be dueto the absence of interactions of this component with soil minerals(Hernández et al., 2012; Rivas et al., 2012). Studies on hydrophobic(HB) and hydrophilic (HI) recalcitrant materials other than black C re-vealed that HB/HI ratio of forest soils was substantially reduced aftercultivation, resulting in an SOM poor in alkyl (aliphatic compounds),except in Andisols (Spaccini et al., 2006). InAndisols, there is a contribu-tion of alkyl C along the soil profile that is resistant to dichromate chem-ical oxidation and that increases C storage (Rivas et al., 2012). It seemsthat alkyl structures can beprotected fromoxidation due to their hydro-phobic nature, possibly through encapsulation into their hydrophobicnetwork (Knicker and Hatcher, 2001). Indeed, Barbera et al. (2008) re-ported that the refractory organic fraction, enriched in aliphatic com-pounds, did not greatly interacted with kaolinite, smectite or poorlycrystalline Fe or Al because part of this fraction (most likely proteins)was bound to crystalline Fe-oxides. Recently, Tonneijck et al. (2010) re-ported that extremely acidic soil pH conditions possibly lead to Al tox-icity along with high microporosity, may enhance the preservation ofplant-derived aliphatic C in Andisols. Other studies related to bio-markers showed that intact biopolyesters may be chemically protectedin the insoluble organic macromolecular network (Naafs and vanBergen, 2002). Recently, Nierop and Jansen (2009) reported that sol-vent extractable lipids preserved their plant-derived signaturesthroughout the soil profile, even if the bulk SOM composition no longerresembled the vegetation growing on these soils. Thus, some recalci-trant component of SOM, such as charred materials, seems to be lesspreserved in comparison with SOM of an aliphatic nature.

The composition of the SOM stabilised by mineral interactions insubsoils of allophanic as well as non-allophanic Andisols was recentlystudied (Rumpel et al., 2012). This SOM was found to be enriched byN-containing compounds, and is different from those of the A horizon.

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This finding could be due either to the soil's specific origin or to the in-situ ageing of the SOM in subsoils. Evidence from studies on volcanicsoils suggests that the presence of SROminerals has a controlling effecton the soil C:N,most likely due to the stabilising effect of highly reactivenon-crystalline clay minerals on the SOM; however, this effect cannotbe generalised for Andisols. An increasing C:N ratio with soil depthwas reported for Andisols dominated by SRO minerals (Conte et al.,2003; Kawahigashi et al., 2003; Lilienfein et al., 2004; Schrumpf et al.,2006). In contrast, most other (non-andic) soil types showed decreasesin the C:N ratio with depth (Rumpel and Kögel-Knabner, 2011), whichwas attributed to the decomposition of plant-derived compounds andgreater contributions of microbial products in the absence of stabilisingSRO minerals (Baldock et al., 1992; Nadelhoffer and Fry, 1988). TheNMR spectral differences between organic matters collected from vari-ous horizons were consistent with the enrichment of material that hasundergone less microbial processing in the deeper Andisols. Duringplant litter decomposition and SOM formation, the dominant signalin the O-alkyl C region from plant cellulose and hemicellulose is pro-gressively replaced by the alkyl C structures (nonpolar aliphatic com-pounds) such as waxes, resins and lipids. The accumulation of thesecompounds in soils is attributed to the selective preservation of plant-derived compounds and/or to new production during microbial pro-cessing (Grandy and Neff, 2008).

Briefly, the non-crystalline minerals of Andisols interact with plantor microbially derived compounds, leading to a preservation of alkyl Cin association with unsaturated C, thus increasing the C:N ratio in thesubsoil. Detailed knowledge regarding organic matter compositionand degradation at different depths in Andisols is lacking.

