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Leaf lipid degradation in soils and surface sediments: A
litterbag experiment
Thanh Thuy Nguyen Tu, Céline Egasse, Christelle Anquetil, Florent Zanetti,
Bernd Zeller, Sylvain Huon, Sylvie Derenne
To cite this version:
Thanh Thuy Nguyen Tu, Céline Egasse, Christelle Anquetil, Florent Zanetti, Bernd Zeller, et al..
Leaf lipid degradation in soils and surface sediments: A litterbag experiment: A litterbag exper- iment. Organic Geochemistry, Elsevier, 2016, 104, pp.35-41. �10.1016/j.orggeochem.2016.12.001�.
�hal-01414796�
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L eaf lipid degr adation in soils and sur face sediments:
A litter bag exper iment
Thanh Thuy Nguyen Tua*, Céline Egasseb, Chr istelle Anquetila, Flor ent Zanettib, Ber nd Zellerc, Sylvain H uond, Sylvie Derennea
a Sor bonne Univer sités, UPM C, Univ Par is 06, CNRS, EPHE, UM R 7619 M etis, 4, place Jussieu, 75252 Par is cedex 05, Fr ance
b UM R 7207 CR2P M NHN-CNRS-UPM C CR2P, M NHN, Case cour r ier 48, 57 r ue Cuvier, 75 231 Par is cedex 05, Fr ance
c UR 1138 Biogéochimie des Ecosystèmes For estier s, Centr e I NRA de Nancy, 54 280 Champenoux, Fr ance
d UPM C, iEES Par is, UM R 7618 CNRS-UPM C-I NRA-I RD-UPEC-Univ.Par is Diderot. Case 120, 4 place Jussieu. 75252 Par is cedex 05, Fr ance
* Corr esponding author. E-mail addr ess: [email protected] (Thanh Thuy Nguyen Tu).
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ABSTRACT
The fate of leaf lipids upon ear ly diagenesis was monitor ed in a two year litter bag exper iment in a soil and at the water -sediment interface of an adjacent pond. The
biomar ker content of degr ading leaves exhibited substantial var iability among litter bags, even for a given time step within a given envir onmental condition, likely r eflecting natur al micr oenvir onmental var iability. Due to this var iability and the oxic condition s in the pond, no substantial differ ence between the soil and the pond could be evidenced in the biomar ker degr adation pattern. An occasional incr ease in the abundance of sever al biomar ker s (- sitoster ol, oleanolic acid, C16 phytyl ester , C27 n-alkane) was also noted dur ing the exper iment, which was attr ibuted to release of bound compounds and/or an exter nal contr ibution. Never theless, absolute quantification showed that the concentr ation of all lipid constituents was r educed, but they exhibited differ ent decay pr ofiles: (i) r apid extensive degr adation (phytyl ester ), (ii) exponential-like decr ease (fatty lipids) and (iii) var iable degr adation pr ofile (polycyclic tr iter penoids). However, all the main constituents initially pr esent in the senescent leaves wer e still detected after two year s of degr adation in both environments. Fatty lipid abundance gener ally decr eased to < 10% of the initial content but their main distr ibution featur es (car bon number maximum and pr edominance) r emained unchanged. The r esults thus tend to validate their use as pr oxy for source and envir onment in ancient organic matter . They also suggest that, on a mid-ter m basis, a plant biomar ker signatur e is not substantially affected by differential degr adation in soil and at the water -sediment inter face, at least for a qualitative appr oach.
K eywor ds: diagenesis, biomar ker , litter bag, leaf lipids, soil, sur face sediment, GC-M S
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1. I nt r oduct ion
Lipid biomar ker s constitute efficient tools for tr acing the sources of organic matter (OM) in mar ine (Har ada et al., 1995; Aquilina et al., 2010), fluvial (Jaffé et al., 1995;
Hemingway et al., 2016) or lacustr ine envir onments (Rieley et al., 1991; Jacob et al., 2007).
