[Advances in Ecological Research] Litter Decomposition: A Guide to Carbon and Nutrient Turnover Volume 38 || Introduction

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  • by itselfyearly burning of stubble on meadows, still a common practice in

    many parts of the world, reveals recognition of the necessity of mineraliza-decomposition, eventually leading to the release of mineral nutrients in-

    dispensable for plants to grow. Some agriculture practices show that

    farmers have known that fertilization with organic manure is not the goaltion

    ens

    min

    ADV

    # 20of organic matter. The burning of organic residues dramatically

    the time needed for release of nutrients and supplements so

    eral nutrients, which can be easily utilized by plants.

    ANCES IN ECOLOGICAL RESEARCH VOL. 38 0065-250

    06 Elsevier Ltd. All rights reserved DOI: 10.1016/S0065-2504sh

    il

    4/06

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    8001-fields arecome, their fields must be supplemented with nutrients. Agricu

    fertilized with manure, which undergoes the natural processural

    ofscien

    yearsce, f

    toecomposition. For centuries, well before the development of m

    armers knew that in order to sustain agricultural production

    ltdern

    formatter d oIntroduction

    I. General Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1A. Decomposition, Nutrient Turnover, and Global

    Climate Change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3B. Biomass Distribution between Soil and AboveGround

    Ecosystem Compartments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9C. The Importance of Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

    I. GENERAL REMARKS

    Very few people without some ecological background turn their attention

    to dead organic matter and its decay. The reason is simple: the processes

    on which this book focuses occur, to some extent in an invisible way,

    without such spectacular events as blooming flowers, singing birds, or color-

    ful butterflies. What more easily attracts our attention is the opposite side

    of the organic matter turnover: the production. The importance of organic

    matter production seems obvious to everybody, not only to specialiststhis

    is the source of our crops and fodder for animals which are, in turn, utilized

    as food for humans; this is the timber used for housing, furniture, and paper

    production. The list can easily be made much longer. Life is production, and

    production means the synthesis of organic compounds from inorganic

    chemical elements. Nevertheless, those of us closer to agriculture or forestry

    are perfectly familiar also with the opposite side of the storyorganic-

    0

    9

  • 2 BJORN BERG AND RYSZARD LASKOWSKIConsidering the cycle of life, there is no exaggeration in the statement that

    decomposition of dead organic matter is a crucial process for sustaining life

    on Earth. Without decay (and fires), with constant production of organic

    matter by plants and a yearly primary production reaching ca. 4 kg m2 inthe most productive ecosystems, the whole land surface of the Earth would be

    soon covered with a metersthick layer of undecomposed organic matter.Nutrients would be fixed in a form unavailable to plants, making further

    production impossible. Thus, even if common connotation of decay is dead

    matter, its rotting and decomposition, in fact, decay is so tightly connected

    to biomass production that neither can exist without the other. They are

    just the two sides of the same phenomenon called life. The most simplified

    description of these two processes making Earth alive can be summarized in

    two wellknown equations:

    6CO2 6H2O! C6H12O6 6O2 photosynthesis; organic matter productionC6H12O6 6O2 ! 6CO2 6H2O organic matter mineralization

    These two equations summarize the initial synthesis and the final mineral-

    ization. The enormous set of processes is much more complicated, of course,

    with an overwhelming variety of organic compounds produced by plants

    from a range of inorganic compounds and mineral nutrients, transformed

    and complicated even further by consumers. The chemical composition of

    litterthe substrate for decomposition processesis described in detail in

    Chapter 2.

    Decomposition undergoes a number of steps, leading from complicated

    organic compounds through simpler compounds to mineral nutrients, and,

    under certain circumstances, not all chemical elements return to their original

    inorganic form (Chapter 4). Actually, under the common term decomposi-

    tion, most scientists understand a whole set of biochemical/microbial

    processes, even those opposite to the strict meaning of the term, such as

    polymerization of long chains of secondary organic matter collectively called

    humus. However, such processes, going in a direction opposite to actual

    degradation, rely on substrates released by earlier partial decay of primary

    organic matter. In that sense, they belong to the long list of complicated

    processes of dead organic matter transformations and cannot be considered

    separately from strict decomposition (cf. Chapter 6). These processes would

    be impossible without the billions of microorganisms per gram soil, either

    directly engaged in microbial enzymatic degradation of dead organic matter

    or indirectly aVecting these processes. The taxonomy of soil organisms,belonging to such divergent groups as bacteria, fungi, protozoans, potworms,

    earthworms, insects, and even vertebrates, exceeds the scope of this book.

