[Advances in Ecological Research] Litter Decomposition: A Guide to Carbon and Nutrient Turnover Volume 38 || Anthropogenic Impacts on Litter Decomposition and Soil Organic Matter

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  • Anthropogenic Impacts on LitterDecomposition and Soil Organic Matter

    I. Introductory Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263ADVAN

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

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

    (05)3$35.0

    8008-II. Fate of Pollutants in Litter and Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . 264

    A. General Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264

    B. Acidic Precipitation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

    C. Heavy Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266

    D. Accumulation of Heavy Metals in Decomposing LitterA

    Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268

    E. Sources of Heavy Metals in Litter. . . . . . . . . . . . . . . . . . . . . . . . . 271

    F. Organic Pollutants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275III. EVects of Pollutants on Decomposition . . . . . . . . . . . . . . . . . . . . . . . . 277

    A. Heavy Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277

    B. Acidic Precipitation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280

    C. Organic Pollutants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281

    D. EVects of Climate Change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283

    E. Changes in Water Regimen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289I. INTRODUCTORY COMMENTS

    In the world of today, with severe anthropogenic impacts on almost every

    single aspect of many ecosystems, our view on litter decomposition would be

    incomplete without considering, at least briefly, how these impacts are

    reflected in this process. In this chapter, we describe the fate of pollutants

    such as heavy metals, organic compounds, and acidic precipitation, on litter

    and soil and give an overview of the present knowledge about their eVects ondecomposition processes. Finally, we will discuss possible eVects of globalwarming and changes in water regimen on litter decomposition.

    The term anthropogenic impacts covers a broad range of human activities

    leading to various eVects on soil processes. Intensive agriculture and forestryfrequently cause massive losses of the most fertile, surface soil layer but, on

    the other hand, reasonable management can turn infertile soils into arable

    acreage. These problems are mostly the domain of intentional activities and

    have been extensively studied by agriculture and forestry practitioners. Here,

    we concentrate on anthropogenic impacts of specific importance for organic

    matter decay in forest ecosystems; impacts that usually are unintentional

    and undesired.0

    1

  • 264 BJORN BERG AND RYSZARD LASKOWSKIAlthough not yet fully understood and explained, some of the impacts of

    pollutants on the degradation of dead organic matter are relatively well

    known. On the other hand, only poor data exists on the eVects of changesin water regimen resulting from forest management practices and even less is

    known about possible eVects of such a global phenomenon as climate changeon decomposition processes. Despite this lack of knowledge and understand-

    ingor, rather, because of thatthese processes deserve special attention

    and it was our intention when preparing this book to include a review of the

    present stateoftheart in research in this area.II. FATE OF POLLUTANTS IN LITTER AND SOILA. General BackgroundDepending on type and chemical composition, pollutants may undergo

    diVerent fates and have diVerent transfer routes in an ecosystem. For exam-ple, heavy metals are deposited mainly with dust particles while nitrogen and

    sulfur oxides react with water in the air and reach the soil as acidic precipi-

    tation. When deposited in a gaseous state on soil and plants, they finally also

    react with water, for example, in soil solution, and turn to acids. Metals,

    as well as NH4 and H ions, may accumulate in ecosystems where they

    can create a threat to an ecosystem in the long run, even at moderate input

    rates. Organic pesticides are intentionally sprayed in ecosystems where, after

    reaching the soil, they can be stored for some time, degraded through

    different physicochemical and microbial processes, or leached to the ground-

    water. The fate of a pollutant in an ecosystem largely determines how

    harmful it can be to the function of the ecosystem.

    Generally, pollutants reach ecosystems with wet and dry deposition,

    mostly with rainfall and snow andto a lesser extentthrough socalledinterception (horizontal deposition; Fig. 1). This latter route, relying on

    horizontal transport of pollutants with clouds and fog, may be important

    in mountains and coastal areas, where significant amounts of water are

    deposited in that way. After reaching a forest canopy layer, part of the

    water evaporates from leaf surfaces so that the amount of water reaching

    forest floor as throughfall and stemflow (Fig. 1) usually is significantly lower

    than the amount deposited as bulk deposition (deposition above the canopy

    layer plus interception). Water chemical composition also changes dramati-

    cally during its passage through the forest canopy: for example, NH4 andH ions are, in part, absorbed directly into leaf tissues while others, such asK or Mg2, are usually leached out from leaves. Many elements are neitherabsorbed nor leached but their concentrations in throughfall increase due

    simply to evaporation of water. As a result of these processes, the water

    reaching the forest floor is rich in a number of chemical components and, in

  • Figure 1 Main routes of input and transfer of chemical elements in forestecosystems. TF, throughfall; LF, litterfall; SF, stemflow.