2.2. Distribution across the soil profile

The strong adsorption potential of the Andisol mineral phase sug-gests that the DOM (Aran et al., 2001) andWEOM are of limited impor-tance. Soil solution studies of volcanic deposits have indicated that theformation of amorphous materials and SOM accumulation that arecharacterised by high concentration of Fe, Al, andDOM in the A horizonsshow little translocation to the B horizons (Dahlgren et al., 1991; Ugoliniet al., 1988). Therefore, it has been long hypothesised that the non-crystalline materials (e.g., allophane, imogolite, and ferrihydrite) inthe B horizons have been formed in situ rather than transported. Re-cently, Kaiser and Kalbitz (2012) proposed a new conceptual model inwhich the DOM in the soil profile resulted from fractional sorption-co-precipitation and from microbial processing as a consequence ofthe C saturation of the protective site and the subsequent delivery of or-ganic matter that was previously sorbed into overlaying horizons. Thiseffect is due to complex reactions of Al with soil organic C from thesoil solution and the subsequent precipitation of insoluble complexesof Al-SOM (Rasmussen et al., 2006). The model also applies to Andisolsand can explain the radiocarbon age of the SOM in the subsoil that con-trasts sharply with the assumption of roots as a major source of deepsoil C (Rumpel et al., 2002). These results corroborate to recent studiesby Kramer et al. (2012) and Osher et al. (2003). The organic mattersequestration in the subsoils of Andisols is controlled by the transportand adsorption of oxidised compounds, which strongly resemble theDOM from plant litter (Kramer et al., 2012). SRO mineral–SOM com-plexes were transported to deeper soil in Andisols after the conversionof tropical forest into grassland (Osher et al., 2003). In high precipitationregions, the C losses from the soil appear to occur via downward trans-port, either as colloids or in solution. However, there are almost nostudies that address this crucial issue.Moreover, changes in the chemis-try solutiondue to agricultural practices can lead to a dissociation of SROmineral–organic matter complexes and C losses due to mineralisation(Osher et al., 2003).

Kramer et al. (2012) suggested that long-term C storage in thesubsoils of Andisols occurs through the SRO minerals to which plant-litter-derived aromatic acids are adsorbed and then transported via

preferential flow. A preferential flow pathway in the delivery of C todeep soils was observed in Hawaiian rainforest volcanic soils (Marin-Spiotta et al., 2011). However, it seems that the dominant drainageprocess in the Andisol is matrix flow and that preferential flow occursonly at certain times of the year, accounting for 16–27% of the annualtotal drainage (Eguchi and Hasegawa, 2008; Hasegawa and Sakayori,2000). Bioturbation may be important for vertical SOM translocationin Andisols (Tonneijck and Jongmans, 2008), despite acidic soil condi-tions. Roots are usually thought to be the primary contributor to deepsoil C (Jobbágy and Jackson, 2000; Rasse et al., 2006). However, inAndisols, the root activitymay be limited by the acidic pH and Al activity.The importance of the three different precursors to SOM (root-derivedorganic matter, DOM and organic matter transported by bioturbation)that contribute to the high C content in the deep horizon of Andisolsneeds to be addressed in future studies.

In summary, it is unclear how plant-litter-derived organic acidscould quantitatively contribute to the high amounts of stabilised SOMin the subsoil of Andisols. A new conceptual model suggests that theDOM andWEOM are adsorbed and desorbed after microbial processing(Kaiser and Kalbitz, 2012). Empirical evidence showing the importanceof SRO and Al for the destabilisation of SOM at a field scale needs to becompleted to determine the type of organic matter that is sorbedunder specific conditions. The question of whether horizontal stratifica-tion is of the same importance for subsoil C-stabilisation as in non-andicsoil types needs to be clarified (Chabbi et al., 2009).

3. Interactions between the organic matter and the mineral phase

3.1. Chemical stabilisation

The composition of the colloidal fraction (b2 μm) of temperateAndisols developed under humid climate conditions forms a continuumbetweenpure Al-humus complexes (which are very rare) and pure allo-phane and imogolite fractions, depending on the pH and organic mattercharacteristics (Mizota and Van Reeuwijk, 1989). This has been ob-served in New Zealand, African and Japanese volcanic-ash-derivedsoils (Parfitt et al., 1983, Parfitt, 2009; Shoji et al., 1993; Wada, 1989;Wada et al., 1987). The Al- and Fe-humus complexes are formedmainlyin environments that are rich in organic C and low pH, but allophane isrestricted to a pH N5 (Pizarro et al., 2003; Shoji and Fujiwara, 1984).

The mineralogical properties of allophanic Andisols, particularlySRO (allophane and imogolite type materials), present a high reactivesurface area and are regarded as the major agent of C-stabilisation(Parfitt, 2009; Parfitt et al., 2002; Torn et al., 1997; Zunino et al.,1982). This feature, as well as the abundance of charged sites on theclay surface, facilitates the adsorption of organic molecules, which isan important mechanism leading to the physicochemical protection ofthe SOM. Recently, studies have shown that allophane, which makesup most of the nanoclay fraction (b100 nm) of Andisols, contains highconcentrations of SOM that is resistant to chemical oxidation (Calabi-Floody et al., 2011). Panichini et al. (2012), however, indicated anoverestimation of SOM in themineral fraction from Andisols as a conse-quence of Al-SOM precipitation that can be transferred during disper-sion and fractionation processes.

