Peat soils: Genesis and classification

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    ISSN 1064-2293, Eurasian Soil Science, 2006, Vol. 39, No. 7, pp. 699704. Pleiades Publishing, Inc., 2006.Original Russian Text L.I. Inisheva, 2006, published in Pochvovedenie, 2006, No. 7, pp. 781786.

    INTRODUCTIONPeat soils consist of 5095% organic substances;

    they are excessively moistened. These features deter-mine their polyfunctional nature. Botanists and geobot-anists study the specific features of bog vegetation onpeat soils and the climatic characteristics of the periodof the peat accumulation based on the stratigraphy ofpeat deposits, and they define peat soils as bogs. Geol-ogists explore peat reserves for industrial purposes andconsider peat bogs as peat fields (economic deposits).Hydrologists study the hydrological regime of bogs anddetermine them as water bodies. Foresters study bogsfrom the position of improving the quality class of for-est stands and call them forest bogs. Soil scientistsstudy peat soils as agricultural highly fertile soils. Eachspecialist has his or her own purposes and methods ofstudying peat soils, but they study the same object.Over the long-term period of the studies, a great bodyof data on peat bogs has been accumulated, their impor-tant biospheric role was proved, and trends in the fieldof their conservation and rational use were determined[18]. However, the essence of the peat formation mech-anism and the place of peat soils in the classificationsystem (and in soil science on the whole) remain uncer-tain. Therefore, the aim of this work is to draw theattention of soil scientists to this problem.

    According to Dokuchaev [6],

    It is impossible toagree with the statement that a natural soil is identicalto a plow layer. However, it is still more difficult to givethe name soil to any rock just because it occurs onthe land and humans got the idea to grow some crop onit. Until the rock does not change to a certain depth dueto the joint action of water, air, and organisms, it is notsoil, it remains only rock

    Even in the soil classificationof 1886, Dokuchaev distinguished the class of typicalpeat bogs with their entire profile down to the mineral

    rock. According to Efimov [7], many scientists, in par-ticular, Glinka, Williams, Vilenskii, and Kravkov, sup-ported Dokuchaevs opinion. In 1937, Gerasimov [4]was the first to divide the whole peat profile into peatsoil and peat-forming rock; the peat-forming rockbeing a material substrate for the peat soil. These viewshave been widely reflected in the work by Skrynnikova[24]. According to this authors definition, a peat soil isthe upper peat layer to the depth of distributing the mainmass of plant roots, which is aerated periodically, and itis a place where plant falloff is decomposed and high-molecular organic compounds are formed. The deeperpeat layers cannot be called soil, since soil-formingprocesses are not recorded there and peat itself is pre-served. This layer was determined to be an organogenicrock. At that time, scientists accepted this notion of peatsoils as an evident one. Thus, 1-m-thick high-moor peatsoil was differentiated into the upper straw-colored orbrown-yellow T1 horizon of sphagnum residues, the T2horizon of brown peat with well visible plant residues,which graded into the T3 layer of dark brown peat.

    The notion of active and nonactive layers (accordingto other authors, functioning and nonfunctioning, oracrotelm and cathotelm) appears partly owing to thehydrologists studying bogs [3, 1012, 19, 20, 22]. Theyassociated this notion with the state of the water regime.It is also known that the boundary between the activeand inert layers is considered as conventional to somedegree. For instance, our investigations showed thatSerdobolskiis gradation accepted for the redox condi-tions [23] was unsuitable for peat profiles, and Eh = 0 mVwas used for a more correct determination of theboundary between its layers [13]. Such a choice madeit possible to hypothesize and prove that, in the naturalsoils, the active layer was much thicker than that pro-posed by Ivanov on the basis of the mean annual mini-

    GENESIS AND GEOGRAPHYOF SOILS

    Peat Soils: Genesis and Classification

    L. I. Inisheva

    Siberian Research Institute of Peat, Siberian Division, Russian Academy of Agricultural Sciences,ul. Gagarina 3, Tomsk, 634050 Russia

    Received December 9, 2004

    Abstract

    This paper considers three topical problemsthe definition of peat soils as naturalhistorical for-mations and the estimation of their profile thickness, the analysis of the genesis of organic soils, and the prin-ciples of the classification of peat soils. Based on the experimental data of long-term studies, it was concludedthat peat soils may include the whole peat layer and the upper horizons of the surface mineral soil. The organicand mineral parts of the natural structures were found to be a genetically homogeneous soil profile, which hasthe same history of development. The upper layer of the peat soils should be considered as the horizon reflectingthe contemporary stage of the soil formation. A hierarchy of peat soils is analyzed for developing their classi-fication.