    ANTHROPOGENIC IMPACTS ON LITTER DECOMPOSITION 265industrialized parts of the world, the input of some of them can be significant

    in comparison to the amounts released by natural turnover. An ecosystem

    may be reached not only by nutrients, but also by elements normally not

    involved in biological processessocalled xenobiotics, for example, heavymetals such as cadmium or lead. A fraction of the elements reaching forest

    floor is leached down the soil profile, eventually leaving the ecosystem with

    streams or groundwater. The remaining part, however, accumulates in or-

    ganic layers andto a lesser extentin mineral soil layers. Some heavy

    metals such as Pb or Cd, being potentially toxic to organisms, may endanger

    the two main ecosystem processes, production and decomposition.B. Acidic PrecipitationAcidification of atmospheric precipitation has become one of the most

    serious and widespread threats to ecosystems, originating from human

    activities. Although natural, unpolluted rainfall is also slightly acidic due

    to atmospheric CO2 dissolving in the rainwater and forming carbonic acid,

    its pH does not drop below 5.6, which is approximately the equilibrium

    point for CO2 in water at normal atmospheric CO2 concentration. Increased

    concentrations of sulfuric and nitric oxides in the atmosphere, originating

    from burning fossil fuels, result in formation of sulfuric and nitric acids in

  • 266 BJORN BERG AND RYSZARD LASKOWSKIwater in clouds, fog, and raindrops. This, in turn, increases the concentra-

    tion of H ions by as much as 1 to 2 orders of magnitude (pH drops to 4.53.5). A large number of these hydrogen ions (50 to 70%) are intercepted by

    forest canopies due to the substitution of alkaline ions (K, Mg, Ca2) inleaves (Lindberg et al., 1986; Stachurski, 1987; Bredemeier, 1988). In fact, at

    stands rich in alkaline nutrients, the rainfall may be completely buVeredduring its passage through the forest canopy (Meiwes and Koenig, 1986). On

    the other hand, in the long term, such a decrease in precipitation pH,

    especially in stands on pure granite sand, leads to increased leaching of

    nutrients, not only from leaves but also from the surface soil layers, leading

    finally to premature foliar litter fall (Lawrence and Fernandez, 1991) and/or

    decrease in tree biomass production (Orze, 1985).

    Changes in litter chemical composition can be expected to be reflected in

    decomposition processes. As we have shown in previous chapters, decompo-

    sition is often initially faster in litters rich in the main nutrients. Acidic

    precipitation may cause increased leaching of alkaline nutrients (K, Ca,

    Mg) and such chemical elements as are more soluble under acidic conditions,

    such as Mn. Such changes in litter may lead to changed decomposition

    patterns, which would be indirectly related to acidic precipitation. Based on

    the discussion in Chapter 4, we may expect that the higher litter N levels

    following N deposition and the leaching of Mn from foliar litter would create

    a litter that leaves larger recalcitrant remains. Thus, we may hypothesize that

    at least moderate acidic precipitation, in general, should decrease the extent

    of the organic matter decomposition in ecosystems and cause a higher humus

    accumulation rate.C. Heavy MetalsThe old statement made by Paracelsus,1 sola dosis fecit venenum, means

    that only the dose makes the poison. This important observation can be

    regarded as one of the foundations of toxicology and ecotoxicology. From

    this point of view, distinguishing toxic metals from nontoxic ones does not

    make much sense. In fact, all metals, even nutritional ones, may become toxic

    above a certain concentration threshold. When researchers today focus their

    attention only on a few selected heavy metals, this is not because of their

    special toxicity but rather due to the simple fact that only a limited number of

    heavy metals are emitted to the environment in amounts that endanger

    normal functions of organisms and ecosystems. The general eVects of someof them (Pb, Cu, Hg, Zn) on organic matter decomposition are relatively well1Philippus Aureolus Theophrastus Bombastus von Hohenheim, 1493-1541, German

    alchemist and physician born in Switzerland.

  • ANTHROPOGENIC IMPACTS ON LITTER DECOMPOSITION 267recognized. However, this does not mean that other heavy metals will not

    become important in the future, for example, if the pollution patterns change.