The primary controllingmechanism leading to the sorption of humicsubstances by variable-chargedminerals is the ligand exchange reaction(Spark, 2003; Yuan et al., 2000). Matus et al. (2009) used a hypotheticalmodel thatwas developed by Yuan et al. (2000) to explain Al-SOMcom-plex formation in allophanic soils (Fig. 1). Hydroxyl groups attract orloose protons depending on the soil pH. The reaction produces a nega-tive charge that is compensated for by electrostatic interactions withNa+ or cations such as Al3+ ions. Therefore, humic acid sorption is pro-moted by the presence of electrolytes such asNaCl and CaCl2. Soil organ-icmatter binds to the spherules of allophane and can interact againwitha cation to form a new complex, mediated by electrostatic attraction.This mechanism is the precursor of further electrostatic interactions

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for a new nucleus of Al-SOM complexes that subsequently precipitate.However, Huygens et al. (2005) showed that electrostatic interactionwith amorphous Al and clay minerals was the main mechanism pro-moting SOM stabilisation in Andisols from southern Chile.

Although the mineralogical properties of SRO (allophane- andimogolite-type materials) are regarded as the key factor of C-stabilisation, Percival et al. (2000) studying New Zealand volcanicsoils, and Matus et al. (2006, 2008a), studying Chilean volcanic soils,supported the hypothesis that Al oxides, rather than clay (having 50%of allophane) and climatic factors, are the most important stabilisingagent for SOM. Experimental evidence of the stabilisation potential ofamorphous Al oxides was provided recently by Schneider et al. (2010)and Miyazawa et al. (2013). Schneider et al. (2010) concluded that themineral availability for the sorption of DOM aswell as the chemistry so-lution determines the sorptive interactions and, thus, the stability andcomposition of the adsorbed DOM and its resistance to microbial decay.

It seems that allophane typematerials can stabilise smaller amountsof C than previously thought. This was indicated by the interaction ofsynthetic allophane and organic acids in laboratory experiments byBoudot (1992).

3.2. Physical protection

The location of physically protected organic matter in stable aggre-gates reduces its accessibility for microbes, fungi or enzymes (Sollinset al., 1996) and is considered another important mechanism that con-trols the stability of the SOM in allophanic Andisols (Huygens et al.,2005) as well as other non-andic soils (Dungait et al., 2012). Severalauthors have noted the extremely high stability of soil aggregates isolatedfrom Andisols (e.g., Candan and Broquen, 2009; Hoyos and Comerford,2005). Baumgarten et al. (2013) reported that Al-humus-complexes pro-mote soil aggregation of hydrophobic organic compounds from alu-andicAndisols, which have a Na-pyrophosphate extractable Alp (Al-humuscomplexes, Parfitt and Kimble, 1989) and acid ammonium oxalateextractable Alo (Al dissolved from imogolite, allophane, and Al-humuscomplexes) ratio of N0.5 (Hernández and Almendros, 2012).

One characteristic of Andisols is that the aggregate hierarchy modeldoes not apply (Huygens et al., 2005). In this model, the binding agentsact at different hierarchical stages of aggregation (Tisdall and Oades,1982). Microaggregates (20 to 250 μm) made up of primary silt andclay particles are bound together by persistent binding (humified or-ganic matter and polyvalent metal cation complexes). These particles,in turn, are bound together as macroaggregates (N250 μm) by tempo-rary (e.g., fungal hyphae and roots) and transient (e.g., microbial- andplant-derived polysaccharides) binding agents. Because of this hierar-chical order, microaggregate stability is higher and less dependent onagricultural management than macroaggregate stability. The reasonallophanic soils have a high capacity to stabilise the SOM and do not re-spond to the hierarchical aggregationmodel was explained byWoignieret al. (2008), who described allophane as a porous material. Theyemphasised that these characteristics confer low diffusivity and perme-ability at the microaggregate level. This hypothesis was recently

Wall hole

Octahedral sheet

>Al<Si OH2

OHoVoid

space

Tetrahedral sheet

Defect structure

+ [-OOC ]

-

HA+carboxylate

Fig. 1. A transversal view of an external hypothetical allophane (model Al2Si2O5·nH2O, specific sugroups (\COOH) show a ligand exchange mechanism and Na+ or Ca2+ electrolytes by electrosAfter Matus et al. (2009).

supported by Chevallier et al. (2010), whodemonstrated that the biode-gradability of the SOM in Andisols was closely related to the allophanecontent and the existence of fractal clusters forming a ‘Nan labyrinth’that protects the SOM within the microaggregates. Recently however,Asano and Wagai (2014) showed that aggregate hierarchy also appliesfor Andisols, and they proposed a conceptual model for Andisol aggre-gate hierarchy.