    However, our feeling is that the book would be incomplete without at least

    a short introduction to soil ecology and a presentation of the principal

  • ic matter

    we are a

    even the

    most important processes, such as formation and structure of humus. One

    INTRODUCTION 3reason for this discrepancy in the level of understanding of the two most

    important processes on Earth lies in the fact that while photosynthesis is

    restricted to a very limited set of possible photochemical and biochemical

    reactions, organic matter decomposition can follow a plethora of pathways,

    each consisting of a virtually indefinite number of possible combinations of

    diVerent physicochemical and biochemical reactions. While organic matterproduction, leading from carbon dioxide and water to a variety of compli-

    cated organic compounds, can take place in a single plant cell, the decom-

    position of these substances back to minerals can be performed at diVerentstages and, to a diVerent degree, step by step, primarily by fungi andbacteria, but also through vertebrate and invertebrate animals and by purely

    physicochemical reactions.

    Although it seems unlikely that we will reach a full understanding of even a

    limited set of the decay processes, substantial progress in decomposition studies

    has been made during the last two to three decades. In spite of numerous

    scientific articles on the subject published every year, there are surprisingly

    few handbooks summarizing the findings in decomposition science, most of

    them quite old and, at least to some extent, outdated. The only newer books

    available on the market are those by Reddy and Reddy (1996), Cadish and

    Giller (1997), and Berg and McClaugherty (2003). With this in mind, we

    decided to summarize contemporary knowledge on organic matter decomposi-

    tion in a formof book that could, in part, serve as a stateoftheart summary ondecomposition for scientists, and also as a textbook/handbook for graduate

    students interested in research on this aspect of ecosystem function.

    A. Decomposition, Nutrient Turnover, and GlobalClimate Change

    As has been stressed, organic matter decomposition is indispensable for

    sustaining life on Earth, as it is the only process enabling massive recycling

    of chemical elements on the scale of ecosystems and the whole biosphere.

    Turnover of these huge quantities of matter requires enormous amounts of

    energy and almost all of it is delivered as photosynthetically active solarSurprisingly, the opposite side of organic matter turnoverorgan

    decompositionis still poorly understood; moreover, it seems that

    long way from a full explanation not only of minor details, but ofdecomposers. This gap is filled to some extent by Chapter 3, devoted entirely

    to soil organisms and their role in organic matter decay.

    Although photosynthesisthe source of virtually all organic matter on

    Earthis an extremely complicated process from a biochemical point of

    view, it has already been understood and explained in detail decades ago.

  • 4 BJORN BERG AND RYSZARD LASKOWSKIradiation. After the fixation of carbon in the process of photosynthesis, the

    sole carriers of this energy are the organic compounds, which usually pass

    through a number of trophic levels before they are completely decomposed.

    Although there are millions of diVerent organic compounds synthesized andused by organisms for various purposes, the energy transfer is generally fixed

    to carbon transformations since it is carbon oxidation that eventually re-

    leases energy from organic compounds. This implies that carbon turnover

    rate is ultimately linked directly to the rate of energy transfer in ecosystems.

    In fact, ecologists use carbon to trace and calculate energy transfers through

    trophic chains in ecosystems. The complete decomposition of organic matter

    means, thus, the release of all energy fixed in organic compounds, which is

    tied to oxidation of carbon to carbon dioxide. As we will see in the following

    chapters, such complete decomposition may take place only in some ecosys-

    tems and, if it happens at all, it can take thousands of years or more. One of

    the commonly known results of incomplete organic matter mineralization is

    one on which our civilization heavily relies, namely, all fossil fuels: coal,

    crude oil, and methane.

