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Volcanic ash soils in Andean ecosystems : unravelling organic matter distribution and
stabilisation
Tonneijck, F.H.
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Citation for published version (APA):
Tonneijck, F. H. (2009). Volcanic ash soils in Andean ecosystems : unravelling organic matter distribution and
stabilisation Amsterdam: Universiteit van Amsterdam
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Download date: 17 Jun 2017
4
The influence of bioturbation on the vertical
distributionofsoilorganicmatterinvolcanicash
soils:acasestudyinnorthernEcuador
PublishedinEuropeanJournalofSoilScience59:1063–1075,2008,
TonneijckFH&JongmansAG.Withadditionalphotographs.
Abstract
Soil faunal bioturbation (‘bioturbation’) is often cited as a major process influencing the
verticaldistributionofsoilorganicmatter(SOM).Theinfluenceofbioturbationonvertical
SOMtransportiscomplexbecauseitistheresultofinteractionbetweendifferentgroupsof
soil faunal species that redistribute SOM through the soil profile in distinct ways. We
performedasemiquantitativemicromorphologicalanalysisofsoilfaunalpedofeaturesand
relatedtheiroccurrencetotheverticaldistributionofSOMandhighresolutionradiocarbon
datinginvolcanicashsoilsundermontaneforestandgrassland(páramo)vegetationinthe
northern Ecuadorian Andes. The páramo soil data suggest that bioturbation was largely
responsible for the vertical distribution of SOM, while illuviation and root input were of
minorimportance.Bioturbationwascausedbyendogeicspecies,whichtypicallymixthesoil
only over short vertical distances. Short vertical distance mixing was apparently enhanced
byupwardshiftingofbioturbationasaresultofsoilthickeningduetoSOMaccumulation.A
changefrompáramotoforestvegetationwasaccompaniedbyachangefromendogeicto
epigeicspecies.Theselatterspeciesdonotredistributematerialvertically,whicheventually
resultedintheformationofthickectorganichorizonsintheforest.
Keywords
bioturbation,organiccarbon,verticaldistribution,radiocarbondating,volcanicashsoils
67
Introduction
The vertical distribution of SOM is principally determined by 1) the input above and
belowground of litter, 2) the output of organic matter through decomposition and 3) its
verticaltransport.Withrespecttothelatter,soilfaunalbioturbation(furtherreferredtoas
‘bioturbation’)isoftencitedasamajorprocessinfluencingtheverticaldistributionofSOM
(e.g. Anderson 1988; Lavelle 1988; Rasse et al. 2006). Elzein & Balesdent (1995)
demonstrated in a modelling study that bioturbation is the dominant mode of vertical
transport,relativetoleaching,inarangeofsoiltypes.
SoilsformedinvolcanicashcontainthelargestamountsofSOMperunitareaofallmineral
soil orders (Dahlgren et al. 2004), making them potential carbon sinks for the greenhouse
gas CO2, which is of global importance. We previously investigated the age and vertical
distributionofSOMinsuchsoilsundermontaneforestandgrassland(páramo)vegetation
inthenorthernEcuadorianAndes(Tonneijcketal.2006;Tonneijcketal.2008a).Thesesoil
profiles consist of a current soil in a thick tephra deposit superimposed on a SOMrich
paleosol in a preceding tephra deposit. We observed a progressive downward increase in
SOMfromtherelativelySOMpoorsubsoilofthecurrentsoilintotheSOMrichtopofthe
paleosol,furtherreferredtoasthe‘overprintedzone’.VerticaltransportofdissolvedOMby
leaching can be assumed insignificant in volcanic ash soils, because of the large metalto
SOM ratios that strongly limit its mobility (Aran et al. 2001; Dahlgren et al. 2004).
Furthermore,rootingistypicallysuperficialinAndeanmontaneforests(Soetheetal.2006)
and páramo (Hofstede & Rossenaar 1995). Therefore, the occurrence of the overprinted
zonestronglysuggestedthatbioturbationplaysanimportantroleduringsoilformation.
TheinfluenceofbioturbationonverticalSOMtransportiscomplexbecauseitistheresultof
interactionbetweendifferentgroupsofsoilfaunalspecies.Thesemaytypicallybedividedin
threemaingroupsaccordingtotheirbehaviourinthesoil(Anderson1988),similartothe
classificationforecologicalgroupsofearthworms(Lee1959):epigeic,anecicandendogeic
species.Thesegroupsofspeciesredistribute SOMthroughthesoilprofileindistinctways
(Anderson 1988; Lavelle 1988; Lee & Foster 1991). Epigeic species inhabit the ectorganic
layersanddonotactivelyredistributematerial.AnecicspeciesfeedmainlyonOMfromthe
earthsurfacebutinhabitdeepersoillayersinwhichtheycreate(semi)permanentvertical
burrowsandthereforeareresponsiblefortransportofOMoverlargeverticaldistancesthat
mayexceed1m.Bycontrast,endogeicspeciesingestbothmineralsoilandorganicmatter
and create extensive networks of subhorizontal burrows thus transporting SOM only over
short vertical distances. Some endogeic species are typically concentrated around 10 – 20
cmdepth,otherendogeicspeciesoccurdeeperinthesoilprofileupto50cmdepth.
To increase understanding of the influence of bioturbation on the vertical distribution of
SOM in volcanic ash soils in northern Ecuador, we performed a semiquantitative
micromorphologicalanalysisofsoilfaunalpedofeaturesandrelatedtheiroccurrencetothe
verticaldistributionofSOMandhighresolutionradiocarbondating.
68
Descriptionofstudyareaandsites
ThestudysitesarelocatedinthenatureprotectionareaofGuanderaBiologicalStationin
northernEcuador,neartheborderwithColombia,onthewestfacingslopesoftheEastern
Cordillera(Figure1.1andTable1.1,0°35’northand77°41’west).Guanderahasa(semi)
naturalupperforestlineatapproximately3650mabovemeansealevel(a.s.l.).Thereare
someforestpatchesabovethecurrentupperforestline.Dominantspeciesintheforestare
ClusiaflavifloraENGL.,WeinmanniacochensisHIERON.andIlexcolombianaCUATREC.and
thepáramoischaracterizedbybunchgrassCalamagrostiseffusaKUNTH(STEUD.)andstem
rosette Espeletia pycnophylla CUATREC. The forest and páramo belong to the globally
leading Tropical Andes biodiversity ‘hotspot’ (Myers et al. 2000). Because deforestation in
the Inter Andean valleys has been particularly severe (Keese et al. 2007), the Guandera
forest constitutes a unique remnant. Mean annual precipitation is around 1900 mm and
meanannualtemperaturerangesfrom12°Cat3000ma.s.l.to4°Cat4000ma.s.l.Both
precipitationandtemperatureshowlittleseasonalvariation.Thesoilclimateisisomesicand
perudic.