Land-use has a tremendous impact on the physical protection ofSOM in Andisol soil structure (Dec et al., 2012; Dörner et al., 2010,2011; Ellies et al., 2000; Hartge et al., 1978; Seguel and Horn, 2005).To determine the quantity of SOM that is physically protected againstdecomposition in Andisols, sequential density fractionation approacheshave been applied (Basile-Doelsch et al., 2007; Panichini et al., 2012;Sollins et al., 2006). In general, organic matter associated with the min-eral phase is found in the high-density fraction (N2.0 g cm3) of soils(Turchenek and Oades, 1979). A density fractionation that indicates aseparation of the low-density fraction of Andisols lies between 1.35and 1.90 g cm−3 (Basile-Doelsch et al., 2007; Huygens et al., 2005,2008; Prior et al., 2007; Sollins et al., 2009). Basile-Doelsch et al.(2007) focused on the heavy fraction (N1.9 g cm−3) of an andic subsoilhorizon when analysing the mineral composition of various organo-mineral fractions. The results showed that imogolite-type mineralsbound to three to six times more organic matter than an orthoclaseand iron oxides. Contrasting results were obtained for the sequentialdensity fractionation of the A horizons of Andisols, where themineralo-gy was not related to SOM composition (Sollins et al., 2006, 2009).Although the contribution of the C associated with the high-densityfraction increases with soil depth, up to 50% of the organic C may bepresent in the low-density and particulate organic matter (POM) frac-tions of the A horizons of Andisols under forest cover (Panichini et al.,2012; Rumpel et al., 2012; Sollins et al., 1983; Spycher et al., 1983).This is more C than is typical in the POM fraction with a low-densityof non-andic soil, even in the A horizons.

Looking at organic matter dynamics in relation to mineral aggrega-tion assemblages of Andisols seems to be a valuable approach. Aggre-gate hierarchy is an essential soil characteristic that regulates variousbiogeochemical and physical processes, and it has been well studiedfor non-andic soils, but not for soils abundant in poorly-crystallineminerals.

3.3. Stabilisation mechanisms as influenced by land-use, climate andvegetation

Land-use changes are hypothesised to have little impact on theC-stabilisation potential of Andisols. The absence of SOM stabilisationby land-use changes can be a result of their high structural stability,nonexistence of aggregate hierarchy and slow stabilisation of freshorganic matter (Buurman et al., 2007; López-Ulloa et al., 2005; Paulet al., 2008). However, Johnson-Maynard et al. (1997) found chemicalchanges occurring in Andisols accompanied by successional communitiesof bracken fern from adjacent undisturbed forest. The pH and organic Cwere reduced by the succession, and it induced a shift from allophanic

+ OH-Na+

Ca2+>Al<Si

OH2

OOC

-

Interactions

rface area ~800 gm−2). The spherule of 3.5–5 nmand humic acids (HAs) with carboxylatetatic interactions.

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106 F. Matus et al. / Catena 120 (2014) 102–110

to non-allophanic properties. The effects of agricultural practices on theSOM in Andisols may be long-lasting. Soil management was found tochange the basic soil properties, such as bulk density, pH, and microbialcommunity structure, after 41 years of no-tillage (Rahman et al., 2008).A recent study indicated that SOM storagewas reduced and its chemistrychanged for as long as 200 years after the abandonment of agriculturalactivities (Cusack et al., 2013). This may be due to Andisols losing theirandic properties upon successive rewetting and drying because volcanicmaterials behave similarly to gels during drying, undergoing majorirreversible shrinkage that can modify the soil physical properties(Woignier et al., 2008). There is also a linkage between microbial com-munity and tillage practices in temperate Andisols (Rahman et al.,2008). Regular burning may intensify C-stabilisation and black colouringand may increase the aromaticity of the humus and the mean residencetime at the surface and in the subsoils (Iimura et al., 2010).