    DOI:

    10.1134/S1064229306070027

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    mal level of the bog water when analyzing the anaero-bicaerobic conditions within the whole peat profile.According to studies of other researchers [8, 16], theorganic matter content and the hydromorphic featuresof peat soils determine their stability to their obligatoryanaerobiosis.

    Specialists in amelioration were also interested inthe 1-m-thick layer, since the rate of drainage was, as arule, limited by this depth. It is quite possible that thementioned circumstances became the reason forexpanding Skrynnikovas concept of peat bogs. Whatactually happens is that such a definition imperfectlyreflects the substantivefunctional and geneticevolu-tionary features of peat soils.

    The authors results and the materials of otherresearchers make it possible to consider that a source ofmineral nutrition for peat-forming bog plants is thewaterlogged mineral substrate (mineral soil). Theinvestigations of oligotrophic soils in the southern taigasubzone of Western Siberia [14] have revealed theirmesotrophic character due to the elevated contents ofcalcium, magnesium, and some other biogenic ele-ments in the oligotrophic part of the soil profile, wherethe mineral nutrients are supplied only with atmo-spheric precipitation. However, these elements werecontained in great amounts in the underlying ancientsoil and migrated following the accumulating peatlayer. The main amount of ash elements accumulates inpeat at the initial stage of its formation, when theirsource is a mineral substrate; in the case considered, itwas calcareous clays. The roots of peat-forming plantsconsumed calcium from this layer. Thus, the redistribu-tion of elements within the profile of these peat soils ischaracterized by the decrease in their concentrationstoward the top layers. As a result, in the territory stud-ied, oligotrophic peat soils with features characteristicof the mesotrophic type of peat formation have devel-oped. Previously, this process was called the biogenicmigration of elements, and it was described by Bakh-nov [1]. The same arguments were also true for the ironcontent, since the territory investigated was located inan area of iron ore deposits. Thus, the ancient soilexposed to waterlogging serves as the soil-formingrock in respect to the developing soil, and a closegenetic connection remains between them in the future.

    The formation of the peat profile in terms of themigration of elements within the soil profile (the soil-forming process) is considered below. The migration ofelements toward the parent rocks in peat soils is consid-ered to be weak because of their high water-holdingcapacity and weak filtration of water. This fact is truefor the soils developing in the area even now. However,the nonuniform topography of the bog causes the redis-tribution of gravitational water within bog bodiesresulting in the formation of autonomous, transitional,and transaccumulative geochemical microlandscapes[15]. The water overflow takes place down the peat pro-

    file and upward within the bog bodies and is determinedby specific features of the soil-forming process.

    In this connection, let us consider peat soil as a sub-aqueous genetically in situ system. According to Tar-gulian [26], the specific properties of the soil are dis-played when it is considered as a surface-planetary,exogenic, polydisperse, multiphase system with a solidsubstrate; it is a bioabiotic, bioproductive, and sub-aerial system that has formed and is functioning in situ.In this case, the soil formation is the accumulation ofresidual products of functioning in the solid, liquid, andgaseous phases. The in situ infiltration processes pene-trate into the deep layers of the rock and transform themin situ without transportation of the main rock mass andthe newly formed products. In mineral soils, the pre-vailing direction of the latter processes is a downwardone due to gravitation forces, whereas peat soilsdevelop upward due to the accumulation of peat. There-fore, the upper layer of peat soils corresponds to thepresent-day environment and reflects the current devel-opmental phase. The lower layers represent the previ-ous stages of the soil development. All the argumentsmentioned above attest that peat soil is an in situ sub-aqueous system with a minus sign (developed upward).The ancient mineral soil is the layer of the biolithos-phere that was formed under the conditions of long-term and permanent excessive moistening under hydro-philous vegetation. As a rule, the upper part of this layeris gleyed and acts as a soil-forming material for the peatsoil growing upward. It is also a zone of functioningflows of matter and energy resulting in the developmentof a peat soil, the properties of which are firstly deter-mined by the botanical composition of the peat. Thepeat soil is composed of layers whose thicknessdepends on the homogeneity of the botanical composi-tion of peat. Thus, the notion of a peat soil includes thewhole peat profile and the upper mineral horizons of theancient mineral soil. The organic and mineral parts ofpeat soils are regarded as a substantive and functionalsystem representing a genetically integrated soil profilereflecting its own history of development. The upper 1-m-thick horizon of peat soils would be more properlyconsidered as a part of the soil profile reflecting the cur-rent stage of soil formation with more intense biochem-ical processes. Moreover, the deeper horizons are alsobiochemically active. The studies carried out on the oli-gotrophic bogs showed that fungal spores, actinomyce-tal mycelium, and bacteria were present within thewhole 3-m-deep profile of the peat soils. The fungalmycelium was found in the soil profile up to a depth of70100 cm [5]. If the bacterial population decreasedgradually with depth, the density of the fungal sporesand actinomycetal mycelium was frequently higher inthe deeper layer of the peat profile. This statement isbased on particular examples (Table 1).