Quantifying biomar ker var iation through sedimentar y sequences facilitates r econstruction of past envir onmental var iability over var ious timescales, fr om centur ies (M cCaffr ey et al., 1991; Or tiz et al., 2016) to millions of year s (Pear son and Obaje, 1999; Tzor tzaki et al., 2013). However , the diagenetic fate of biomar ker s must be assessed befor e their
distr ibution and abundance can be used as quantitative pr oxies. Fr om their str uctur es, it would be anticipated that differ ent biomarker s would exhibit var ying degr ees of
susceptibility to degr adation, which would ultimately affect their pr eservation in sediments (M eyer s and I shiwatar i, 1993). Sever al stability scales have been established for lipids in sediments; n-alkanes ar e gener ally considered as the most stable components, while functionalized compounds such as pigments or n-alkanols as the most r eadily degr aded (Cr anwell, 1981; Colombo et al., 1997). Ster ols and fatty acids (FAs) exhibit inter mediate stability that var ies accor ding to functionality and degr ee of unsatur ation (Cr anwell, 1981;
M eyer s and I shiwar ati, 1993; Colombo et al., 1997). Environmental par ameter s in the water column, at the water -sediment interface and within the sediment also influence lipid degr adation/pr eservation (M eyer s, 1997; Rontani and Volkman, 2003).
Vascular plants ar e major contr ibutor s to sedimentar y OM and plant biomar ker s often ser ve as proxies for continental OM in mar ine sediments (Volkman, 1986; Hedges and Oades, 1997). The plant biomar ker signature in sediments can be biased by differ ences in pr oductivity among taxa, and biomar ker solubility or tr anspor t mode (Diefendor f et al., 2014). Among polycyclic ter penoids, Gir i et al. (2015) showed that these pr e-sedimentation pr ocesses tend to favor dit er penoids at the expense of tr iter penoids, leading to an
overestimation of conifer input vs. angiosper m input in paleovegetation r econstr uction.
Befor e deposition in mar ine sediments, plant detr itus may remain for varying extents of time in soil and/or in fr esh water. The contr asting degr adation conditions in these
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envir onments may in tur n affect the plant biomarker signature. Sur face degr adation of a given plant on land was shown to modify bulk leaf chemistr y to a differ ent extent than degr adation in bur ied conditions (Webster and Benfield, 1986; Stear t et al., 2009). Although the fate of molecular biomar ker s was independently studied in sever al envir onments via var ious appr oaches (e.g. soil litter bags, soil/sediment pr ofiles/sections, labor atory
incubations; Cr anwell, 1981; M eyer s and I shiwar ati, 1993; Rieley et al., 1998; Zech et al., 2011, Wiesenberg et al. 2012; Wang et al. 2014), compar ative studies addr essing the fate of biomar ker s in diver se natur al envir onments are r ar e. However, the plant biomar ker s r ecover ed fr om sedimentary OM may have undergone different pr e-bur ial histor ies that may have influenced their r obustness as a quantitative pr oxy.
As the compar ative fate of plant lipids in differ ent envir onments is not fully documented, the aim of the pr esent study was to compar e the degr adation of lipids fr om a given plant species in two differ ing environments: soil and the water -sediment inter face.
The investigation focused on leaf lipids, as epicuticular wax is the main sour ce of the fatty lipids commonly used as continental biomarker s in mar ine sediments. The study was based on a litter bag exper iment , in or der to (i) allow the r ecovery of the small leaf debr is
gener ally obtained after sever al months of degr adation and (ii) pr ecisely monitor the changes undergone in the leaves initially intr oduced into the bags, without potential bias due to contamination by other plant mater ial. Eur opean beech (Fagus sylvatica L.) was chosen because it commonly occur s in temper ate for ests. The lipid composition of the leaves fr om two litter bag exper iments conducted simultaneously in a soil and in a near by pond was investigated to test whether these differ ent envir onments might lead to differ ent biomar ker signatur es. The key questions wer e: Do differ ent lipid classes degr ade similar ly in these two environments? I s the distr ibution of fatty li pids modified similar ly in the two envir onments?