Land-use changes and management strategies were also found toaffect SOM storage and the microbial biomass activity (Dube et al.,2009; Zagal et al., 2009, 2013). Generally, the conversion of grasslandto forest plantation led to a decrease in the SOM stocks associatedwith disturbances to the site when the grassland was established(Dube et al., 2009). Conversely, the conversion from native forest tograssland led to an increase in the SOM stock in allophanic Andisols(Huygens et al., 2005). The direction of these changes was similar tothose observed by Guo and Gifford (2002) for a range of non-andicsoil types and was attributed the effect of vegetation cover and thesoil disturbance.

Broquen et al. (2005), studying a bioclimatic sequence of Argentini-an Andisols, observed that cover grass-shrub steppe with a xeric soilmoisture regime contributed to the formation of humic substancescompared with a Nothofagus spp. forest with a udic soil moisture re-gime. According to this finding, the type and quantity of organic matterinput, with a higher organic matter content and lower humic C/fulvic Cratio underNothofagus spp., indicate amore reactive organicmatter thatmay be somewhat impact weathering. The authors indicated that highC-mineralisation and moderately acidic soil pH under a xeric moistureregime comparedwith anudicmoisture regime, andwith a lower annualtemperature, produced the highest SOM storage, the lowest pH and thehighest aluminium activity.

In summary, it seems that land-use and soil management tech-niques, such as no-tillage or minimum tillage, can be implementedand developed to preserve the andic properties and initial SOM contentupon cultivation. However, it is unclear how implementing these man-agement techniques can preserve the andic properties.

3.4. SOM destabilisation processes in Andisols

Adsorbed and subsequently desorbed DOM and WEOM transloca-tion in the soil matrix as a mechanism of soil C destabilisation wereaddressed in Section 2.2. Here, we present another important mecha-nism, namely, the preferential C uses by soil microorganisms, whichcan lead to a change in the SOM turnover that is induced by the additionof labile C compounds (Sparling et al., 1982). Rasmussen et al. (2006,2007) studied the influence of the mineralogical composition of soilon this priming action in temperate rain forests. Amorphous clay-like(volcanic) soils tend to exhibit a negative priming (retardation of theSOM decomposition) in the surface soils, so the chemical interactionbetween the SOM and metals (Al and Fe oxides) promotes SOM-complexes. This result is important because complex formationsincrease the energy needed for the decomposition of the SOM. Likenon-andic soils, the decomposition of the SOM may be energy-limited(Fontaine et al., 2007). For example, Crow et al. (2009) showed thatthe priming action of topsoil SOM was due to a doubling of the litteradditions to Andisols under forest cover, accounting for 11.5 to 21.6%of the annual CO2 flux. Few studies exist concerning how priming mayserve as a possible destabilisation mechanism of SOM in Andisols. The

response of SOM stabilisation and destabilisation processes to climateand vegetation change has not been addressed in detail.

