[Advances in Ecological Research] Litter Decomposition: A Guide to Carbon and Nutrient Turnover Volume 38 || Changes in Substrate Composition and Rate‐Regulating Factors during Decomposition

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  • Changes in Substrate Compositionand RateRegulating Factors

    during Decomposition

    I. Introductory Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102ADVAN

    # 2006CES IN ECOLOGICAL RESEARCH VOL. 38 0065-250

    Elsevier Ltd. All rights reserved DOI: 10.1016/S0065-25044/06

    (05)3$35.0

    8004-II. OrganicChemical Changes During Litter Decomposition . . . . . . . . . . 104

    A. Decomposition of Single Chemical Components and Groups

    of Compounds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

    B. Relationships between Holocellulose and Lignin

    during Decomposition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

    III. Concentrations of Nutrients and Heavy Metals During Litter Decay. . 114A. Nitrogen (N) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

    B. Phosphorus (P) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

    C. Sulfur (S). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

    D. Potassium (K) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

    E. Calcium (Ca) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

    F. Magnesium (Mg). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

    G. Other Metals and Heavy Metals in Natural Concentrations. . . . . . 118IV. A Threephase Model Applied to Litter of DiVerent InitialChemical Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

    A. Overview of the Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

    B. Initial Decomposition Rates for Newly Shed LitterThe Early

    Decomposition Stage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

    C. Decomposition in the Late StageA Phase Regulated by

    Lignin Decomposition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

    D. Link between the Retardation of Litter Decomposition, Lignin

    Degradation Rate, and N Concentration . . . . . . . . . . . . . . . . . . . 136

    E. Comments on Spruce Needle Litter Decomposition versus the

    ThreePhase Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

    F. The Litter Close to the Limit Value and at a HumusNear Stage . 142

    G. Do Limit Values Indicate a Stop in the Litter

    Decomposition Process? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

    V. Lignin Dynamics in Decomposing Litter . . . . . . . . . . . . . . . . . . . . . . . 150A. Repeatability of Patterns in Lignin Concentration Changes . . . . . 150

    B. Variation in the Increase in Lignin Concentration Relative to

    DiVerent Initial Lignin Concentrations in the Litter . . . . . . . . . . . 153

    C. Variation in Lignin Concentration Increase Rate as Compared to

    DiVerent Concentrations of N in Litter. . . . . . . . . . . . . . . . . . . . . 153

    VI. Does the Litter Chemical Composition Influence Leaching of

    Compounds from Decomposing Litter? . . . . . . . . . . . . . . . . . . . . . . . . 1540

    4

  • 102 BJORN BERG AND RYSZARD LASKOWSKII. INTRODUCTORY COMMENTS

    In the course of decomposition, the litter is subject to considerable chemical

    changes when being converted from fresh litter to humus. Only some of these

    chemical changes are known; most remain to be discovered. Those chemical

    changes that have been described are known for only a few species of foliar

    litter and a few ecosystems and, even today, we can not say that the chemical

    changes described in this chapter have general validity. Regarding the dy-

    namics of nutrients and metals, these have been studied mainly for nutrient

    release and cycling on the ecosystem level (Anderson and Macfadyen, 1976;

    ONeill et al., 1975) and apparently less to reveal the finer details of

    the chemical composition of litter, such as when it approaches humus, or

    details in quantitative uptake or release. Still, several studies also provide

    concentration changes during decomposition (Dwyer and Merriam, 1983;

    Dziadowiec, 1987) of the major plant nutrients (Berg and Staaf, 1981; Blair,

    1988a,b; Laskowski et al., 1995; Rashid and Schaefer, 1988).

    Although a number of scientists focus their studies either on major plant

    nutrients or on heavy metals, the distinction between these two groups is

    not clear. The term heavy metals is often used for pollutants, although a

    number of elements from this group also belong to nutrients (such as Zn and

    Cu). In this chapter, we treat selected heavy metals as nutrients in unpolluted

    systems and discuss their dynamics in that context.

    The microbial decomposers of litter organic components are selective

    toward diVerent compounds, which results in clear patterns in chemicalchanges in the course of litter decomposition. Each such pattern may be

    related to the initial chemical composition of a given litter type. In this

    chapter, we describe detailed chemical changes for Scots pine needle litter

    as a case study and, in applicable parts, we also present data from other

    boreal and temperate species. The patterns discussed here have been found

    mainly in boreal systems but probably have higher generality and even such

    diVerent systems as decomposing chaparral shrubs show similar decomposi-tion patterns as litter from boreal tree species (Schlesinger and Hasey, 1981).

