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Nutrient and biotic properties of mormoder and leptomoder humus forms in the coastal western hemlock.. Klinka, Karel; Fons, Jaume; Chourmouzis, Christine 1997-04-17

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Scientia Silvica Extension Series, Number  8, 1997Nutrient and Biotic Properties of Mormoder and LeptomoderHumus Forms in the Coastal Western Hemlock ZoneIntroductionIn British Columbia, humus form identification is widely useto  infer  the  level  of plant-available soil  nutrients. Thisidentification is based on field-observable (morphological)features.  We recognize  three  major humu forms:  Mors,Moders, and Mulls -  which are differentiated according tothe type of F horizon, and the presence/absence of organicmatter-enriched Ah horizons.Mors represent  humus forms  where decomposition isdominated by fungi,  with slow  decomposition rates  andaccumulations of organic matter on the  soil surface. Morsare characterized by the presence of a Fm (m - mycogenous)horizon. In contrast, Mulls represent humus forms with highrates  of decomposition and faunal  activity resulting  inorganic matter being intimately incorporated into the uppermineral soil  layer  instead  of accumulating on its  surface.Intermediate on the humus form gradient from Mors to Mullare the  Moders.   Moders are similar  to  Mors in  that  theyhave accumulations of organic matter on the surface of themineral soil but decomposition is not fungus dominated, sothey lack the diagnostic Fm horizon.  The central concept ofthe  Moder is  represented  by the  Leptomoder,  which ischaracterized by a Fz (z - zoogenous) horizon with an activepopulation of soil meso- and microfauna, fungal mycelia arenot present or present in small amounts. When both fungalmycelia and faunal  droppings can be found,  but neitherclearly predominate over the other, an Fa (a - amphimorphic)horizon  results.  Fa  horizons  are  characteristic  ofMormoders, an integrade between Mors and Moders.Considering the prevalence of  Mormoders and Leptomodersin  B.C. and the  difficulties in  identifying  Fa horizons, theaim  of  this  study  was  to  determine  whether  themorphological features  used to  differentiate these  twoModer humus forms  reflects  differences in  their physical,chemical and biotic properties.Study StandsThe study was conducted in the submontane variant of theVery Wet Maritime Coastal Western Hemlock (CWHvm1)subzone.  The study  stands  represented  the  stem  exclusionFigure 1. Humus form profiles of a typical Leptomoder andMormoder.;;;;;LFzHz/h;;;;;LFaHhLeptomoder Mormoderand understory reinitiation  developmental stages  on siteswith fresh  soil  moisture regime,  medium soil  nutrientregime, and good soil aeration). Two stands were located ineach of three adjacent watersheds  (Capilano, Seymour, andCoquitlam), for a total of 6 stands.Differences in field-observable propertiesMormoders and Leptomoders were differentiated by thepresence of Fa and Fz horizons, respectively (Figure 1). InFz horizons, recognizable plant residues are aggregated in amatted structure  with a  tenacious consistency and fungalmycelia and fine  roots  are abundant. In  Fa horizons partlydecomposed plant residues  are aggregated into  a weak tomoderate, non-compact matted structure  and both fungalmycelia and faunal  droppings are present but neitherdominate.No significant  differences were detected in  the  thicknessof LF and H horizons between the  two  humus forms:  themean thickness  of the  LF and the  H horizon, respectively,was 3.4 and 5.1 for  Mormoders, and 3.6 and 5.0 cm forLeptomoders.Differences in nutrient and bioticpropertiesNutrientsMormoders are nutrient-poorer than  Leptomoders. Twoindices  of easily available plant nutrients -  C:N ratio  andmineralizable-N - which were strongly correlated, accountedfor most of the differentiation. In Mormoders, the C:N ratiowas significantly higher and concentrations of mineralizable-N significantly  lower.    