Weselectedasitecoveredbyforest(G1)at3501ma.s.l.,asiteinaforestpatchabovethe
currentupperforestline(G5a)at3697ma.s.l.,asitejustnexttoitwithpáramovegetation
(G5b)andfinallyasiteat3860ma.s.l.inthepáramo(G7).Wewereabletoidentifysoils
with erosional/depositional features by anomalous grain size and geochemical data
(Tonneijcketal.2008a)andonlyinvestigatedundisturbedsoils.Thesoilprofilesatoursites
wereeachformedinthreedistincttephradepositsofHoloceneage(Tonneijcketal.2008a).
Relatedtothistephrastratigraphyallsoilscontainedamultisequumconsistingofacurrent
soil, a paleosol and a second heavily truncated or immature paleosol, each at least 40 cm
thick.Betweenthecurrentsoilandthefirstpaleosolanoverprintedzonewithtransitional
organiccarboncontents,grainsizedistributionandelementratioswaspresent.Inthesoil
horizondesignationsweused‘1/2’asaprefixtoindicatethisoverprintedzone.
ThemineralhorizonsequencecanbesummarisedasAh1–1/2Ah2or1/2Bw2Ahb2Bsb–
3BCb. In addition, the forest profiles had ectorganic horizons (LFH) with a combined
thicknessofupto75cm.Bycontrast,ectorganichorizons(L)barelyexceeded1cminthe
páramo profiles. A thin placic horizon was present at the boundary between the first and
second paleosol. This placic horizon is ascribed to a strong textural contrast hindering
drainage of the otherwise well drained soils. The soils classified as a Histosol overlying a
mineralsoilwithandicproperties(G1),asanAndicCambisol(G5a)andasanAndosol(G5b
and G7) according to the World Reference Base (FAO 2006). The ectorganic horizons
classifiedasresimor(G1)andmor(G5a)intheforestandasmullinthepáramo(G5band
G7), according to Green et al. (1993). We observed that the ectorganic horizons changed
abruptlyfrommortomullattheboundarybetweenforest(patch)andpáramo.
69
Materialsandmethods
Samplingprocedures
Soilpitsofapproximately1.5m2surfaceareaandadepthofatmaximum2mweredug.We
tookbulksoilsamplesforphysicalandchemicalanalyses(n=35,forhumicsubstancesn=
28),ringsamplesforbulkdensitydetermination(n=35)andundisturbedboxsamples(with
dimensionsof7cmheightx5cmwidthx4cmdepth)forthepreparationofthinsections(n
=30,uptothetopofthefirstpaleosol),inthesamesoilpitatregulardepthsbutrespecting
horizonboundaries.Bulkandringsamplesweretakenoveraverticalintervalof5cm,the
middleofwhichwasnotedasthedepthofthesample.Additionally,wetookundisturbed
vertical soil samples (monoliths) using one or two metal gutters of 75 cm height x 5 cm
width x 4 cm depth. For radiocarbon dating (n = 35) and additional estimation of organic
carboncontents(n=182)andhumicsubstances(n=19,beingaselectionofthesamples
usedforradiocarbondating),sampleswerecoredfromthemonolithsbymeansofacorer
of0.5cmand0.75cmdiameterrespectively.SincethecontinuousinputoffreshOMinto
soilscausesthemeasured 14CagesofbulkSOMtobealwayssomewhatyoungerthanthe
depositionalage(Wangetal.1996)itisnecessarytousesuchsmallsamplingintervalsfor
radiocarbon dating. All samples were stored at 2 °C under field moist conditions prior to
analysis.
Laboratoryproceduresandstatisticalanalysis
Laboratory procedures for the measurement of total carbon content, dry bulk density,
pHCaCl2 andallophanecontentweredescribedinTonneijcketal.(2006)andTonneijcketal.
(2008a). Total carbon equals organic carbon, since carbonates were absent. Humic
substance fractions of SOM were obtained by alkali and subsequent acid extraction
according to Schnitzer (1982). After each extraction, carbon content in the extract was
measuredwithaShimadzuTOCAnalyser5000(Kyoto,Japan),toobtaintheorganiccarbon
content present in humic acids (HA). Pearson and Spearman correlations (bivariate)
between total organic carbon and carbon in HA were calculated (n = 47) using the SPSS
Correlateprocedureandconsideredsignificantwhenp<0.01.Fieldmoistundisturbedbox
samples were impregnated after replacing water by acetone, according to Miedema et al.
(1974).ThinsectionswerepreparedaccordingtoMurphy(1986).
Radiocarbondating
Inadditiontotheradiocarbondataalreadypublished(Tonneijcketal.2006;Tonneijcketal.
2008a),weperformeddating forprofilesG5a,G5bandG7tofurtherincreasethevertical
resolutionandacquiredafirstsetof 14CdatesforprofileG1.Radiocarbondatingfollowed
proceduresoutlinedinTonneijcketal.(2006).Inshort,humicsubstancefractionsofSOM
were radiocarbon dated with an Accelerator Mass Spectrometer. The fulvic acid fraction
70
was discarded because of its inherent mobility in most soils. We previously discussed
(Tonneijcketal.2006)thatinAndosolslackingathickectorganichorizon,datingofHAgave
the most accurate results since roots tend to accumulate in the residual (humin) fraction
ratherthanintheHAfractionandHAwasimmobileduetothelargemetal/SOMratiosand
acidicpH.Contrarily,inmineralsoilsamplesjustbeneaththickectorganichorizons,humin
ages were more accurate, because HA was then contaminated by younger HA illuviated
from the ectorganic horizons while roots were concentrated in the ectorganic horizons
rather than in the mineral topsoil. Consequently, we dated HA in all cases, except for
samples just beneath a thick ectorganic horizon where we dated both the HA and humin
fractions.Radiocarbonageswerecalibrated,accordingtoproceduresoutlinedinTonneijck
etal.(2006),toacalendaryearprobabilitydistribution(calAD/BC).Calibratedradiocarbon
ages are expressed as the range of calendar years of the 1 sigma peak with the largest
relativeareaundertheprobabilitydistribution.Sincetherangesaresmall,calculationsare
performedwiththemeanandthedotsdisplayedinthegraphsrepresentthewholerangeof
calendaryearsatthescaleofthegraph.