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2. M at er ial and met hods 2.1. Exper imental site
The exper iment was set up in the "Breuil -Chenue" exper imental for est site, in Burgundy, Fr ance (47°18'10"N, 4°4'44"E). The for est is par t of the "Par c Natur el Régional du M orvan", a pr otected area wher e ur ban pollution is minimal. The exper imental ar ea is on a shelf 650 m above sea level and exhibits a slight nor thwest facing slope. M ean annual temper ature and precipitation at the site ar e 9 °C and 1,280 mm, r espectively. The bedrock is the alkaline gr anite of "La Pierr e qui Vir e", cover ed by a thin layer of eolian silt. The incubations took place in a 30 yr old Eur opean beech (Fagus sylvatica L) stand planted after a clear -cut in 1976, and in a ca. 300 m2 adjacent pond, r espectively. The soil is
classified as a dystr ic cambisol (WRB, 2006) with moder -type humus. I t is acidic with pH 4- 4.5 and has a poor ly satur ated cation exchange complex. M or e infor mation on the site is available elsewher e (M oukoumi et al., 2006; Andr ianar isoa et al., 2010). The pond had a maximum depth of 1.5 m. The main char acter istics wer e deter mined using a CTD
(conductivity – temper atur e – depth) pr obe equipped with additional sensor s and external fluor ometer s (ASD sensor s). Dur ing the cour se of the study the water column displayed near ly constant temper atur e, electr ical conductivity and pH with water depth
(Supplementary Table A.1). The water had a slightly acid pH (ca. 6.1), low dissolved ion load (low electr ical conductivity, ca. 25.7 µS/cm), low tur bidity (FTU < 20), high CDOM content (chr omolenic dissolved OM, > 200 µg/l) and satur ated or close to satur ation dissolved O2 concentr ation (ca. 98%). Over all, this is over all typical of sur face and subsur face water s of forests soils on gr anitic substr atum (Beaucair e and M ichard, 1982).
Ther mal str atification only occur s dur ing summer , when photosynthetic activity is enhanced (high chlor ophyll-a concentr ation and oversatur ation for dissolved O2; Supplementary Table A.1 and Fig. A.1).
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2.2. Litter bag incubations
At the end of October 2004, senescent leaves wer e collected fr om the beech stand wher e the soil incubation took place. The soil litter bag exper iment has been descr ibed elsewher e (Nguyen Tu et al., 2011). The leaves wer e collected fr om the same height (ca.1.5 m) fr om the br anches of sever al differ ent tr ees in or der to pr ovide an aver aged composition and to minimize bias linked to intr apopulation var iability. After har vesting, the leaves wer e r insed with distilled water to remove extr aneous par ticles and dr ied at 45 °C to constant weight. Each batch was composed of 10-15 leaves and corr esponded to ca. 1 g OM.
Batches wer e assembled using leaves fr om as many differ ent tr ees as possible to provide equivalent batches.
Litter bags (9.5 14.5 cm) wer e made of Al wir e; mesh size was 1.4 1.8 mm, i.e.
sufficient to r etain small leaf litter debr is but sufficiently large to per mit fr ee entry of mesofauna (main functional group at the site) and micr obial activity (bacter ia and fungi).
Pond litter bags wer e ballasted with glass sticks to maintain them at the bottom of the pond. Befor e being enclosed in litter bags, leaves were weighed and then r e-moistened with distilled water to r ecr eate natur al conditions. The litter bags wer e placed in the field in December 2004. Litter bags (in duplicate) wer e then regular ly r etr ieved over 2.5 yr: after 5, 10, 15, 20, 30, 40, 52, 79 and 129 weeks for the soil, and 9, 27, 66 and 100 weeks for the pond. The soil litter bags cor r espond to the unlabeled leaves analyzed before for their specific alkane isotope composition (Nguyen Tu et al. (2011).
2.3. Lipid extr action and fr actionation
The r ecover ed mater ial was fir st r insed with distilled water to r emove obvious extr aneous par ticles such as moss, r oots or ar thropods. Leaves wer e t hen dr ied at 45 °C to constant weight and weighed. Samples wer e stor ed dar k in Al foil at 5 °C until analysis. All the leaves for each litter bag were combined and gr ound fine enough to pass thr ough a 500
m mesh. Leaf powder was ultr asonically extr acted for 20 min with CH2Cl2/M eOH (2/1; v/v).
The mixtur e was centr ifuged (10 min, 4,000 r pm) and the lipids wer e r ecover ed in the
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supernatant. The extr action procedure was r epeated 6 with the centr ifugation residue.
The combined extr acts wer e concentr ated via rotar y evapor ation and dr ied under N2. The extr act was fr actionated using column chr omatogr aphy on alumina (Sigma-Aldr ich 507C, ca. 150 mesh) deactivated to Br ockmann gr ade I V by adding 0.1 wt % distilled water . The non-polar fr action (1) was r ecover ed with heptane. The slightly polar fr action (2) was r ecover ed with toluene. The medium polar ity fr action (3) was r ecover ed with CH2Cl2 and CH2Cl2/M eOH (9/1; v/v). Each fr action was concentr ated via r otary evapor ation and dissolved in ca. 3 ml heptane, toluene, and CH2Cl2, respectively. The main components of the thr ee fr actions wer e quantified using inter nal standar ds: tetr atr iacontane for fr action 1 and tr icosan-1-ol for fr actions 2 and 3.