    The peat profile (point 3) is of the marshy type, andit is composed (from the bottom to top) of a low-mirehorse-tail and sedge peat layer of 1 m thick and a tran-sitional woodysphagnum (0.5 m thick) layer overlain

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    PEAT SOILS: GENESIS AND CLASSIFICATION 701

    by thick (1.5 m) raised-bog peat (

    Sphagnum magellani-cum

    and

    S. fuscum

    ) layers. The profile at point 5 (2.5 m)is composed of sedge and woodysedge peat (1.5 m)underlain by transitional

    ScheuchzeriaSphagnum

    (0.4 m) and high-moor sphagnum peat layers. Themicroscopic fungi are distributed within the profilerather evenly. In the 0.5-m layer, the share of fungiaveraged 35% of the total number of micromycetes; inthe 100- to 300-cm horizon, it averaged 27%.

    The results of the long-term studies on the enzy-matic activity showed that it was determined by thepresence of microbiological and plant enzymes andreflected more fully the biochemical activity of the peatsoils. In the high-moor peat soils, an elevated invertaseactivity was recorded only in the upper 1-m-thick layer;the other enzymes were distributed more evenly withinthe profile (Table 2). In the low-moor peat soils, theenzymatic activity increased (according to the activityof the catalase, the polyphenol oxidase, and the nitrate

    reductase) in the 0- to 25-cm layer; with depth (even inthe 75- to 100-cm layer), it decreased. Thus, the bio-chemical processes took place in the deep layers of thepeat soils, but the biochemical characteristics of theselayers differed from those of the upper ones.

    The given viewpoint is supported by the changes inthe chemical composition of the peat along the profilesof peat soils. In the same kinds of peat, the concentra-tions of water-soluble and easily hydrolyzable com-pounds were shown to decrease with depth, and that ofhumic acids increased [21]. These phenomena couldnot be accidental, since they took place due to the trans-formation of easily hydrolyzable compounds. This factemphasizes once more that, under the anaerobic condi-tions, the chemical composition of the peat-formingsubstrate continues to change. In the deep peat layers,instead of the microbiological processes proceedingunder aerobic conditions (mainly hydrolysis), otherbiochemical processes promote the organic compounds

    Table 1.

    The limits of the variation for the number of microscopic fungi (A), fungal biomass (B), and the carbon content (C)in the oligotrophic peat

    ObjectA, mg/kg B, kg/m

    2

    C, %

    The depth of the peat deposit, cm

    50 100 300 50 100 300 50 100 300

    3 28 1021 1314 0.050.1 0.20.4 0.30.5 0.21 0.40.8 0.10.25 226 530 1136 0.050.7 0.20.9 0.51.2 0.23 0.32 0.31

    Table 2.

    The enzymatic activity of virgin peat soils

    Depth,cm

    Botanical composition Invertase

    1

    Catalase

    2

    Polyphenol-oxidase

    3

    Nitrate-reductase

    4

    Nitrite-reductase

    5

    High-moor peat soils, the middle taiga2575 Sphagnumwaterlogged 65.92 0.63 0.00 4.94 3.5275125

    ScheuchzeriaSphagnum

    44.32 0.42 0.62 4.91 5.26125150

    Scheuchzeria

    24.41 0.71 0.93 3.60 4.37150175

    "

    22.37 0.22 0.49 3.73 7.01175200

    "

    17.24 0.56 1.81 3.63 5.89200225 Transitional woody

    Scheuchzeria

    27.50 0.44 3.41 4.80 4.70Low-moor peat soils, the southern taiga subzone

    025 Woody 74.45 3.30 1.33 19.33 4.6975100

    "

    37.56 0.90 0.67 8.09 3.85150175 Woodysedge 49.15 0.78 0.61 5.41 2.03225250 Sedge 33.75 0.87 0.99 7.88 13.03

    1

    mg of glucose per 1 g for 4 h.

    2

    ml of O

    2

    per 1 g for 2 min.

    3

    mg of 1.4 n-quinone per 1 g for 30 min.

    4

    mg of reduced per 1 g for 24 h.

    5

    mg of reduced per 1 g for 24 h.

    NO3

    NO2

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    transformation toward their humification. The aerobicand anaerobic microorganisms are different in their cat-alytic and thermodynamic action, as well as the charac-ter of the decomposing of the organic matter. Therefore,the primary decomposition of the dead peat-formingplants (mainly...