Micromorphologicalanalysis
ThinsectionswereanalysedusingaLeitzM420makrozoommicroscope(foramagnification
of8x)andaLeitzWetzlarpetrographicmicroscope(formagnificationsof25xand63x).Thin
sections were described following the micromorphological terminology of Stoops (2003).
Abundanceclasseswereasfollows:veryfew(<5%),few(5–15%),common(15–30%),
frequent(30–50%),dominant(50–70%)andverydominant(>70%).Todisplaythese
classes in bar graphs we used class midpoints. Rather than using Stoops’ grades of
coalescenceofmicroaggregates,wepreferredthemoreillustrativeterminologyofwelding.
Our single excrements correspond to Stoops’ very porous microaggregates and our very
stronglyweldedexcrementstoverydensemicroaggregates.Wegroupedtheterms‘organ
residues’,‘tissueresidues’and‘cellresidues’ofStoopsinoneclassof‘fragmentedOM’,to
increase readability. Fragmented OM includes both fresh and more decomposed (less
birefringent)OM.
Wedefinedsoilfaunalpedofeatures,i.e.excrements,looseinfillingsanddenseinfillings,as
‘bioturbation features’. In addition, we included zones with a vughy microstructure (i.e.
zones containing frequent vughs) in our definition, since these zones were interpreted to
resultfromverystrongweldingoflooseinfillings,whichisinaccordancewithobservations
byPawluk(1987).
Results
Soilproperties
GeneralsoilpropertieswerereportedinTonneijcketal.(2008a).Briefly,theuppermineral
horizonswerecharacterisedbyaverylargeorganiccarboncontent(8%to22%),acidicpH
71
(pHCaCl2 3.2to4.4)andsmalldrybulkdensity(0.3to0.7gcm3).Themineraltopsoilsdidnot
containallophane,whilethesubsurfacehorizonscontainedupto5%allophane.
VerticaldistributionofSOMandHA
TheverticaldistributionoftotalorganiccarbonandcarbonintheHAfractionispresented
foreachsoilprofileinFigure4.1a.Asexplainedintheintroduction,organiccarboncontent
decreased with depth in the current soils, but instead of an abrupt increase in organic
carbon content at the boundary with the paleosol, organic carbon content increased
gradually. Carbon in the HA fraction followed a similar distribution with depth as organic
carbonandbothPearsonandSpearmancorrelationsbetweencarbonintheHAfractionand
totalorganiccarbonwerelarge(+0.82and+0.86respectively,p<0.001inbothcases).
Radiocarbondating
Conventional and calibrated radiocarbon ages are presented in Table 4.1 and Figure 4.1c
additionallyshowscalibratedageversusdepthforeachsoilprofile.Weusedthecalibrated
radiocarbon ages of HA for the agedepth relationship in all cases, except for the mineral
soils just beneath the ectorganic horizon of the forest profiles where we used calibrated
radiocarbonagesofhumininstead,accordingtoTonneijcketal.(2006).Ther2valueofthe
linearregressionwaslargein allcases(>0.84).Theslopeoftheagedepthrelationshipin
themineralsoilofforestsiteG1wassignificantlydifferentfromtheslopesoftheothersoil
profiles,whileforestpatchprofileG5aresembledthepáramoprofiles.
Figure4.1Verticaldistributionof:
a)Totalorganiccarboncontent(%):…=bulksample(5cminterval),…=monolithsample(0.75cminterval),
14
„ =
C sample (0.5 cm interval) and organic carbon content in humic acids (%) : × = bulk sample (5 cm
interval),×=monolithsample(0.75cminterval).
b)Bioturbationfeatures:+=veryfew,++=few,+++=common,++++=frequent,+++++=dominant,++++++=
verydominant.
c) Calibratedradiocarbonages(calAD/BC)forthemineralsoilprofiles.
72
Figure4.1continued
73
Table4.1Conventional(BP)andcalibrated(calAD/BC)radiocarbonagesofthehumicacidfraction(except
forsampleslocatedjustbeneathathickectorganichorizonwherethehuminfractionwasdated,asindicated
inboldfont).
Site
Codea
GrA30094e
GrA30092
GrA34933
GrA34934
GrA34935
GrA30109
GrA30112
GrA34938
GrA28101
GrA34939
GrA30113
GrA30114
GrA28104
GrA30133
GrA30134
GrA28108
GrA34943
GrA35329
GrA35330
GrA30138
GrA35354
GrA35355
GrA28111
GrA30139
GrA30107
GrA28113
GrA30238
GrA30232
GrA30234
GrA28116
GrA28130
GrA28134
GrA28126
GrA34944
G1
G5a
G5b
G7
a
b
c
d
e
f
g
h
74
=
=
=
=
=
=
=
=
Horizonb
H
H
Ah
Ah
2Ahb
F
F
Ah
Ah
Ah
Ah
Ah
Bw
1/2Bw
1/2Ah1b
1/2Ah1b
2Ah1b
Ah
Ah
Ah
Ah
Ah
Ah
Bw1
Bw1
1/2Bw2
1/2Bw2
1/2Bw2
2Ahb
2Ahb
Ah1
Ah2
1/2Ahb
2Ahb
14
Depthc
Cage
13C
[cm]
[BP]
[‰]
30.5
40±35 25.73
1
380±35 25.68
0
1225±40f 28.15
10
2475±40 27.18
39
6155±45 25.31
20.5 623±35 27.46
1
28±35 26.39
5
325±35g 26.91
10
775±35h 25.93
20
755±35 26.51
24.5
355±35 26.03
39.5 1855±40 26.65
49.5 2170±35 25.92
69
3650±40 25.71
70.5 3880±40 25.62
79.5 4355±40 25.10
85
4870±40 25.57
5
200±35 25.86
10
400±35 25.88
14.5
590±35 25.33
20
685±35 25.62
25
855±35 25.75
34.5 1350±35 25.45
49.5 2070±40 26.45
64.5 3045±40 25.79
75
3465±35 25.51
94.5 4270±40 25.81
114
5260±40 24.81
115.5 5405±40 25.20
124.5 5730±40 24.91
10
180±35 25.24
46.5 1690±40 25.22
89.5 4910±45 24.98
98
5985±45 25.47
Calibratedage
Range Mean
[calAD/BC] [calAD/BC]
calAD1877to1912 calAD1895
calAD1449to1517 calAD1483
calAD769to830 calAD800
calBC727to606 calBC667
calBC5135to5049 calBC5092
calAD1996to2002 calAD1999
calAD1956to1957 calAD1957
calAD1538to1602 calAD1570
calAD1247to1271 calAD1259
calAD1251to1281 calAD1266
calAD1467to1523 calAD1495
calAD124to217 calAD171
calBC352to293 calBC323
calBC2041to1956 calBC1999
calBC2406to2337 calBC2372
calBC3015to2944 calBC2980
calBC3667to3638 calBC3653
calAD1766to1801 calAD1784
calAD1441to1494 calAD1468
calAD1311to1359 calAD1335
calAD1276to1300 calAD1288
calAD1157to1224 calAD1191
calAD648to682 calAD665
calBC116to43 calBC80
calBC1323to1266 calBC1295
calBC1780to1740 calBC1760
