Classification of forest humus forms: a French proposal

Classification of forest humus forms: a French proposal

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In: Annales des Sciences Forestières, 1995, 52 (6), pp.535-546. A 2-way classification grid and a nomenclature are proposed for French forest humus forms but which could include mountain, Mediterranean and tropical forms as well. This proposal takes into account our present knowledge of biological mechanisms that take place in plant litter decomposition, transformation of soil organic matter, linkage of the latter to mineral particles and building of the structure in the A horizon. Basically, by adjoining free qualifiers, humus forms may be defined by accounting also for their chemical and physical particularities.

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article
Original

Classificationofforesthumusforms:
aFrench
proposal

ABrethes

JJ Brun

BJabiol

J Ponge

FToutain

citéadministrative
1
desrecherchestechniques,Coligny,
ONF,département
ruedu45042Orléanscedex;
131,Faubourg-Bannier,
2
CEMAGREF,
la
domaineuniversitaire,2,ruedePapeterie,
BP76,38402Saint-Martin-d’Hèrescedex;
3
54042
ENGREF,
14,rueGirardet,Nancycedex;
4
labor
Muséumnaturelld’écologiegénéra
nationald’histoiree,atoirele,
1800
4,avenueduPetit-Château,9Brunoy;
5
ru
centredeue,BP5,1 7,eNotre-Dame-des-Pauvres,
CNRS,pédologiebiologiq
France
54501Vandceuvrecedex,

218March
1994;
(ReceivedJanuaryaccepted1995)

forFrenchforest
classificationandanomenclaturearehumusforms
Summary —A2-waygridproposed
stakesinto

butwhichcouldn,tropical
includemountaiMediterraneanandformsaswell.Thiproposal
inlitter
thattake
accountourofmechanismsplant
presentknowledgebiologicalplacedecomposi-
sformationofsoilmatteofthelattertomineralandofthe
tion,tranorganicr,linkageparticlesbuilding
horifreehumusform

structureintheAzon.Bassbedefinedby
ically,byadjoiningqualifiers,may
alsofortheirchemicaland
es.
accountingphysicalparticulariti

humusform/ forest/ classification/ nomenclaturebiology
/ soil

Résumé —Laclassificationdes formesd’humusforestières :Unegrille
propositionfrançaise.
tréesetnomntleshumusrs
forestie
declassificationà2enuneenclaturesoproposéespourdeFrance,

maisaussiauxformesd’altitudproposi-
e,etméditerranéennes.Cette
pouvants’appliquertropicales
intervenant
tionenlesconnaissancesactuellesconcernantlesogiques
prendcomptephénomènesbiol
dansdécomdelalitière,latransformationdelamatièresesliensavecles
la
positionorganique,par-
etlastructurationl’horizonA.Parsonà
sminéralesdeprincipemême,grâcel’adjonctde
ticuleion
lesformesd’humusêtreàleurscaractères
dontlechoixestlibre,peuventpréciséesquant
qualificatifs
chimiquesouspa
physiquerticuliers.

dusol
formed’humus/forêt/ classification/ nomenclature / biologie

*
author
Corresponding

INTRODUCTION

Thehumus
isofdifferent
profilecomprised
scaleswhichbe
mayintegrated:regional
soil
climate,k,
parentrocvegetation,organ-
ismsb;Bemier and
(Toutain,1987a,Ponge,
for
disHumusmsareu
1993).nevenly
the
tributedoverwoforclimaticandhis-
rld,
toricalreasons.Asavarious
consequence,
classificationshavebeeninuseuntil
now,
eachfocusedon
regionalaspects.
Indescribed
Kubiëna
Europe,(1953)
numerousawide
humusforms,covering
ofrocksand
rangeclimates,parentvege-
tationHiscriteriawerederived
types.mainly
fromhisownobservations.
morphological
DuchaufourandBabellater
(1956)(1971)
differentchemicaland
investigated
microstructuralascientific
aspects,giving
basisformorerefinedclassifications.Dele-
courthenanidentification
(1980)proposed
formosthumusformswest-
in
keypresent
ern
Europe.
InNorththeneedforanother
America,
ofclassificationwas
typeemphasizedby
Wildeadetailed
(1954,1971).Morerecently,
ofhumusformswasachieved
taxonomyby
KlinkaandGreen1993).
et al (1981)et al (
Inoftheothers
otheregionsworld,
r
endeavouredtodescribeand
tropical
Mediterraneansoilsonthesamebasis