    The turnover rate of a chemical element in the biosphere, that is, the time

    needed to complete the cycle from inorganic form through fixation to

    organic matter and its decomposition back to mineral form, determines its

    retention time in a particular pool. While the turnover rate depends on rates

    of organic matter synthesis and decomposition only, the retention time in a

    particular pool is a net outcome of the turnover rate and the pool size. For

    example, all terrestrial ecosystems fix approximately 1.05 1017 g carbonper year, which stands for approximately 12% of the total atmospheric pool

    of CO2. Assuming no change in atmospheric CO2 concentration (which is

    now not entirely true due to human activity), the average retention time of

    a carbon atom in the atmosphere is 1=0:12 8:3 years (Rickelfs, 1979).Although oxygen release rate is fixed strictly to photosynthesis (two oxygen

    atoms are released per each carbon atom fixed), its retention time in the

    atmosphere is very diVerent from that of carbon, due simply to the diVerencein pool sizes. The atmospheric oxygen pool is estimated to be approximately

    1.1 1021 g. Knowing the amount of carbon fixed yearly by terrestrialplants, the amount of oxygen released to the atmosphere can be calculated

    as 2 16/12 1017 g. This produces approximately 1/4000 of the atmo-spheric oxygen pool, thus the average retention time of an oxygen atom

    in the atmosphere equals approximately 4000 years. The retention time of

    both carbon dioxide and oxygen gives us an appropriate perspective on the

    importance of nutrient cyclingand this means decomposition of dead

    organic matter. Both numbers are indeed low in ecological and geological

    perspectives, but the 8yearlong retention time for carbon in the atmosphereis particularly striking: if carbon fixed in organic compounds was not

    released quickly to the atmosphere, its whole pool would be consumed in

  • INTRODUCTION 5just 8 years. Even if such a dramatic event is improbable (especially as we

    neglect here the carbon exchange between the atmosphere and the huge

    carbon deposits in oceans), everybody is familiar nowadays with the prob-

    lem that even minor changes in concentration in the atmospheric CO2 can

    cause. Carbon dioxide is one of the main greenhouse gases in the atmo-

    sphere, which are responsible for maintaining the global temperature at a

    certain level. The public has become familiar with the danger of global

    warming due to the increase in CO2 level in the atmosphere caused by

    massive fuel combustion and deforestation. Still, it has to be remembered

    that only due to the warming eVect of carbon dioxide and other greenhousegases the life on Earth is possible in the form we know it. To put it another

    way, present climatic conditions on Earth are controlled to a large extent by

    the balance between primary productivity and organic matter decomposi-

    tion rate. Any deviation from the present balance between carbon fixation

    and its release back into the atmosphere must inevitably lead to climatic

    changes.

    Considering organic matter decomposition from the point of view of

    balancing the atmospheric CO2 pool, its other function crucial for life on

    Earth is apparent: without decomposition, the atmospheric CO2 concentra-

    tion would continually decrease. This would be followed by a decrease of

    the atmospheric greenhouse eVect and decreasing Earth surface temperaturetoward the level resulting from purely physical balance between the input

    of solar radiation and escape of energy from Earth back to space. The

    latter is proportional to the Earth surface, and calculations estimate the

    resulting average Earth surface temperature without any greenhouse

    eVect to be approximately 18C. The current average global temperatureis 15C, and it is not hard to imagine consequences of a tempera-ture decrease of 30C that would be caused by removing main part of thegreenhouse gases (CO2, CH4, N2O, water vapor) from the atmosphere, with

    carbon dioxide being the most important of them. Of course, this scenario is

    not very probable even if decomposition were completely halted, first, be-

    cause primary productivity would gradually proceed at a lower rate and a

    point would be reached at which no more CO2 would be fixed in organic

    matter, and secondly, because other atmospheric gases, such as CH4 and

    water vapor, add their eVects to climate warming. Nevertheless, it has to berealized that even minor changes in the balance between rates of production

    and carbon mineralization can cause significant climate shifts simply due to

    the diVerence in atmospheric pool sizes between O2 and CO2. For example,moving the balance toward increased carbon dioxide evolution due to, for

    example, burning fossil fuels would use atmospheric oxygen proportionally

    to CO2 production, but would cause a significant change in the carbon

    dioxide pool only. A change in the balance between oxygen production

    and carbon fixation that would cause only a negligible 0.001% change in

  • 6 BJORN BERG AND RYSZARD LASKOWSKIO2 atmospheric concentration would be accompanied by a parallel change in

    CO2 concentration by as much as 0.7%. As carbon dioxide is the main

    greenhouse gas, such a change in concentration would inevitably cause

    climatic eVects at a global scale. Thus, detailed knowledge of organic matterdecomposition and the eVects of anthropogenic activities on these processesare of prime importance for understanding such problems as predicted

    global climate...

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