calBC2913to2879 calBC2896
calBC4071to4034 calBC4053
calBC4327to4280 calBC4304
calBC4614to4515 calBC4565
calAD1761to1789 calAD1775
calAD329to407 calAD368
calBC3712to3643 calBC3678
calBC4935to4860 calBC4898
Referenced
*
*
*
**
**
*
**
**
*
*
*
**
**
*
**
**
**
*
*
*
*
LaboratorycodeoftheCentreforIsotopeResearch,GroningenUniversity,theNetherlands
horizonnomenclatureofectorganichorizonsaccordingtoGreenetal.(1993)
depthofsample,verticalsampleinterval0.5cm,themineralsoilsurfacewassetat0cm,samplestaken
fromsoilmonolith
*Tonneijcketal.(2006),**Tonneijcketal.(2007)
inthiscasethehuminfractionwasdatedbecauseHAwaslost
14
CageofHAwas840±35BP
14
CageofHAwas255±35
14
CageofHAwas490±35BP
Mineralandorganiccomponents
Throughout the mineral horizons of all soil profiles mineral components (data not shown)
were represented by plagioclase, amphibole, biotite, vesicular rhyolitic volcanic glass and
rhyolitoid rock fragments. Opaque mineral grains (e.g. magnetite and hematite) were
difficulttorecognizeduetothepresenceofveryfine(<20m)opaqueOMfragments.In
general, minerals and rock fragments showed only initial pellicular and/or dotted
weathering.Finemineralcomponentsinthemineralhorizonsofallsoilprofilesconsistedof
dull lightto darkbrown, isotropic materialwithundifferentiatedbfabric.Thepresenceof
veryfine(<20m)opaqueOMresultedinadottedappearance.
DistributionoffreshrootsandfragmentedOM(asarea%)asobservedinthethinsectionsis
presented in Figure 4.2. In the ectorganic horizons of the forest profiles, OM was mainly
present as common fresh roots, frequent to common fragmented OM (both roots and
leaves) and fine organic groundmass. In the mineral horizons of all soil profiles, the
abundanceoffreshrootsandfragmentedOMdecreasedquicklywithdepthfrom(very)few
tocommoninthemineraltopsoilstoveryfewfurtherdownthesoilprofiles.Veryfewintact
and fragmented sclerotia occurred throughout the mineral soil profiles. Additionally, very
fewirregularshapedopaque OMfragments ofvarioussizeswerepresent, mostlywithout
recognizablecellstructure.
Typeanddistributionofbioturbationfeatures
Theverticaldistributionofbioturbationfeatures(asarea%),typeofbioturbationfeatures
(as proportions of total) and microstructures (as area %) are presented in Figure 4.2 and
described in detail for each soil profile in the following sections. Figure 4.1b also shows
bioturbationfeaturesversusdepthincombinationwithcarboncontentsand14Cages.
x G7páramo
Withinthefirst3cmoftheAh1horizon,bioturbationfeatureswerefrequentandoccurred
mainly as loose discontinuous infillings containing clustered spherical brown matric
excrements(50100m‡)invaryingstagesofwelding.Weldingresultedintransformation
to mammilated excrements, occurring again in clusters (Figure 4.3). In addition, very few
organic/matricdarkbrownmammilatedexcrementsupto~8mm‡werepresentnearthe
surface.Thiszonehadacrumbmicrostructureandshowedanirregulartonguingboundary
alongwhichmassivedensecompletematricinfillings(~5mm‡)wereobserved,composed
offinebrowngroundmasswith(very)fewmineralgrainsandveryfewOMfragments.These
denseinfillingsweresimilarinsizeandcompositiontotheonesoccurringatgreaterdepth
buthadamammilatedoutlineowingtothegreaterporosityofthetopsoil.
75
Figure 4.2 Bar graphs of bioturbation features (area %), type of bioturbation features (as proportions of
total),microstructures(area%)andorganicmatter(area%)versussampledepth(cm,topofmineralsoilset
atzero)forallsoilprofiles.
76
Bioturbation features decreased with depth to few in the Ah horizon (at 30 cm depth),
initially mainly occurring as loose discontinuous infillings with clustered spherical to
mammilated excrements similar to the ones in the topsoil. These infillings were often
welded to such an extent that a vughy microstructure developed (Figure 4.3). In addition,
veryfewloosecontinuousinfillingswithweldedmatricsphericalexcrements(~100m‡)
occurredinclusteredandbandedbasicdistributionpatterns.From30cmdepthonwards,
bioturbation features were mainly present as brown dense complete infillings instead of
looseinfillings.
Figure4.3Currentsoilwithpáramovegetation:Loosediscontinuousinfillingswithweldedspherical(example
encircled in the middle of the graph) and mammilated excrements (example encircled at top), eventually
gradingintoavughymicrostructure.
Strikingly, dense complete infillings increased to dominant in the 1/2Ah2 and 1/2Ahb
horizons(theoverprintedzone),whileloosediscontinuousinfillingswereabsent.Thecolour
contrast between different dense infillings increased and they were now composed of
either dark brown fine groundmass or brown to light brown fine groundmass (Figure 4.4a
andb).Themineralgrainssometimesshowedacrescentdistributionpatternfollowingthe
outlineofthedenseinfilling.Intheoverprintedzoneachannelmicrostructure(Figure4.5)
becamedominant.Therandomlydistributedchannelsrangedfrom60–600m‡anddid
notcontainexcrements,somechannelsdidcontainroots.
Figure 4.4 Current soils with páramo vegetation and overprinted zones of all soil profiles: a) Brown dense
completeinfillingandb)Darkbrowndensecompleteinfillingwithcoarsertexture;denseinfillingsresultedina
massivemicrostructure.
A
B
77
Figure4.5Overprintedzonesandtopofpaleosol:
Channelmicrostructure.
N.B.dottedappearanceduetoabrasivepowder.
x
Figure 4.6 Ectorganic profile forest: Ellipsoidal
organic excrements (example encircled at the top
ofthegraph)andmammilatedorganicexcrements
(exampleencircledleftbottomcorner).