2.4. Analysis
The hydr ocar bon fr action was analyzed as such, while the other two fr actions wer e conver ted to tr imethylsilyl der ivatives pr ior to analysis. Aliquots (30 µl) of the fr actions wer e dr ied under N2 and then silylated with 30 µl N,O-bis-
(tr imethylsilyl)tr ifluor oacetamide (BSTFA) for 1 h at 80 °C. After cooling, the aliquot was dr ied under a gentle str eam of N2 and dissolved in 30l of appr opr iate solvent (toluene for fr action 2 and CH2Cl2 for fr action 3). Assignment and quantification of lipids wer e carr ied out using gas chr omatogr aphy coupled-mass spectrometr y (GC-M S) and GC with flame ionization detection (FID, 350 °C), r espectively. Separ ation was with an Agilent 6890N gas chr omatogr aph equipped with a split/splitless injector (350 °C) and fitted with a fused silica column, coated with VF5-M S (50 m 0.32 mm i.d., 0.12 µm film thickness). The GC
conditions wer e: 80 °C to 100 °C at 10 °C/min, and then to 325 °C (held 30min) at 4 °C/min.
He was the carr ier gas at a constant 2 ml/min. For GC-M S an Agilent 5973N electr on impact mass spectr ometer (70eV, scan r ange m/z 35-800) was used. Components wer e assigned accor ding to libr ary and published mass spectr a. Quantification was achieved fr om the r esponse factor in GC-FI D of the measur ed component s and compar ison of GC-FID peak ar eas with those of the inter nal standar ds, introduced to the fr actions in known
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amount. Response factor s wer e obtained fr om calibr ation curves of the standar d and the component, or compound with similar str uctur e, polar ity and molecular weight , if no r eference compound was available. Coelution in GC-FI D occasionally pr evented
quantification of some lipid components. Biomar ker content was quantified as µg/g dr y biomass and fur ther expressed in % r elative to initial content in Fig. 1 for compar ison pur poses.
3. Result s and discussion 3.1. Biomarker behaviour
The composition of lipids fr om senescent leaves of Fagus sylvatica has been descr ibed by Nguyen Tu et al. (2007). The lipids were dominated by ster ols, the most abundant being -sitoster ol, and to a lesser extent, by acyclic isopr enoids, including phytadienes and phytyl ester s (C6-C20, max. C16). Oleanolic tr iter penoids, mainly oleanolic acid, and fatty lipids wer e also abundant. The latter compr ise an homologous ser ies with even chain length pr edominance: mainly n-alkan-1-ols (C14-C30, max. C28), n-acids (C8-C28, max. C16, C28) and, to a lesser extent , n-aldehydes (C20-C32, max. C28). Predominantly odd number ed ser ies also occurr ed in substantial amount s: mainly n-alkanes (C12-C33, max. C27), and to a lesser extent n-alkan-2-ols (C23-C29, max. C27) and n-alkan-2-ones (C25-C29, max. C27).
The lipid fingerpr int of degr ading beech leaves r evealed no major qualitative changes dur ing 2 yr of diagenesis in the soil or at the water -sediment inter face. M ost of the initial components were still pr esent at the end of the experiment and no degr adation products wer e evidenced. Quantification of the main components of each lipid class for each
degr adation stage showed that the concentr ation of all the biomarker s studied was (i) ver y var iable but (ii) substantially r educed at the end of the exper iment , although to var ying extent (Fig. 1).