3.5. SOM storage changes over time

The SOM storage and residence times of SOM in Andisols are chang-ing over time. This is related to the weathering and ageing processesthat affect the mineral phase and, in turn, the SOM stabilisation (Tornet al., 1997). Torn et al. (1997) studied the soil organic matter accumu-lation along a 4-million year soil chronosequence formed on basalticlava in Hawaii. The noncrystalline material concentrations were relatedto a maximum accumulation of SOM after 150,000 years and then de-creased along more stable crystalline minerals leading to a reducedSOM. Inoue and Higashi (1988) studied the radio C age of soil. The 14Cages of humic acids extracted from the A horizons of Andisols rangedfrom the present to 30,000 years before present (YBP), with the major-ity in the range of 1000–5000 YBP. The mineralogical changes ofAndisols were also studied along weathering sequences in the Etna re-gion in southern Italy (Egli et al., 2008). The main mineral transforma-tions over 115,000 years of soil development were from volcanic glassto imogolite and kaolinite. The highest C-accumulation was observedin young soils b2000 years old (Egli et al., 2008). The researchersnoted, however, that after 15,000 years, the C-accumulation seemedto reach a maximum. In this region, the stabilisation of SOM in the soilcan be attributed to the proportion of aromatics in the humic acidsin the presence of noncrystalline Al and Fe phases and kaolinite concen-trations (Egli et al., 2007). Lilienfein et al. (2003) studied the rates oforganic matter accretion and the increase in allophane, specifically thesurface area changes in a young andesitic chronosequence from 77 to1200 years in California. The C-accumulation rates were highest at theinitial stages of soil development and ceased after 600 years. Concur-rently, the allophane concentrations increased to a maximum rate of0.14 g kg−1 year−1. The evolution of the chemical composition ofstabilised organic matter through time has been intensively studiedusing Andisols from Hawaii covering 0.3–4100 ky (Chorover et al.,2004; Mikutta et al., 2009) and from New Zealand (1–10 ky) (Prioret al., 2007). In both sequences, SOM composition changeswere record-ed. In the early weathering stages, the mineral phase was composed ofprimary, low-surface area minerals (olivine, pyroxene, feldspar) associ-ated with low amounts of SOM and a high contribution of microbialcarbohydrates. From 2000 to 40,0000 years, the SOM content increasedsharply and was associated with a concomitant increase in poorlycrystallised minerals and showed a higher contribution of lignin in theorganomineral complexes. The third weathering stage was char-acterised by a transformation from a poorly crystallised to a well-crystallised mineral phase and to a decrease in the SOM associatedwith soil minerals. Peña-Ramírez et al. (2009) studied the lava flowsof different ages covered by volcanic ashes of humid temperate soilsunder forest. The results showed a linear increase of C associated withthe formation of Al-organo mineral complexes between 1835 and10,000 YBP. From 10,000 to 30,500 years, the amount of C stored de-creases by approximately 30% of the maximum soil C accumulation.Zehetner et al. (2003) studied the altitudinal weathering sequencein the Andean valleys of northern Ecuador. In pedons N3200 masl, allo-phane and humus complexes dominate the colloidal fraction (Andisols),while in pedons b2700 masl, halloysite is the predominant mineral,with b1% organic C (Inceptisols and Entisols). This sequence wasthe same in 3000-year-old soils and in buried paleosols older than40,000 years, indicating that the time of pedogenesis does not causemarked differences in the composition of the colloidal fraction. Anexcellent overview of soil developmental processes occurring in soilderived from volcanic materials is presented by Ugolini and Dahlgren(2002).

In summary, the chemically stabilised C pool in young Andisols isreduced by pedogenic processes at different times. As long as the pedo-genic processes continue, a more crystalline mineral phase interacts

Page 6: Soil carbon storage and stabilisation in andic soils: A review

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 200 400 600 800 1000 1200 1400

Met

al:C

prat

io

Radiocarbon age (years)

0-5 cm

10-20 cm

20-40 cm

Fig. 2. The relationship between the radiocarbon dating and the molar ratio of Alp and Fepextracted in Na-pyrophosphate and the soil organic C (SOC) at different soil depths in anAndisol from Chile. The horizontal bars are the standard error of the mean.See Rivas et al. (2012).

107F. Matus et al. / Catena 120 (2014) 102–110

with newly formed organic matter, as influenced by the climate and thevegetation cover.

4. Carbon storage capacity

The prediction of the SOM storage capacity of Andisols and how thisevolves over time in connection with the mineral phase is a key issueand was presented in the preceding section. Climate, vegetation typesand soil mineralogy and particularly the elevation gradients have beendemonstrated to be a simple and effective predictor of SOM stock(Gamboa and Galicia, 2012; Tsui et al., 2013). The storage capacityand, ultimately, the maximum (saturation) protected C pool are closelyrelated to the SOM steady-state concept. The latter is mainly controlledby the C-input rate, the climate and soilmineralogy,whereas the formerdepends on the chemically protective sites that are occupied by SOM. Itis crucial to distinguish whether SOM has reached a steady-state or sat-uration because a soil approaching themaximumwill accumulate smalladditional amounts of SOM, as it has less protective sites (Six et al.,2002). Thus, a soil becomes a C-sink or a C-source depending on the ex-tent to which the reactive mineral fractions approach C-saturation(Matus et al., 2008b). The saturation of SOM in the reactivemineral frac-tion has never been evaluated in volcanic soils. What we know is thatthe storage capacity develops over time and that weathering is muchslower and less intense in non-andic soils compared with the changesthat occur in Andisols (Dahlgren et al., 2004; Egli et al., 2008). Recent ev-idence of saturation of SOM in the fine fraction (silt + clay) has beenpresented by Arai and Tokuchi (2010). They found no significant differ-ences in the SOM in the silt + clay fraction of a 55-year-old C of japonicaplantation (with a greater SOM stock) than natural forest comprisedmainly of Abies firma and Tsuga diversifolia in Japanese volcanic soils.The SOM differences resulted from larger C-accumulations in the coars-er and WEOM fractions.