G5bpáramonexttoforestpatch
ThetypeanddistributionofbioturbationfeaturesinprofileG5bwasverysimilartoprofile
G7 (see Figure 4.2). Within the first 2 cm of the Ah horizon, bioturbation features were
frequent and the microstructure was crumb with few massive zones. Similar to G7,
bioturbation features decreased with depth to very few at the bottom of the Ah horizon.
Loose discontinuous infillings remained present until 35 cm. Microstructure changed to
massive,withveryfewvughyzoneswherelooseinfillingsbecamesostronglyweldedthat
theywerenolongerrecognizableassuch.Theproportionofdenseinfillingswassomewhat
higherthanthatoflooseinfillingsthroughouttheAhhorizon.AsinprofileG7,fromtheBw
horizononwardsbioturbationfeaturesincreasedandoccurredonlyaslightbrowntodark
browndenseinfillings.However,thefrequencyofbioturbationfeaturesintheoverprinted
zone was somewhat lower than in G7. Colour contrast between the dense infillings
increasedwithdepththroughouttheprofile.Atdepth,asimilarchannelmicrostructureas
inprofileG7becamedominant.
x G1forest
Within the first 5 cm of the thick (75 cm) ectorganic profile of G1, bioturbation features
werefrequentandrepresentedbybothsingleandweldedexcrements.Microstructurewas
crumb. We encountered small spherical/ellipsoidal organic excrements, ranging from 50 –
200m ‡. Such excrements were composed of light brown to reddish brown organic fine
groundmasswithoutrecognizableOMfragments,oftenassociatedwithleafandroottissues
(Figure 4.6). In addition, larger mammilated organic excrements (up to 800 m ‡)
composed of light brown to reddish brown organic fine groundmass with common OM
fragmentsoccurred.Weldingincreasedquicklywithdepth.
78
Further down the ectorganic horizons, the mammilated organic excrements became so
stronglyweldedthattheyturnedintofineorganicgroundmassmostlywithoutrecognizable
excrements (Figure 4.7a and b). The microstructure became massive in large parts of the
thin sections (Figure 4.7c). These massive zones did not qualify as ‘bioturbation feature’
according to our definition, because (welded) excrements were no longer recognizable as
such. Bioturbation features decreased with depth to very few. Still, very few single to
stronglyweldedorganicspherical/ellipsoidalexcrementswerepresentevenatthebottom
oftheectorganichorizons,oftenassociatedwithrootsandleaffragments.
Figure4.7EctorganicprofileG1:a)Mammilatedorganicexcrements;b)becomingmoreandmorewelded
with depth; c) Until they grade into an organic fine groundmass with massive microstructure. N.B. dotted
appearanceduetoabrasivepowder.
A
B
C
In the Ah horizon, very few bioturbation features were present as loose discontinuous
infillingscomposedof(verystrongly)weldedbrightreddishbrownsphericalorganic/matric
excrements,andmicrostructurewasmassive.Thefinegroundmasswasofsimilarcolouras
theseexcrementsandtheboundariesbetweeninfillingsandfinegroundmasswerediffuse.
The transition between ectorganic and mineral horizons was gradual, the Ah horizon still
containingcommonfreshrootsandfragmentedOM.
In the 1/2Bw horizon (overprinted zone), bioturbation changed drastically with regard to
typeofactivity,andincreasedinabundancetoverydominant.Bioturbationfeatureswere
onlypresentasdensecompletematricinfillingsofthesametypesasencounteredinprofile
G7 and G5b. Bioturbation features in the 2Ahb horizon were very similar to that in the
overlying1/2Bwhorizon.
x G5aforestpatch
IntheectorganichorizonsofprofileG5a,similartypesofbioturbationfeatureswerepresent
asinforestprofileG1.Weobservedthatthemammilatedorganicexcrementswereoften
composedofsmallerexcrements,whichwasnotclearinprofileG1probablyduetostronger
weldingthere.ContrarytoG1,wedidnotobservezoneswithmassivemicrostructureand
excrements remained recognizable. Bioturbation features remained frequent to dominant
throughouttheectorganichorizons.
79
ThemineraltopsoilofforestpatchG5aresembledpáramoprofilesG7andG5bratherthan
forest profile G1. At the top of the Ah horizon, bioturbation features were frequent,
occurring as loose discontinuous infillings similar to the ones found in páramo profiles G7
and G5b. They were again often welded to such an extent that a vughy microstructure
developed.ContrarytoprofileG1,thetransitionbetweenectorganicandmineralhorizons
was abrupt, the Ah horizon containing only very few fresh roots and fragmented OM and
lacking the reddish staining. With depth, the loose infillings decreased in abundance until
theydisappearedinthe1/2Bwhorizon.Instead,veryfewdensecompletematricinfillings
similar to the ones encountered in profile G7 appeared at the bottom of the Ah horizon.
ThesedenseinfillingsbecamedominantintheBwhorizonandverydominantinthe1/2Bw
and 1/2Ah1b horizons (overprinted zone). Simultaneously, microstructure changed to
massive with depth. Finally, in the 2Ah2b horizon dense infillings decreased somewhat in
abundance to frequent and a channel microstructure developed. The colour contrast
betweendenseinfillingsincreasedwithdepth.
Discussion
Mineralandorganiccomponents
Micromorphological observations of the mineral components confirmed the mineralogical
assemblage as determined by Xray diffraction and presented by Tonneijck et al. (2008a).
ThedulllighttodarkbrowncolourofthefinegroundmassindicatedstainingbyOM,since
organic carbon contents were high (> 6 %) in all mineral horizons. A high organic carbon
contentistypicalforAndosolsandcanberelatedtotheformationofamorphousorgano
mineral/metalcomplexes(Dahlgrenetal.2004).Ourconclusionwithrespecttostainingby
OM was confirmed by the undifferentiated bfabric of the fine mineral components that
indicated masking by humus and/or the presence of amorphous materials (Stoops 2003).
TheredcolouringofthefinegroundmassinthemineraltopsoilofforestsiteG1wasmost
likely due to OM staining and not to the presence of iron, since amorphous Fe contents
werelowthereandsimilartotheothertopsoilslackingthisredcolouring(Tonneijcketal.
2008a).TheirregularshapedopaqueOMfragmentsmaywellbecharcoalfragments,since
páramo vegetation in the study area is known to be subject to burning and charcoal
fragmentswerefoundbyDiPasqualeetal.(2007)insoilsfromthesamestudyarea.