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3.2. Var iability in biomarker concentr ation
I n this 2 yr exper iment, the biomar ker content of degr ading leaves exhibited substantial var iability among litter bags, even for a given time step within a given envir onmental condition (Fig. 1). Ther efor e, we address the potential or igin of such var iability befor e discussing degr adation patterns. The differ ence in biomar ker content between r eplicate litter bags maximized in soil at 70% and 100% of initial leaf content, for oleanolic acid and C16 phytyl ester , r espectively. Other quantified compounds (-Sitoster ol, C16 acid, C28 alcohol and C27 alkane) exhibited maximum differ ences between r eplicate litter bags of around 40%. Such var iability may r elate to (i) initial var iability of leaf lipid content and/or to (ii) var iability in microenvironmental conditions. The concentr ation of leaf lipids is known to vary substantially between species, within species, within individual s and even within a given leaf (Her bin and Robins, 1969; Gülz et al., 1991; Gao et al., 2015). In
agr eement with the lower var iability of in vitro exper iments vs. field studies, the var iability in lipid decay is higher her e than r epor ted for beech lipids in a labor ator y exper iment (Rieley et al., 1998). Soils, and especially for est soils, ar e r ecognized as highly
heter ogeneous environments (Hedges and Oades, 1997; Bélanger and Van Rees, 2008;
Lehmann et al., 2008). A number of biogeochemical char acter istics of soils commonly vary within a few decimeter s, so that differ ent decay pr ofiles between litter bags separ ated by ca.
20 cm is not unexpected (Witkamp and Olson, 1963; M ur phy et al., 1998; Suchewabor ipont et al., 2011). Additionally, the main dr iver s of litter decomposition in Br euil -Chenue exper imental for est ar e sapr ophytic fungi , which are r epr esented mainly by thr ee species (M ycena inclinata, Mycena galopus and Phallus impudicus) that gener ally exhibit a heter ogeneous spatial distr ibution (Zeller et al., 2007). Decay conditions within sediments ar e believed to exhibit smaller spatial var iability, although leaf degr adation at the water - sediment inter face under a substantial water column has seldom been investigated in the field. Never theless, leaf decomposition in seawater and str eam water was shown to be var iable over a few months monitor ing and/or within small ar eas (Suber kr opp et al., 1976;
Rublee and Roman, 1982; Webster and Benfield, 1986; Boulton and Boon, 1991; M achás et
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al., 2006; M or etti et al., 2007). Hydr ocar bon and FA content of deciduous leaves degr ading in a blackwater str eam were shown to vary substantially over thr ee months (M ills et al., 2001).
3.3. Occasional incr ease in biomar ker concentr ation
The lipids occasionally exhibited a higher content in degr ading leaves than in initial senescent leaves. That was the case for 4 of the 6 classes investigated: -sitoster ol r eached 112% of its initial content in pond leaves after 27 weeks of degr adation, oleanolic acid had a content of between 114% and 141% for some litter bags in the soil and in the pond between 9 and 40 weeks of degr adation, 123% of the initial C16 phytyl ester was detected after 5 weeks in a soil litter bag, and the n-C27 alkane reached 124% and 103% of its initial content in pond litter bags after 27 and 103 weeks, r espectively. Labor ator y incubation of beech leaves by an estuar ine microbial community (Rieley et al., 1998) also showed an initial ster ol incr ease that was attr ibuted to the release of fr ee compounds fr om a bound state. Gr ass degr adation in soil litter bags similar ly exhibited an occasional incr ease in n-alkane content over 10 months, r eaching up to 230% of the initial content (Wang et al., 2014). Initial incr ease in alkane content was also noted in maple, ash and beech leaves dur ing a 2 year litter bag degr adation, and their deuter ium content suggested a micr obial contr ibution (Zech et al., 2011). I sotope char acter ization (13C) of the alkanes from the soil litter bags of the pr esent study showed that micr obial alkanes also contr ibuted to the alkane pool in degr ading leaves (Nguyen Tu et al., 2011). The occasional incr ease in some of the biomarker s may thus r eflect either an external contr ibution, especially for r ather ubiquitous compounds such as fatty lipids, and/or moiety release from bound str uctur es.
3.4. Biomarker degr adation pr ofiles
Degr adation at the water -sediment inter face would be expected to be less extensive than in a soil. However, in the present exper iment the high var iability in biomar ker concentr ation pr ecludes such a conclusion (Fig. 1). I ndeed, this high var iability pr event ed
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calculation of r epr esentative decay r ates for most biomar ker s. Consequently, no differ ence in degr adation r ates could be evidenced between the soil and the pond litter bags. This lack of slower degr adation in the pond than in the soil may also be par tly due t o the oxic
conditions at the bottom of the pond (Section 2.1), which ar e known to be mor e favor able for degr adation than anoxic ones. The var iability of the pr esent r esults calls for caution when compar ing decomposition patterns, but the quantified lipids seemed to degr ade accor ding to thr ee main pr ofiles, at least dur ing the fir st 6 months (Fig. 1).