(Marin1985;Ferry,1992;Leroy
et al,et al,
1993).
inthescientificof
Progressknowledge
theinchemicaland
soil,both
its
biological
nowallowsustoafunc-
aspects,propose
tionalclassificationofforesthumusforms.A
worldwidetoolneededtoenablesoilsci-
is
entistsandforesterstodescribekind
every
ofhumusForthat
existingprofile.purpose,
weaclassificationwhichisnottax-
propose
onomical.Rather,aimsanswerthe
to
it
ofwtosee
questionho when
processes
thesoilwiththenakedand
observingeye
usethistohumusforms
knowledgeidentify
withmore
certainty.
Theclassificationisbasedon
proposed
thesameastheFrenchPedo-
principles
logicalReferenceBasefors
oil
types(AFES,
Thisisareferenceandnota
1992).system
hierarchicalandexhaustiveclassification.
Humusformsaredescribedbylinkingthem
tosomewell-definedreferenceformsand
asas
freelyadjoiningmanyqualifiers
needed.Aneffortwasmadetomakethis
classificationscientificand
pragmatic,pre-
flexible.
cise,but,nevertheless,

soilsand
Waterlogged(gleypseudo-gley)
andtheirhumusforms
will
(AFES,1992)
notbediscussedhere,asabetterknowl-
of
edgebiologicalmechanismspoor
in
ly
aeratedhorizonsisneeded.

STUDIESONFUNCTIONALASPECTS

Transformationof leaf litter

Recentstudiesthe
emphasizedimportance
ofeof
thechemicalnaturplantmacro-
moleculesinthefateofhumusIn
layers.
earlierobservations
particular,followingby
Handley(1954),Reisingeret al (1978),
and
Toutain(1981 )Françoiset al (1986)
describedthefollowedtannin-
pathwayby
foodwebs
soil
proteincomplexesthrough
indifferenthumusSeveralcritical
types.
wereatfirstthesenes-
phasesrecognized,
cenceoftreewithof
foliage,appearance
stabledarkwherewas
pigmentsnitrogen
70%
oftotalin
sequestered(egnitrogen
beechunavailablefor
foliage),rending
it
nutrition
plant(Toutain,1981).Investiga-
tionsondifferentscalesleaf
(soilslides,
trastructure,etc)showedthat
ul oafew
nly
biota,suchasearthwormsandwhite-rot
abletosuch
fungiweredisintegrate
(fig1),
recalcitrantmolecular
assemblages
these
(Toutain,1981).Whenorganisms
wereandthe
presentactive,disappearance
ofleafwasWhen
litter
rapid(mullhumus).
werelitterdis-
theyabsent,however,slowly
theofsmallani-
appearedthroughactivity
suchasand
mals,microarthropodsenchy-
traeidworms
(moderhumus).
thereisathresholdfornutrient
Thus,
thatnotbea
y
cyclesmay ormalockup,
ontheorbsenceof
a
dependingpresence
efficientTheir
organisms.presencedepends
onenvironmentalandhis-
(Toutain,1987a)
toricalconditionsthatleadtoaof
variety
functionalHereistheborderline
types(fig2).
betweenmullandmoder.moderhumus
In
iseffectivebutsoil
forms,biologicalactivity
foodchainsareTherinc
discontinuous.
p
ivisibletothenakedisthe
palactivity,eye,
transformationofalfae-
litterintoanim
plant
ceswherealotofmatterremains
organic
atleast
977;
untouched,temporarily(Webb,1
Toutain
et al,
onge,
1982;P1991 a,b).

Theborderlinebetweenmoderandmor
humusformsmoredetermine,
isdifficultto
andmoreon
knowledgebiologicalpro-
cessesisneededbeforecleartrendscan
beOurownobservationsindi-
perceived.
catedtha
twormswerepartic-
enchytraeid
abundantmorhumusforms
in
ularly(Bernier
et al,1993;thus,
Ponge,unpublisheddata);
theirdominancecouldbemorethanan
exclusionoftheotherthe
groups;however,
truemechanismsareunknown.

ofmatter
Assemblageorganic
withmineral
particles

Thechemicalnatureofmatterand
organic
intheA
itswithmineralmatter
assemblage
horizonontheaforementioned
dependpro-
cesses.We3main

aydistinguishpathm
et al,,
ways(Berthelin1994;Duchaufour
1995):
-
BiomacrostructuredAhorizon:
Clay-min-
eralbe
complexesmaycementingmacroag-
duetotheofsoil-
gregates,mixingactivity
earthwormsand
dwelling(BernierPonge,
1994);
-
"Insolubilisation"Ahorizon:Soluble
metabolicofwhite-rot
productsfungimay
on
precipitateclay-ironparticles;
-
horizon:Inherited
"Juxtaposition" Aorganic
mattermadeofcellwallsrec-
plant-fungal
transmission
in
ognizable(Toutain,1981)
oreven1991
lightmicroscopy(Ponge,a,b)
beinfaecalof
maypresentpelletsmany
smallanimals
(litter-dwellingearthworms,
sidesidewith
arthropods,enchytraeids),by
mineral
grains.