Soilfaunalgroups
Basedonthedistributionandcompositionofbioturbationfeatures,wededucedwhichsoil
faunal group probably created them. This is of great importance, since it may clarify the
verticaldistributionofSOM(Anderson1988),asexplainedintheIntroduction.Bothinthe
mineralsoilprofilesandintheectorganichorizonsoftheforestsites,theheterogeneityof
the bioturbation features encountered indicated the activity of several soil faunal species.
The small organic excrements occurring in the ectorganic horizons of the forest profile
80
obviouslywereformedbyepigeicspecies.Thematriccompositionandrandomdistribution
of the small (50 – 100 m ‡) excrements occurring as loose infillings in the Ah horizons
clearlysuggestthatthesewereproducedbysmallendogeicspecies.Similarly,thelarger(~5
mm‡)matricdenseinfillingswerecreatedbylargerendogeicspecies,asindicatedbytheir
compositionanddominantoccurrenceinthesubsoil.Sincewedidnotobservepermanent
verticalburrowsinthethinsectionsnorinthefield,anecicspeciesmusthavebeenscarce
or absent. The dominant soil fauna is most likely represented by Lumbricidae, Acari,
Collembola, Enchytraeidae, and Nematoda, similar to Colombian (van der Hammen &
Beglinger 1989) and Mexican Andosols (Barois et al. 1998). Detailed classification of soil
faunalspeciesisnotpossibleduetothemorphologicalsimilarityofsoilfaunalpedofeatures
(Pawluk1987).
Verticaldistributionofroots
The dominant occurrence of both fresh roots and fragmented OM (including roots and
leaves)intheectorganichorizonsand/ormineraltopsoilconfirmedourfielddescriptionsof
superficial root distribution (Tonneijck et al. 2006) and are in accordance with data from
Soethe et al. (2006) for Ecuadorian montane forest and data from Hofstede & Rossenaar
(1995) for Colombian páramo ecosystems. Páramo ecosystems have the highest
aboveground:belowground biomass ratio (typically 1:0.35) of a wide range of grassland
ecosystems(Hofstede&Rossenaar1995).
Sincethechannelmicrostructurethatwaspresentintheoverprinted zoneandtopofthe
paleosol (except in profile G1) was not accompanied by an increase in fresh roots (Figure
4.2)andchannelswereoftenempty,weconcludethattherootsystemthereisnolongerin
use.Becausethechannelmicrostructure occurredin bothpartofthecurrentsoilandthe
paleosol, it was probably formed after the last tephra deposition. Possibly, roots have
shiftedupwardsovertime.Alternatively,adeeperrootingvegetationtypeinthepastcould
have created the channel microstructure, although this seems unlikely in light of the
superficialrootsystemsofbothcurrentforestandpáramovegetation.
Nieropetal.(2007)foundwithanalyticalpyrolysistechniquesthatthesubsoilsofoursoil
profiles exhibited more of a root than of a leaf derived SOM type. Root litter may have
been transported by bioturbation to the subsoil instead of being deposited there in situ
and/orpriortoupwardshiftingrootsmayhavereachedthesubsoil.
VerticaldistributionofHA
The strong correlation between carbon in the HA fraction and total organic carbon in the
mineralsoilprofilesdemonstratesthatHAwastransportedthroughthesoilprofilebythe
same mechanisms as bulk SOM. Radiocarbon dating of HA therefore offers an additional
clueregardingtheverticaltransportofbulkSOM.RadiocarbondatingofHAmatchedvery
81
wellwithdatingofcharcoalfragmentsinmineralsoilprofilesfromthesameresearcharea
by Di Pasquale et al. (2007), confirming the reliability of our datings. In a previous study
(Tonneijck et al. 2006) we already indicated that HA in these soils was most likely not
rejuvenated by roots or illuviation of younger OM. Only the mineral topsoils just beneath
thethickectorganichorizonsoftheforestprofilesformedanexception,astheyoungerage
ofHAthanofthehuminfractiontheredidindicatesomeilluviationofyoungerHAfromthe
ectorganichorizons.Becauseofthis,inthosespecificcaseshuminwasusedfordating.
VerticaldistributionofSOM:ectorganichorizons
OMaccumulationontopofthemineralsoilsurfacedemonstratedthatbioturbationdidnot
transportOMverticallyintheforestsites(G1andG5a).Indeed,thesoilfaunalpedofeatures
intheectorganichorizonswereproducedbyepigeicspeciesandthesedonotredistribute
OM(Anderson1988).IntheectorganichorizonsofforestprofileG1,thestrongincreasein
weldingwithdepthtotheextentthatamassivemicrostructuredeveloped,suggestedthat
bioturbationisnolongeractiveatdepth.Still,thefrequentoccurrenceoffragmentedOM
tissueandthedecreaseinsizeoffragmentedOMshowedthatbioturbationmustoncehave
beenmoreactive.Apparently,bioturbationmainlyaffectsfreshOMcontinuouslysupplied
at the surface, rather than older OM (up to 1483 cal AD, Table 4.1) remaining at depth.
Additionally,theveryacidicpH(pHCaCl2 <2.5)andperiodicallywetandanaerobicconditions
inthethickectorganichorizonsareunfavourableforsoilfaunalactivity.AtforestpatchG5a,
theectorganichorizonwasthinner(35cm)thanatG1andmuchyounger,itsbasedatedto
1957 cal AD (Table 4.1). Because OM was still relatively fresh, bioturbation features were
morefrequentthanatG1.Thesitealsohadabetterdrainage,resultinginlessweldingof
excrementsascomparedtoprofileG1.Bioturbationthroughouttheectorganichorizonsof
G5aisprobablystillactive,althoughitdoesnotresultinverticaltransportofOM.
VerticaldistributionofSOM:currentsoils
Pessendaetal.(2001)andRumpeletal.(2002)foundasimilarincreaseofagewithdepth
accompaniedbyadecreaseinorganiccarboncontentasinourcurrentmineralsoilprofiles
andrelatedthistoalargerinputofyoungermaterialtothetopsoilthantothesubsoil.In
ourcasethiscouldhavebeencausedbythedecreaseinbioturbation(looseinfillings)with
depth combined with dominant OM input from the soil surface (aboveground litter) and
topsoil (superficial rooting). Additionally, at the forest sites (G1 and G5a) some OM
illuviationinthemineraltopsoiloccurredasexplainedpreviously.