As mentioned previously, the organic matter storage capacity ofAndisols, regardless of land-use or climate, was found to vary mainlywith the interactions of soil minerals, particularly the Al oxides in mod-erately acid Andisols (Miyazawa et al., 2013) and Podzols (Bardy et al.,2007). The process that controls the soil C stability along the soil profileand that influences the pattern of nutrient limitation in response toseveral factors, including the effect of climate change, such as elevatedCO2 and temperature, remains elusive.

4.1. Evaluation and control of carbon storage capacity in the chemical pool

Recently, researchers have proposed that the molar metal (Alp andFep):Cp ratio can be used as an indicator of the chemical C pedogenictransformations of Andisols (Garrido and Matus, 2012; Matus et al.,2009). According to these authors, in any soil exceeding a molarAl + Fel:C ratio N0.12 (Matus et al., 2009) or N0.18 (Garrido andMatus, 2012), the SOM is saturated with metals (Fe and Fe). This ratiois remarkably similar to the value of 0.17 that can be estimated from atheoretical metal-humus complex unit of a melanic Japanese Andisolproposed by Nanzyo (2002). This author determined that for each 40C, three carboxyl groups are complexed with Al and only one exhibitsa negative charge. Similar metal:C ratios have been estimated inPodzols. A strong linear relationship between Alp + Fep and Cp (R2 =0.97; P b 0.001) was established by Dahlgren and Ugolini (1991).Their data indicated a slope of 0.18, similar to the early estimation of0.16 byHigashi et al. (1981) in similar soils. Once the complexing capac-ity of humic substances is saturated with metals, excess Al will reactwith Si to form allophanic materials (Ugolini and Dahlgren, 2002);this would explain the poor relationship between allophane contentand SOM inMatus et al. (2006) and Percival et al. (2000). Such relation-ships do exist in saturated humus complexes because the SOM binds tonon-crystalline-type materials (Garrido and Matus, 2012). The metal:Cratio is remarkably similar between Podzols and Andisols, and, giventhe large variability of soil acidity (e.g., 4.9–6.5) among Andisols, the

complexing ability of humic substances may vary, making the satura-tion ratio somehow variable. However, for any given Andisol and hori-zon, a ratio may exist that corresponds to the saturation of the humiccomplex with metals (Dahlgren and Ugolini, 1991) related to the soil'sC age (Neculman et al., 2013). The latter can be derived from datasetof Rivas et al. (2012). A relationship between the radiocarbon age andthe ratio values (Alp + Fep:Cp) is calculated (Fig. 2). This result suggeststhat: 1) the oldest subsoil showed a higher molar metal:C ratio, i.e., to-ward saturation, while the youngest topsoil was far from this maxi-mum; and 2) the stabilising capacity of Andisols evolves over time,and the C-stabilisation is controlled by the presence of allophane inthe subsoils.

Sequestering C in Andisols depends not only on the storage capacitydetermined by the mineral interaction and time but also on the avail-ability of the stabilising elements of nitrogen (N), phosphorus (P) andsulphur (S), which are known to be essential components for stableSOM. Therefore, the stable pool of SOM with constant ratios of C:N:P:Sfor andic and non-andic soils can be used as proxy to evaluate C-sequestration strategies (Kirkby et al., 2011). This finding is of particularinterest for Andisols that show a large reservoir of organic P but haveserious limitations in their available inorganic P in forests and agricul-tural soils (Borie et al., 1989). Further research is needed to more accu-rately determine the amount of organic P in the stable SOM pool andhow the C:N:P:S ratio relates to the SOM storage capacity. Anothergood indicator of C-sequestration in Japanese soils over 30 years ofconversion from grassland to forest was the disappearance of themelanic epipedon, with decreasing aromatic C and an increasing alkylC proportion of humic acids (Iimura et al., 2010).

In summary, we know that the SOM storage capacity of Andisolsevolves over time as influenced by mineral pedogenic direction. More-over, we urgently need to assess, as in non-andic soils, the C input andthe potential stabilisation capacity of Andisols.We also need to evaluatethe distribution of chemically and physically protected SOM and howthis impacts the saturation of SOM in the top and deep soils.