    One of the major problems with several heavymetals is their high aYnity tosoil organic matter and to mineral particles. Because of this, they tend to

    accumulate in soil andeven at moderate inputsmay eventually exceed the

    toxicity threshold to soil microorganisms and invertebrates. The discovery

    made by Paracelsus almost five centuries ago acquires new meaning as

    regards the dose: in the long run, not only the input rate of metals (the dose)

    to an ecosystem is important but also the rate of their accumulation in soil,

    which, to a large extent, depends on soil properties. Soil properties also

    determine the chemical form in which metals are present, which is as impor-

    tant for their toxicity as the magnitude of the input and the accumulation

    rates. It has been shown in a number of studies that it is mostly the ionic

    form of metals which is toxic to invertebrate and microbial decomposers,

    mycorrhiza, and plants.

    Because concentrations of some heavy metals increase during litter de-

    composition (Fig. 6, Chapter 4) (Ruhling and Tyler, 1973; Berg et al., 1991b;

    Laskowski et al., 1995), they can reach relatively high concentrations in

    more decomposed fractions of forest litter, even in clean and moderately

    polluted ecosystems. Laskowski and Berg (1993) made a similar finding

    for Fe, Zn, Pb, and Cd in unpolluted Scots pine and oakhornbeam forest

    stands. In the Berlin area, Kratz and Bielitz (1989) found that, after 19

    months, decomposition concentrations of lead in leaf and needle litter had

    increased 3 to 14fold, and those of Cd 1.3 to 6.5fold.Furthermore, a net accumulation has been seen and McBrayer and

    Cromack (1980) and Staaf (1980) found significant accumulation of Fe,

    Zn, and Cu in unpolluted decomposing litter in beech and oak forests. Net

    accumulation of heavy metals in soil and litter can be strongly modified by

    the pH in the soil environment (Livett, 1988). Generally, soils at approxi-

    mately neutral pH and with a high content of clay minerals and/or organic

    matter can immobilize large amounts of heavy metal ions. A consequence is

    that the amount of heavy metals can increase considerably without neces-

    sarily aVecting ecosystem functions, unless a decrease in soil pH occurs.Under such conditions, with neutral pH, the heavy metals are inactive

    from a toxicological point of view. However, a drop in pH below approxi-

    mately 6.0 to 5.5 will cause a rapid increase in solubility of most heavy

    metals. For instance, Christensen (1984) found that decreasing pH by two

    units increased the solubility and lowered the equilibrium isogram for cad-

    mium by more than 75%, and Boekhold and Van der Zee (1992) proved that

    the eVect of pH on the behavior of Cd is the most important among all sofarinvestigated soil factors. In an experiment by Tyler (1978), less than 10%of the total amount of cadmium and less than 20% of total amount of zinc

    was leached from soil using a solution of pH 4.2. Decreasing the solution pH

  • 268 BJORN BERG AND RYSZARD LASKOWSKIby one unit (to 3.2) resulted in leaching of more than 40% of the cadmium

    and above 55% of the zinc. KabataPendias and Pendias (1979) have re-ported zinc mobility in acid soils to be 10fold higher than at pH above 6.4.In their study, lead is clearly the least mobile heavy metal and only about

    10% was leached even at a pH of 2.8. Christensen (1984) identified another

    important mechanism triggering desorption of Cd from soil: higher concen-

    tration of zinc or calcium in a leaching solution significantly increased the

    solubility of cadmium in soil solution.

    The importance of heavy metal accumulation in soils and a possible de-

    layed deleterious eVect on ecosystems was recognized many years ago.Some authors suggested that metals accumulated in soil organic layers may

    become a sort of timebomb which will be triggered by acidification orother as yet unknown phenomena. As a consequence, by the end of the last

    century, some countries proposed extremely restrictive limits on allowable

    total inputs of heavy metals, aiming at a zero accumulation of heavy metals

    in soils. Although this may seem excessive (as we noted before, some heavy

    metal accumulation can be observed also at low pollution levels), it can

    be argued that even at very low accumulation rates, toxic concentrations

    will be reached eventually. The problem was discussed in 1996 by Witter,

    who wrote that:

    With the possible exception for Cd, there is apparently no scientific

    evidence at the moment to suggest that zero accumulation of metals in soil is

    required to adequately protect soil productivity, the environment, and

    human and animal health. A policy which steers towards zero accumulation

    may therefore seem excessively cautious. It is, however, also a policy which

    recognizes the practically...

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