TherejuvenatingimpactoffreshOMincreaseswithincreasingagedifference(Mook&Van
de Plassche 1986). Apparently, bioturbation seldom transports fresh OM directly to the
oldersubsoil,eventhoughsomebioturbationfeatureswerefoundthere.Indeed,freshOM
was concentrated in the topsoil and loose and dense infillings in the current soils were
mainlycomposedofmatricratherthanorganicmaterial.Theendogeicspeciesobservedto
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bedominantinthestudiedsoilprofilesfeedonSOMratherthanfreshOMfromtheearth
surface (Anderson 1988; Lavelle 1988; Lee & Foster 1991). Mixing of SOM with small age
differences over short distances does not prevent a linear agedepth relationship even if
bioturbation extends to considerable depth. Thus, downward SOM transport must have
occurredlargelyinthechronologicalorderinwhichitwassuppliedatthesurface,gradually
movingdeeperintothesoil.
VerticaldistributionofSOM:overprintedzones
Intheoverprintedzone,theunusualincreaseoforganiccarbonwithdepthsuggestedthat
soilfaunamixedthesubsoilofthecurrentsoilwiththetopsoilofthepaleosol(Tonneijcket
al. 2008a), which was confirmed by our micromorphological observations. Material with a
relativelyloworganiccarboncontentandrelativelyyoungage(bottomofthecurrentsoil)
was mixed into material with a higher organic carbon content and relatively high age
(paleosol),assuggestedbytheoccurrenceofdenseinfillingsofcontrastingcolour.Current
rootsweremainlyconcentratedinthetopsoil(páramo)andectorganichorizons(forest),but
pastrootsmayhavecontributedtotheincreaseinorganiccarbonintheoverprintedzone,
as suggested by the channel microstructure. However, root input alone cannot readily
explain the gradual nature of this increase and therefore bioturbation is probably of
overridingimportance.
We discarded two remaining possibilities that in theory could have caused the unusual
distribution of organic carbon contents in the overprinted zone (Tonneijck et al. 2008a).
First, while burial of a páramo vegetation would probably not result in elevated organic
carboncontents,onecanimaginethatburialofathickectorganichorizoncould.However,
in that case the lithology should be that of the tephra deposit burying it, which was not
supportedbygrainsizeandgeochemicaldata.Moreover,buriedectorganichorizonswould
be recognizable as such with micromorphological analysis. Second, as mentioned before,
leachingandsubsequentilluviationofOMishighlyunlikelyinAndosolsbecauseofthelarge
metaltoSOMratios(Aranetal.2001;Dahlgrenetal.2004)andindeedwedidnotobserve
anysignsofOMilluviation(e.g.coatings)inourmicromorphologicalanalysis.Rumpeletal.
(2002) presented data on a Podzol profile with leaching as the dominant mode of OM
transport and illuviation of younger OM resulted in a radiocarbon age inversion. Such age
inversionswerenotobservedintheoverprintedzone.
In view of the magnitude of bioturbation in the overprinted zone, one would expect the
entireoverprintedzonetohavebeenhomogenised,whichclearlycontradictstheobserved
gradualincreaseinorganiccarboncontent.Inaddition,ifbioturbationindeedhadresulted
incompletehomogenisationoftheoverprintedzone,onewouldnotexpecttofindalinear
agedepth relationship but rather one average age. At the very least one would have
expected to find some age inversions, which was not the case. Apparently, analogous to
overlying horizons, mixing in the overprinted zone occurred only over short vertical
83
distances.Theobservedincreaseincontrastbetweendenseinfillingswithdepthindeedfits
shortverticaldistancemixing;sincethecolourcontrastbetweenthebottomofthecurrent
soil and top of the paleosol must have been greater than the contrast between different
positionswithinthecurrentsoilitself.Indeed,theendogeicspeciesdominantintheprofiles
studied mainly move horizontally and are not responsible for transport over large vertical
distances(Anderson1988).
In the top of the paleosol, a channel microstructure was superimposed on the dense
infillings.Becausethechannelmicrostructurewasrelatedtotheformerpresenceofroots
asexplainedpreviously,thisimpliesthatthedenseinfillingsarenotrecentlymadeanymore.
Thesimultaneousoccurrenceofchannelmicrostructuresandmassivemicrostructuresinthe
overprintedzonebetweenthecurrentsoilandpaleosol,suggestthateithertherootsystem
waslessextensiveorthat partofthedenseinfillingswereformed relativelyrecently.The
latter explanation seems more likely since roots generally decrease rather than increase
with depth. Possibly the zone with the dense infilling type of bioturbation has shifted
upwardsovertime.
Shortverticaldistancemixing
Bothinthecurrentsoils(mineralhorizons)andtheoverprintedzones,ourresultssuggest
that mixing by endogeic species occurred only over short vertical distances. Even though
bioturbation differed greatly in magnitude throughout the soil profiles, being much more
extensive in the overprinted zone, the slope of the linear agedepth relationship was not
much affected. This implies that the vertical interval of mixing, i.e. the age differences
mixed,weresmallandsimilarthroughoutthesoilprofile.Sincebothradiocarbonagesand
organiccarboncontentswerenothomogenisedoververticalintervalsof5cm,weconclude
thatmixingmusthaveoccurredoverevensmallerdistances.
An additional process explaining mixing over short vertical distances could be an upward
shift of the soil fauna, rendering deeper positions in the soil out of reach of bioturbation
over time. The channel microstructure superimposing the dense infillings and vice versa
indeedsuggestedupwardshifting.Upwardshiftingmayhavebeencausedbysoilthickening
duetoSOMaccumulation(Tonneijcketal.2008a).Suchsoilthickeninghasbeenobserved
in Mexican volcanic ash soils as well (Barois et al. 1998). Another explanation for upward
shiftingcouldbethatsoilfaunaitselfmigratedupwardsinresponsetochangesinhydrology
over time, e.g. such as caused by the formation of an impermeable placic horizon as
encounteredinthesoilsstudied.
Spatialvariabilityofbioturbationfeatures
Micromorphologicalstudiesaretypicallynotsuitedtoaccountfullyforthespatialvariability
of pedofeatures, due to the small amount of samples and number of replicates involved.
84
However, we do not expect great variability of bioturbation features within the soil
horizons,sinceweobservedinthefieldthatthevariabilityofsoilpropertieswithinhorizons
(e.g. very fine to medium root distribution, soil moisture, porosity, structure, and colour)
was small. Our micromorphological results indeed showed that the variability of
bioturbation features in the overprinted zone was small, since bioturbation features
occupied a high proportion of the thin sections in all soil profiles studied (see Figure 4.2).