- Phytyl ester s were r apidly extensively degr aded, as shown by the phytyl hexadecanoate content, which dropped to ≤ 10% after 15 weeks in the soil, as well as in the pond.
- Fatty lipids (n-alkanes, n-acids and n-alcohols) under went exponential-like decr ease, r eaching ca. 7% of the initial content at the end of the exper iment. M ass loss of bulk leaves is often appr oximated by an exponential decay model in soils and str eams, as the r ate of mass loss is assumed to be a constant fr action of the amount of mater ial r emaining (Olson, 1963; Webster and Benfield, 1986). The pr esent r esults may thus suggest similar
degr adation kinetics for fatty lipids.
- Polycyclic tr iter penoids exhibited a var iable degr adation patter n and r emained at 15%
of the initial content after 2 yr , as shown by the -sitoster ol and oleanolic acid profiles (Fig.
1).
These r esults mainly mat ch those repor ted for a thr ee month labor ator y incubation of beech leaves, although the mentioned labor ator y exper iment was mor e favor able for phytyl ester pr eservation and less for alkanes (Rieley et al., 1998). The pr esent r esults also differ slightly fr om the stability scales often deduced fr om sediment studies since n-alkanes ar e gener ally consider ed as the most stable lipid components and functionalized polycyclic ter penoids as being moder ately stable (Cr anwell, 1981; Colombo et al., 1997). Our r esults thus emphasize the complexity and var iability of l ipid degr adation in natur al
envir onments.
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3.5. Distr ibution patter n of fatty lipids
Despite the substantial degr adation of fatty lipids, their main distr ibution pattern at the end of the exper iment was similar to the star ting one (Table 1, Fig. 2): they remained mar kedly dominated by a single homologue (n-C28 for alcohols, n-C16 for acids, and n-C27 for alkanes), with a slight narr owing of the distr ibution r ange. Chain length distr ibution indexes r emained similar to the initial values (Table 1): the aver age chain length (ACL) center ed ar ound the dominant homologue and the car bon pr efer ence index (CPI ) close to zer o for acids and alcohols, and mar kedly >1 for alkanes. The only noticeable change in lipid ser ies distr ibution concerns FAs, for which the long chain homologue (n-C28) is r apidly degr aded (i.e. no longer detectable after 5 weeks). H owever , such degr adation did not modify chain length distr ibution indexes (Table 1). On the contr ary, sever al studies have r epor ted ACL and CPI modifications thr ough degr adation (Meyer s and I shiwar ati, 1993;
Huang et al., 1996; Celer ier et al., 2009; Zech et al., 2011; Wiesenberg et al., 2012), which wer e attr ibuted to var ious causes, including contr ibution from micr obial lipids, r oot lipids or else anthr opogenic pollution. Never theless, n-alkanes from sever al gr ass species in soil litter bags showed var iable degr adation pr ofiles but no change in ACL and CPI (Wang et al., 2014), similar ly to the pr esent study. Taken together, the r esults suggest that, in spite of degr adation and potential contr ibution fr om external lipids, the or iginal plant lipids can r emain the main sour ce of lipids in degr aded plant debr is and can typically r etain their chain length char acter istics. The results thus tend to validate the use of chain length char acter istics of aliphatic ser ies for source and paleoenvir onment r econstruction, at least on the mid-ter m scale (i.e. a few year s).
4. Conclusions
Compar ative degr adation of leaf lipids in a soil and at the water -sediment inter face of a well oxygenated pond r evealed that most of the biomar ker s exhibited highly var iable content, likely r eflecting natur al microenvir onmental var iability. Due to this var iability and the oxic conditions in the pond, no substantial differ ence between the soil and the pond
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could be detected in biomarker degr adation patter ns dur ing the 2 yr litter bag decay. All the constituents initially pr esent in the senescent leaves wer e degr aded, although to var iable extent. Phytyl ester s wer e the most extensively degr aded, while polycyclic
tr iter penoids appear ed relatively better pr eser ved, with fatty lipids exhibiting inter mediate degr adation patterns. Never theless, the main lipid constituents of initial senescent leaves wer e still detected after two 2 yr degr adation in both envir onments. Dur ing the exper iment, an occasional incr ease in the abundance of sever al biomar ker s was noted, implying moiety r elease from bound str uctur es and/or an exter nal contr ibution. Although fatty lipids concentr ations were markedly r educed, the distr ibution patterns wer e not substantially affected, validating their use as source and envir onmental biomar ker s in ancient OM . The r esults suggest that, for the mid-ter m, a plant biomar ker signature is not substantially affected by differ ential degr adation in soil and at the water -sediment inter face under oxic conditions, at least for a qualitative approach.