    The chemical changes taking place during initial decomposition stages

    expose compounds of diVerent kinds and diVerent biological degradabilityto further decomposition. The decomposition dynamics in most sofarinvestigated needle and leaf litter species follow the model presented in

    Fig. 1. In fact, it seems that the model covers not only diVerent types offoliar litter, but probably also, to some extent, root litter, as well as litter

    from grass and herbs. Thus, the model may have relatively broad generality.

    On the other hand, some litter types show specific behaviors, and, for

    example, spruce needle litter deviates from the model. A possible explana-

    tion to that diVering decomposition pattern is that spruce trees produce

  • Figure 1 (A) Model for chemical changes and rateregulating factors duringdecomposition, modified from Berg and Matzner (1997). The decomposition ofwatersoluble substances and unshielded cellulose/hemicellulose is stimulated by highlevels of nutrients such as N, P, and S (early stage, phase 1). When all unshieldedholocellulose is decomposed, only ligninencrusted holocellulose and lignin remain.In this late stage (phase 2), the degradation of lignin rules the litter decompositionrate. The degradation of lignin is hampered by N, and higher N levels suppress itsdecomposition whereas Mn has a stimulating eVect on the degradation of lignin.Finally, in the humusnear stage (phase 3), the lignin level is about constant, the litterdecomposition rate approaches zero, and the accumulated mass loss reaches its limitvalue. (B) Lignin concentration increases up to a level of 50 to 55%, N concentrationsincrease, and the litter decomposition rate approaches zero as the accumulated massloss approaches a limit value (Section IV.F).

    CHANGES IN SUBSTRATECOMPOSITIONANDRATEREGULATINGFACTORS 103

  • 104 BJORN BERG AND RYSZARD LASKOWSKImo re heterog eneo us folia r litter whi ch, in addition , is in a late deco mposi-

    tion stage (Sect ion IV.C alrea dy when shed, as a co nsequence of an ad-

    va nced decomposi tion of the needles while sti ll attached de ad to the twigs.

    The deco mposition pr ocess normal ly reaches a fina l stage at which it

    almos t stops or goes so slowly that this stage may be approxim ately de-

    scri bed mathemati cally by an a symptote. We have con sidered this to be a

    lim it value for decomposi tion, which for foliar litter of di Verent specie snor mally ranges from 50 to 100% mass loss (Section IV.F ). The level of

    this limit value has been negati vely related to initial litter N levels, whi ch

    mean s that the richer the lit ter is in N, the less it will decompo se under

    co mparabl e con ditions. This relat ionship , which has been general ized for

    foli ar litter types, is de veloped and discus sed in this ch apter as well as in

    Chapter 6.

    Most litter species leach carbon compounds to diVering extents. Suchleachi ng may star t in the early pha se (Section IV.B ) an d continue through-

    out the following decomposition stages. Recent findings have indicated that

    raised N concentrations in foliar litter may support the leaching process of

    carbon compounds. The reaction mechanisms are still unknown. When litter

    is transformed to humus, this property of the litter/humus remains and it has

    been observed that, under some circumstances, the release of C compounds

    can be emphasized and accelerated. There are actually extreme cases re-

    ported with a very high reaction rate, causing an actual disintegration

    of very Nrich humus with a very fast degradation and leaching ofNrich compounds taking place. It has been speculated that this could bedue to changes in the microflora. These findings will be further discussed in

    Chapter 6. The intention of this chapter is to demonstrate and systemize

    decomposition patterns as well as the eVects of several chemical componentsand the chemically changing litter substrate on decomposition rates.II. ORGANICCHEMICAL CHANGES DURINGLITTER DECOMPOSITIONA. Decomposition of Single Chemical Components andGroups of CompoundsMicroorganisms start degrading plant litter as soon as it has fallen to the

    ground and been invaded by decomposers, that is, by fungal mycelium and

    bacteria. The microorganisms that can utilize the soluble components

    start degrading them first and normally at a relatively high rate. The reason

    is that normally small soluble molecules are more easily available to micro-

    organisms since they may be transported directly into the cell and metabo-

    lized. There is thus no need for the additional enzymes that are used to

  • Figure 2 Degradation pattern of Scots pine needle litter. Remaining amounts oflitter (upper full line) solubles, cellulose, hemicellulose, and lignin are given (fromBerg et al., 1982). We see that the degradation of solubles and hemicellulose start inthe first year, whereas a net loss of the sulfuricacid lignin fraction does not start untillater, in this case, the end of the second year.