Mormoders also had significantlyhigher  total  C  concentrations  in  H  horizons  thanLeptomoders.Fungi and BacteriaIn  the  LF horizon, Leptomoders had greater bacterialbiomass than Mormoders. Fungal biomass was not different,even though  fungal presence (i.e.  Fa horizon) is  used as adiferentiating feature between the two Moders. Mormodershad significantly higher amounts of fungi in the H horizon,which is  not a diagnostic feature  for  identification.  Thediscrepancy between estimates of abundance of fungalmycelia  from  visual  examination  and  analyticaldetermination may be due to  the  subjective  nature andinaccuracy of visual inspection.BiotaThirty-one major groups of soil fauna were extracted duringthree  sampling  periods, with no significant  differences infaunal density between sampling times.  Roundworms werethe most abundant group in both humus forms, followed bymites and springtails - these 3 groups accounted for approxi-mately 97% of the total number of soil fauna. Some differ-ences between the Moders were observed in the  less  abun-dant groups. For example, spiders  were only observed inMormoders, the  density of pseudoscorpions was higher inLeptomoders, and the density of annelids (Enchytraeidae) wasslightly higher in Mormoders.Relationship between bioticand nutrient propertiesIn  the  LF horizon there  was a correlation between thefungal:bacterial  biomass ratio  and the  nutrient properties;the fungal:bacterial biomass ratio was higher in Mormoders.This ratio was strongly positively correlated with C:N ratioand inversely  related  to  mineralizable-N:total N ratio,  anexpression of plant-available  soil  N. These relationshipsconfirm that  fungus-dominated  humus forms  have slowerlitter  decomposition and greater nutrient retention.Can field-identified Mormoders andLeptomoders be seggregated by theirnutrient and biotic properties ?The chemical and biotic properties of   the  24 plots (12 perhumus form: 3 stands x 4 samples per stand) were used in acluster analysis. Three clusters were idendified.    Cluster  1(11 plots) included most of the Leptomoders (10 plots) andonly one Mormoder plot.   Cluster  3 contained onlyMormoders (9  plots). Cluster 2 (4  plots) was the  smallestcluster,  containing the  same  number of Mormoders andLeptomoders (2  plots each). Based on the  memberships ineach cluster, we considered that 83% of Leptomoders in clus-ter  1 and 75% of Mormoders in  cluster 3 had well definedmorphological, chemical, and biotic properties,  and hencecould be well segregated by any of these  properties. How-ever,  17% of samples identified  as Leptomoders (2  plots)and 25% of samples identified as Mormoders (3 plots) hadsimilar nutrient and biotic properties.Summary and ConclusionsMormoders and  Leptomoders  identified  by  visualexamination were different in  their  nutrient and bioticproperties; however,  the  differentiation  based on theseproperties was not completely successful as 25% of the studysamples overlapped in  their  nutrient and biotic properties.This implies that there is an imperfect relationship betweenfield-observable  and biotic properties; however,  bioticproperties appear to be closely linked to nutrient properties.These results  suggest  some  improvement of  visualidentification  criteria would be beneficial, such  asdeveloping measures for  more precise estimates of fungaland bacterial biomass in the both F and H horizons.ReferenceFons, J. and K. Klinka. 1998. Chemical and biotic propertiesand temporal  variation of Moder humus forms  in  the  rainforest near Vancouver, British Columbia. Geoderma 86: 83-98.Scientia Silvica  is published by the Forest Sciences Department,The University of British Columbia, ISSN 1209-952XEditor: Karel Klinka (klinka@interchange.ubc.ca)Research: Jaume Fons (jfons@filnet.es) and K. KlinkaProduction and design: C. Chourmouzis (chourmou@interchange.ubc.ca)Financial support: Forest Renewal British Columbia and the SpanishMinistry of EducationFor more information contact: K. KlinkaCopies available from:  www.forestry.ubc.ca/klinka, orK.Klinka, Forest Sciences Department, UBC,3036-2424 Main Mall, Vancouver, BC, V6T 1Z4


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