5. Conclusions

• There are many studies in Andisols reporting the chemical composi-tion of NaOH-soluble SOM, but few of them study the SOM composi-tion of the solid phase SOM fractions. NMR spectra in the solid phasehave revealed the important contribution of alkyl C in deep Andisolsresistant to biodegradation.

• The chemical characterisation of the SOM fractions of Andisols com-pared with non-andic soils indicated the stabilisation of microbiallyderived products rather than plant-derived and recalcitrant organicmatter compounds.

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108 F. Matus et al. / Catena 120 (2014) 102–110

• Although the most important controlling mechanism driving thesorption of SOM by SRO amorphous minerals is the ligand exchange,the strength of the SOM stabilisation through different minerals hasbeen scarcely evaluated.

• There are conflicting results showing that the clay fraction may playa minor role as a stabilising agent of SOM compared with Al and Feoxides in Andisols.

• Empirical evidence showing the importance of DOM, SRO and Al-humus translocation to deep soils at the field scale contrasts withthe classical hypothesis of in situ formation of amorphous materialsand SOM accumulation. This needs to be further studied.

• The stabilisation and destabilisation of SOM are functions of soil pHand the stoichiometric metal (Al and Fe) to C ratio. The destabilisationprocesses of the SOM in Andisols have not been addressed.

• A positive priming effect occurs in the top soil, but it is unknownwhether it affects SOMmineralisation negatively across the soil pro-file. However, the intensity of the priming on Andisols has not beenevaluated to the extent of the non-andic soils.

• Soil architecture and the relative contribution of root materials, DOMand bioturbation for SOM transport, drainage and accretion across thesoil profile upon cultivation need to be further investigated.

• Soil structure and aggregate hierarchy have beenwell studied for non-andic soils, but more research is required for Andisols.

• The impact of soil disturbances and climate change on C sequestrationof SOM in Andisols requires further attention. Physical and chemicalproperties of Andisols may be irreversibly changed once the soil isaltered. Therefore, it is unclear how implementingmanagement tech-niques such as no-tillage can preserve these andic properties.

• Themetal-humus-complexmodel with amolar Al+ Fe:C ratio of 0.18has been applied for the first time in Andisols. This value representsthe upper boundary of saturation of SOM with metals extractable inpyrophosphate. Laboratory experiments are necessary to confirmthis hypothesis, which affects the transport of the DOMand C deliveryinto deep soils.

• The SOM storage of Andisols evolves over time; however, their poten-tial stabilisation capacity has not been evaluated as in non-andic soils.The distribution of chemically and physically protected SOM and howthis impacts the saturation of SOM in the top and deep soils needsfurther attention.

• Lastly, it is essential to develop working hypotheses that express thecurrent understanding and to identify critical experiments to validatethese ideas. The perspective of this new insight could be used to framesuch hypotheses that highlight some of themajor areas of uncertaintywithin known mechanisms and factors. Several of these areas areoutlined in the following section.

6. Future directions

• The hypothesis that Andisols store large amounts of SOM for a longtime requires further attention. Based on current knowledge, researchregarding SOM in Andisols also needs quantification in terms of SOMstabilisation over different time scales throughout soil development.Changes in the SOM composition following changes in the mineralphase over time also need to be addressed in future studies.

• The hypothesis that the organic matter structural composition is sus-ceptible to stabilising differently in various parts of the soil profileshould be addressed along with destabilisation over different timescales.

• The C saturation, as in non-andic soils, needs to be studied in Andisols.An experimental approach to observe SOM accretion, including the Cinput, under different soil managements is required.

• To understand the processes and to be able to develop managementpractices, we need to understand the nature of stabilised organicmatter. In particular, we need to address the question of how SOM-types are stabilised in tropical and temperate andic soils and whetherthe mechanims are similar to that of non-andic soil types.

• We hypothesised that SOM mineralisation can be triggered throughpriming effects across the soil profile. The nature of the microbialcommunities of Andisols needs to be examined and compared withthe microbial communities present in non-andic soil types.

Acknowledgements

The authors thank CONICYT-Chile for their financial contributionsthrough FONDECYT projects 1080065, 1130193 and 3130647. Weacknowledge ECOSSUD-CONICYT C08U01 and ECOSSUD-CONICYTC13U02 for their financial support of the French and Chilean researchgroups. We want to thank Professor Peter Loveland for his critical com-ments on an early version of this review.

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