Thespatialvariabilityofbioturbationfeaturesinthecurrentsoilsmaybesomewhatgreater
becauseoftheirlowerabundance.However,thecumulativeeffectoflocalisedbioturbation
featuresovertimedecreasesthespatialvariabilityiftheyarerandomlydistributed,whichis
likely because they were formed by endogeic species. Still, the possibility exists that we
missedbioturbationfeaturesthatwerelessabundant,butthesescarcefeatureswouldthen
probably not exert a strong influence on the vertical distribution of SOM. Overall, the
generaltrendsintheverticaldistributionofSOM,i.e.decreasinginthecurrentsoilprofiles
andincreasingintheoverprintedzones,canlargelybeexplainedbyourmicromorphological
observationsofbioturbationfeatures.
Vegetationtypeandsoilfauna
Vegetationtypeindirectlyinfluencesthesoilfaunaduetothepalatabilityofitslitter.(Swift
et al. 1979). The sites studied were located along an altitudinal transect intersecting the
upper forest line. Altitudinal fluctuations of this upper forest line result in a complex
vegetation history at our sites (Di Pasquale et al. 2007; Bakker et al. 2008; Jansen et al.
2008),whichisreflectedbychangesinsoilfaunalpedofeatures.
Atthepáramositesabovethecurrentupperforestline(G5bandG7)bothlooseinfillings
anddenseinfillingsproducedbyendogeicspecieswerepresentuptothesoilsurface,while
theywereabsentatthesurfaceoftheforestsites.Thissuggeststhatendogeicspeciesare
stillactivetodayinthepáramoecosystemwhiletheyarenotactiveanymoreintheforest.
Instead, epigeic species dominate the forest soils, eventually resulting in the formation of
thickectorganichorizonsbecauseepigeicspeciesdonotredistributeOM(Anderson1988).
Site G5a was only relatively recently colonised by forest vegetation (Jansen et al. 2008),
likely explaining why the mineral soil profile of G5a resembled the mineral soil profiles of
páramo (G5b and G7) rather than forest (G1) with regard to the soil faunal pedofeatures
encounteredandtheslopeoftheagedepthrelationship.
The deviating slope of the agedepth relationship in the mineral soil of forest site G1 was
likelyrelatedtodominantaccumulationofOMonthesurfaceofthesoilinsteadofinthe
mineralsoilforaprolongedperiodoftime.Indeed,theagedepthrelationshipintheforest
profile of Di Pasquale et al. (2007), with an equally thick ectorganic layer, had a similar
deviatingslope.Still,thesuddenappearanceofdenseinfillingsatdepthinforestprofileG1
could indicatetheformerpresenceofpáramovegetationevenatthis lowaltitudesite.In
viewofthethicknessoftheectorganicprofileandthegradualtransitionfromectorganicto
mineralhorizonsinG1,suchpáramovegetationwouldpredatethatofsiteG5a.DiPasquale
85
et al. (2007) indeed suggested that páramo was present before ~3900 cal BC at their
currently forested site at similar low altitude (3540 m a.s.l.) in the same study area.
Similarly, pollen analysis of a small mire in our study area indicated that the upper forest
linewaslocatedatalowaltitude(between3100and3300ma.s.l.)bothbefore~4300cal
BCandbetween~210calBCand~1430calAD(Bakkeretal.2008).Thus,siteG1couldwell
havebeencoveredbypáramovegetationduringtheseperiods.
Implications
Vertical transport of SOM by bioturbation is often modelled with diffusion equations that
use uniform diffusion coefficients with depth (e.g. Elzein & Balesdent 1995; Bruun et al.
2007).However,inourcasethiswouldhaveresultedinaverticaldistributionofSOMwith
smooth turning points rather than in the observed sharp turning points. Modelling of the
verticaldistributionofSOMinoursoils,andprobablyinvolcanicashsoilsingeneral,maybe
greatly enhanced by applying diffusion coefficients changing with depth as based on
observedbioturbationfeatures,incombinationwithsoilthickeningandsubsequentupward
shiftingofbioturbation.
The studied soil profiles contain paleoecological proxies (e.g. pollen and biomarkers) that
are used to reconstruct the vegetation history in the study area (Jansen et al. 2008).
Because the use of paleoecological proxies contained in soils depends inter alia on their
preservation in chronostratigraphic order, the degree and type of bioturbation is of key
importance. The results from the present study suggest that as long as paleoecological
proxiesweretransportedthroughthesoilprofilesimilarlytobulkSOMandHA,theywillbe
distributed in a (crude) chronostratigraphic order. However, a major consequence of the
presentresearchwithrespecttopaleoecologicalinvestigationsisalsothatanysampletaken
inevitablywouldproduceamixedsignal.
Conclusions
The páramo soil data suggest that bioturbation was largely responsible for the vertical
distribution of SOM, while illuviation and root input were of minor importance. In the
mineraltopsoils,adecreaseinbioturbationbysmallendogeicspecieswithdepthcombined
withdominantOMsupplyfromthesoilsurfaceandtopsoilresultedinadecreaseinorganic
carboncontentandanincreaseofagewithdepth.Intheoverprintedzone,ourdatasuggest
that extensive bioturbation by larger endogeic species mixed the initially relatively SOM
poorsubsoilwiththeSOMrichunderlyingpaleosol,whichresultedinagradualincreasein
organic carbon content with depth. Our data additionally suggest that mixing throughout
thesoilprofileoccurredonlyovershort(<5cm)verticaldistances.Shortverticaldistance
mixing was apparently enhanced by upward shifting of bioturbation as a result of soil
thickening due to SOM accumulation. A change from páramo to forest vegetation was
accompanied by a change from endogeic to epigeic species. These latter species do not
redistribute material vertically, which eventually resulted in the formation of thick
ectorganichorizonsintheforest.
86
Acknowledgements
WeacknowledgetheEcuadorianMinisteriodelAmbienteforissuingthenecessarypermits,
Jatun Sacha for their support in Guandera Biological Station and Ecopar for their office
assistance. Furthermore, we acknowledge the fellow members of the RUFLE program: Jan
Sevink, Koos Verstraten, Boris Jansen, Marcela Moscol Olivera, Henry Hooghiemstra and
AntoineCleeffortheirvaluablecontributionstothisresearch.Inthisrespectwealsothank
EmielvanLoon,MishaVelthuis,MirjamPulleman,JanPeterLesschenandKasperdeRooy.
Finally,wewishtothankLeoHoitingaforhisassistanceinthechemicallaboratoryandFrans
Backer and Wijnanda Koot for thin section preparation. This research was funded by
WOTRO (WAN 75405) and the University of Amsterdam and generously sponsored by
Fjällravenintheformofclothingandgear.
87