Ack nowledgement s
We ar e indebted to J. Ranger for access to the exper imental site of Br euil-Chenue for est and D. Gelhaye, J.P. Calmet and P. Richar d for assistance in the field. Thanks ar e also due to C. Fichot for access to the pond. J. Br outin is acknowledged for encour agement and suppor t to build the or ganic chemistr y labor atory in his depar tment. We ar e gr ateful to B. David and S. Lemettr e for help in lipid pr epar ation. We also thank S. Peigné and S.
Nguyen Tu for technical assistance. The wor k was funded by a gr ant fr om the Fond National pour la Science of Fr ance (ACI JC 10051 - notification 035214). Thanks ar e also due to S. Feakins and an anonymous r efer ee, who provided constr uctive r eviews.
Associate editor – K .G.J. Nier op
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Appendix A. Supplement ar y dat a
Geochemical char acter istics of the water column ar e given: the r ange of values measur ed in the water column (Table A.1) and examples of typical depth pr ofiles for envir onmental par ameter s (Fig. A.1).
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Figur e capt ions
Fig. 1. Fagus sylvatica leaf lipid degr adation pr ofiles. Concentr ation (% r elative to initial concentr ation) of major lipid components vs. time: , soil; , pond water -sediment inter face.
Fig. 2. Distr ibution of n-alkanes, n-acids and n-alcohols in senescent leaves and after 2 yr degr adation in a soil and at the water -sediment interface of a pond. Arr ows indicate when a homologue was detected but was below quantification level.
Nguyen Tu et al. Figure 1
sitosterol
0 20 40 60 80 100 120
0 50 100 150
0 20 40 60 80 100 120 140 160
0 50 100 150
oleanolic acid
C16 phytyl ester
0 20 40 60 80 100 120 140
0 50 100 150
0 20 40 60 80 100 120
0 50 100 150
C16 acid
C28 alcohol
0 20 40 60 80 100 120
0 50 100 150
0 20 40 60 80 100 120 140
0 50 100 150
C27 alkane
time (weeks) time (weeks)
% preserved% preserved% preserved
Figure 1
Nguyen Tu et al. Figure 2
16 18 20 22 24 26 28 30
16 18 20 22 24 26 28 30
16 18 20 22 24 26 28 30
n-alcohols
senescent leavespondsoil
chain length
relative abundance
16 18 20 22 24 26 28 30 32
16 18 20 22 24 26 28 30 32 16 18 20 22 24 26 28 30 32
n-alkanes n-acids
9 11 13 15 17 19 21 23 25 27 29
9 11 13 15 17 19 21 23 25 27 29
9 11 13 15 17 19 21 23 25 27 29
Figure 2
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Table 1
Distribution pattern of main fatty lipid series in F. sylvatica leaves.
n-Alkanes n-Acids n-Alcohols
Crange (Cmax) ACLa CPIb Crange (Cmax Csubmax) ACLa CPIb Crange (Cmax) ACLa CPIb Initial leaves C12-C33 (C27) 26.4 9.9 C9-C28 (C16, C28) 16.1 0.0 C14-C30 (C28) 25.3 0.0 Pond C25-C29 (C27) 27.0 54.5 C9-C24 (C16) 15.8 0.0 C16-C30 (C28) 26.4 0.1 Soil C12-C33 (C27) 26.9 14.7 C9-C24 (C16) 16.6 0.1 C16-C30 (C28) 26.2 0.0
a n[Cn]) / [Cn]
b [C17+19+21+23+25+27+29+31+33]/[C16+18+20+22+24+26+28+30+32]
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Highlights
•Biomarker content varied substantially in soil and at water-sediment interface. •Almost all lipids still present after 2 yr degradation but had different decay profiles. •Although fatty lipid content decreased to <10% after 2yr, main distribution pattern unchanged.
Nguyen Tu et al. Graphical abstract
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0 50 100 150
O H
% remaining
time (weeks) SOIL
POND
Graphical Abstract