BASESFORANEWCLASSIFICATION
INTEGRATINGMORPHOLOGICAL
ANDFUNCTIONALFEATURES

Theexistenceofaclose
relationship
between and
morphologybiochemistry

hasbeenoften
(Duchaufour,1995)ques-
tioned.Thesamehumuswitha
typehigh
humusfromamor-
biologicalactivity(mull
ofwellcorre-
may
phologicalpointview)
toawideofvaluesand
spondrangepH
indicator
plantspecies(Duchaufour,1995).
different
Conversely,morphologicaltypes
havethesameorC/N.
maypH
Giventhesewiththecur-
discrepancies
rentviewthatofthehumus
morphology
isthereflectionofitschemical
profileprop-
erties,weusethelattequal-
decidedtoras
ifiersforbasedonmor-
groupsprimarily
thusthe
phology,traducingimportance
totheofthe
givenbuildingprofilebyliving
organisms.
TableI themainfeatu
resofthe
presents
differenthorizonsusedinourclassification.
Thesehorizonshavebeenused
previously
variousauthoindertohumus
rsor
byclassify
formsFor
(fig3).delineatingmorphological
wetookintoaccountnotmor-
types,only

butalsofunctionala
phologicalthresholds,
borderlinebasedonthethatmor-
principle
issenselesswithout
phologyanybiologi-
if
cal
support.
Theresultsofourobservationsare
pre-
sented4,whichconcerns
in
figuremainly
OandAhorizonswerecon-
lowlandsites.
sidereddistinct
asentries,
allowingapparent
conflictingfsame
eaturestooccurinthe
humusnamescanbe
profile.Thus,given
observedFrench
tomosthumusformswein
sitesAt
underlanticinfluence(fig5).Our
classificationalsobe usedasfor
a
maykey
he
theidentificationoftprincipalhumus
forms(fig6).

Theadditionof
qualifiers(tableII)may
todefinehumusformthatto
needs
helpany
becharacterizedfeatures.For
byparticular
tableindicatesthefea-
III
instance,principal
turesofahumusformthatcouldbecoined
"Aciddesaturatedoakmeso-
clay-loamy
mull"toournomenclature.
according

DISCUSSION

Thethatusto
argumentshelpedseparate
moderandmullthenatureoftheAhori-
by
zonhavebeennotedDuchau-
alreadyby
fourbutwerederivedfrom
(1965)mainly

ourownstudiesontheoriginofsoil
organic
matterBernier
(Brun,1978;Toutain,1981;
andto
Ponge,1994).Contraryprevious
classificationsbasedbothonOand
horiA
zons),wedecidedto
(Duchaufour,1965
theoftheAhorizon.
promotemorphology

Ourmethodallowedustosome
classify
humusformswithoutclearrela-
puzzlingany
betweenwhatcouldbeobserved
tionship
theandthestruc-
inlitter
layers(Ohorizon)
tureoftheAhorizon.Thisisthecasefor
thehumusformwhichhas
amphimull(fig5)
beenoverlookedtimeand
again,being
describedeitherasamoder
morphologically
orasamull.Theexistenceof2
chemically
horizwiththeonehori-
superposed(O
ons,
ofthemodertheother
zon)typeoverlying(A
ofthemullhasbeen
horizon)type,reported

in
mountainsites,as
previously,particularly
wellonalkalineasonacid
(Bottner,1971)
substrateet al,
(Bernier1993).
Inthe
agriculturalassemblage
soils,
betweenandmineralmaterials
in
organic
theAhorizonweresimilarlyby

usedBarand
ratttodis-
(1964)Jacquin(1985)help
betweendifferent
tinguishtypes.
Ourfunctionaltohumusmor-
approach
makestobetterunder-
it
phologypossible
into
standwhichclimate,
dynamicprocesses
andhumusprofilareinvolved.
vegetationes
Forinartificialornatural
changes,
instance,
thehumusprofilbe
theevolutionof
emay
followedanddescribedwithabettercer-
up
thatdifferenthorizons
tainty,acceptingmay
evolveatdifferentrates.Thiswasobserved

afterandfertilization
experimentallyliming
et al,mountainsites,
In
(Toutain 1988).
Bernier anddescribed
Ponge(1993,1994)
humusformthedevel-
in
changesduring
offorestsan
opmentbilberry-sprucealong
altitudinaletal
gradient.Similarly,Leroy
observedmicroscalein
(1993)changes
humusspe
formunderdifferenttree
cies
inrainforests.
growingt
ropical
withstudiesorobserva-
Comparisons
tionsmadeinotherbioclimaticzones
(moun-
tain,evidence
tropical,Mediterranean)gave
thatournomenclaturecanbe
successfully
usedinothercountries
(Toutain,1984;
Bernier andfor
Ponge,1993).Nevertheless,
northernandborealclimates,thereaneed
is
formoreonthemor
investigationsgroup
if