    CHANGES IN SUBSTRATECOMPOSITIONANDRATEREGULATINGFACTORS 105depolymerize the larger molecules. The degradation of hemicelluloses, cellu-

    lose, and lignin starts later. We describe the process for Scots pine needle

    litter in more detail and comment on other litter species. Figure 2 provides

    an overview to the main decomposition pattern, including some main groups

    of compounds.1. Water SolublesThe fraction of water solubles, being chemically complex, is far from a

    homogeneous substrate and the degradability of diVerent components variesa great deal. Generally, in newly formed foliar litter, this fraction contains

    high levels of compounds such as simple sugars, lower fatty acids, and

    protein remains, such as amino acids and peptides. Such simple molecules

    can easily be taken up by microorganisms and metabolized. The fraction of

    water solubles thus should, at least in part, decompose rather quickly and its

    concentration should decrease (Fig. 3). Leaching may play a role, too,

    decreasing the concentrations of water solubles in the litter. The extent of

    leaching may vary among litter species and may range from less than 1% in

    Scots pine needle litter to approximately 28 to 30% of the water solubles

    being leached from willow and maple leaf litter (Table 1). When the leaching

    is low, as in Scots pine litter (Table 1), we may assume that a large part of the

    soluble material is degraded within the litter structure.

  • Figure 3 Changes in concentrations of water solubles, ethanol solubles, cellulose,hemicelluloses, and lignin in decomposing Scots pine needle litter.

    Table 1 Leaching of water soluble substance from some leaf and needle litterspecieslaboratory measurements

    Litter type

    Potentially leachablewatersoluble(% of d.w.)

    Actually leachedsubstance(% of d.w.) Reference

    DeciduousAsh 26.4 22.3 (2)Ash 20.8 16.5 (3)Black alder 12.2 12 (1)Black alder 28.1 21.3 (2)Common beech 6.2 3.8 (1)Common oak 13.3 7.1 (1)Downy birch 26.3 16.3 (2)European maple 35 29.4 (2)Mountain ash 26.9 22.8 (2)Silver birch 13.7 10.7 (4)Trembling aspen 27.7 25.2 (2)Willow 31.4 27.9 (2)

    ConiferousNorway spruce 12.5 1.1 (4)Scots pine 9.2

  • CHANGES IN SUBSTRATECOMPOSITIONANDRATEREGULATINGFACTORS 107For our case study needle litter from Scots pine in a boreal system, the

    concentration of the watersoluble fraction was found to decrease fromapproximately 100 to 57 mg g1 in about a year, whereas for the subfractionof simple sugars and glycosides alone, the concentration decreased from 31

    mg g1 to not detectable amounts in the same period. For some deciduousspecies that have been investigated, the decrease may be even more drastic

    (Table 1) and for silver birch leaf litter, the total water solubles decreased in

    one year from 321 to 45 mg g1, with part of the solubles being leached.Finally, the level of water solubles reached 40 mg g1 after 4 years (Table 2).For Norway spruce needles, of which at least part is considered to start

    decomposing while still attached dead on the twigs, the concentration

    decreased from 114 to 38 mg g1 (Table 2).However, in the course of decomposition, new soluble compounds are

    formed during the decay of polymer compounds, such as holocellulose and

    lignin, and a low level of watersoluble compounds is almost always found indecomposing litter containing simple sugars from degrading polymer carbo-

    hydrates. In fact, even a compound as easily decomposable as glucose

    has been found in concentrations of up to 1% in Scots pine needle litter

    decomposing in the field for up to 5 years (Berg et al., 1982a).2. Ethanol Soluble FractionIn fresh litter, rather small molecules, not being water soluble, are often

    analyzed as ethanol solubles or acetone solubles. These solvents extract,

    among others, lower phenolics and higher fatty acids. This fraction some-

    times contains compounds that suppress microbial growth, as seen for single

    fungal species (Berg et al., 1980), and we can expect also that mixed micro-

    bial cultures degrade these compounds more slowly than they degrade

    water solubles. All single components of this fraction have not yet been

    analyzed, not even for newly shed litter of one species (Chapter 2) and their

    degradability is thus not known.

    The original components of this fraction are degraded but new com-

    pounds are added as the degradation of more complex compounds, such

    as lignin, proceeds and the concentration of ethanol solubles is often high

    even after some years of decomposition, as found, for example, for Scots

    pine and lodgepole pine (Table 2). For Scots pine, the concentration of

    ethanol solubles after 3 to 5 years decomposition could be of a similar

    magnitude as in the initial litter. An example (Table 2) gi...

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