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

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  • Nitrogen Dynamics inDecomposing LitterI. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157ADVAN

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

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

    (05)3$35.0

    8005-II. The Dynamics of NitrogenThree Phases in Decomposing Litter. . . . 159

    A. General Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

    B. The Leaching Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

    C. Nitrogen Accumulation PhaseA Phase with a Net Uptake

    and a Retention of N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

    D. A Release Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

    E. The Final Release Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176III. Nitrogen Concentration Versus Accumulated Litter Mass Loss . . . . . . 177

    IV. Nitrogen Concentration in Litter Decomposing to the Limit Value

    and in Humus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

    A. Background and Some Relationships . . . . . . . . . . . . . . . . . . . . . . 181

    B. AModel and a Case Study for Calculating N Concentrations

    in Humus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181I. INTRODUCTION

    As the chemical composition of litter, together with climate and environ-

    mental factors, governs the decomposition process, it also rules the dynamics

    and release of nutrients from litter in diVerent decomposition stages. Nu-merous studies have been carried out on the dynamics of nutrients in

    decomposing litter but mainly in the early stage of decomposition, and

    relatively few cover the late phases (see Chapter 4). A good general concep-

    tual model of the processes of leaching, accumulation, and release of nu-

    trients is still missing, probably because of the complexity of the processes.

    Although there have been attempts to distinguish subprocesses, such as

    leaching from and uptake to litter in the N dynamics during the course of

    the main decomposition process (Berg and Staaf, 1981), we still do not have

    a good description of the dynamics, much less a good explanation of several

    observed subprocesses. In this chapter, we focus on nitrogen, since there

    appears to be more knowledge generated on N dynamics in litter and humus

    than on other nutrients, making it possible to create a conceptual model for

    its dynamics. The details of the dynamics and the release mechanism are still

    not well explained, though, and are often related to litter species, giving the0

    6

  • 158 BJORN BERG AND RYSZARD LASKOWSKIobservations an empirical character. We therefore focus on a common

    pattern for foliar litter.

    Nitrogen becomes available to the ecosystem basically through the N2fixation process and other sources of N, such as deposition of NOx, which is

    part of the low background N deposition of approximately 2 kg ha1

    yr1. In natural, unpolluted forests, the input of litter N to forest floor is ofconsiderable magnitude. A boreal coniferous forest may shed between 2 and

    20 kg N in foliar litter per ha and year (B. Berg and V. Gauci, unpublished

    data), and a temperate deciduous forest 20 to 40 kg N per hectare in annual

    foliar litter fall (B. Berg and V. Gauci, unpublished data). In the newly shed

    litter, a main part of the N is in the form of proteins and nucleic acids. When

    N is in high excess in the litter, for example, in forests under extremely high

    N deposition, it can be present also in the form of arginine, an amino acid

    that normally is a storage form of N.

    It appears that the N dynamics pattern may vary not only among ecosys-

    tems and environments but also with properties of diVerent litter species.Examples of factors influencing its dynamics are litter pH, and the ratio of N

    to P and S, the nutrients that normally may be limiting for microbial growth.

    A further influencing factor is the availability of the energy source, normally

    indicated by the litter lignin concentration, influencing N dynamics in a way

    that still needs to be explained but probably, among other functions, acting

    as a sink for N, binding N in covalent bonds as part of the humus formation

    process. A further factor is the litters cation exchange capacity (CEC).

    Often, N is limiting in ecosystems, both to the vegetation and to the

    microbial decomposers. Furthermore, N is available only from the atmo-

    sphere and could thus be expected to have entirely diVerent properties forretention and availability as compared to nutrients such as K, which nor-

    mally is not limiting, is available through weathering, is highly mobile, and

    has a solubility that is not pH dependent.

    Often when element dynamics is studied in decomposing foliar litter, the

    total content of a given nutrient is measured, which includes not only

    the amount of the nutrient originally present but also that transported

    into the litter. This means that only the net changes are measured and not

    the actual movements of the nutrient. In addition, not only is the N in litter

    measured but also the amount of N in the microbial biomass and, unless

    accounted for, this part is also included in the dynamics. Even when isotopes

    are used as tools, it may be diYcult to estimate the magnitude of thisphenomenon, especially during a longterm experiment.

    In this chapter, we attempt to create a system for describing N dynamics in

    decomposing litter. To do this, we have used several case studies which we

    consider to be representative, at least for litter in boreal and temperate

    ecosystems. We present a system for N dynamics in decomposing litter,

    describing diVerent phases of the dynamics as well as a suggested release

  • NITROGEN DYNAMICS IN DECOMPOSING LITTER 159mechanism. Finally, starting with newly shed litter, we calculate the N

    concentration in humus. Please note that part of the N dynamics, namely,

    its sequestration in humus and calculations of amounts released in the forest

    floor, is presented at the end of Chapter 6.II. THE DYNAMICS OF NITROGENTHREE PHASESIN DECOMPOSING LITTERA. General CommentsAs mentioned in Chapter 4, the concentration of N increases as litter

    decomposes and the increase may be at least threefold compared to the

    initial concentration. This increase in concentration is a general phenome-

    non, also described as a decrease in the CtoN ratio. The increase isnormally linearly related to accumulated litter mass loss, usually with a

    high R2 value (Berg et al., 1995), irrespective of the initial N concentration

    and of how the absolute amount of N changes during decomposition (Fig. 1;

    see also Section III).

    There are some rules of thumb presented in the literature regarding N

    dynamics in ecosystems. Such simplified rules are normally intended and

    useful for practical purposes and give general relationships, which may be

    applied in agriculture and forestry. Still, they have very little to do with

    ecosystem research and, from a scientific point of view, they are sometimes

    directly wrong. For example, a general and fixed initial CtoN ratio in litteras a limit for net release or net accumulation in decomposing litter has been

    proposed (see, for example, Lutz and Chandler, 1947; Mulder et al., 1969)

    given as a CtoN ratio of 25, which means an N concentration of about 20mg g1 in the litter organic matter. There appear to be either no or very fewexperimental data to support the generality of such a statement, and when

    applied to a nutrientpoor Scots pine ecosystem, we see that it is wrong: a netrelease from decomposing needle litter could take place initially at CtoNratios of about 125 (N concentration of about 4 mg g1) (Berg and Ekbohm,1983). We see from Fig. 2 that for four Scots pine litter types, incubated

    simultaneously in the same forest stand, a net release was dependent on N

    concentrations and started at an initial CtoN ratio of ca 80.In this section on N dynamics, we present and discuss diVerent cases of net

    uptake and net release as well as three phases for N dynamics and their

    importance in the N budget of decomposing foliar litter. Nitrogen in decom-

    posing litter is not just released but, since it is often limiting to the decom-

    posing microorganisms, it may be taken up actively to the litter, and thus its

    absolute amount in litter increases (Fig. 1). Such an uptake may take place

    through ingrowing fungal mycelium, which also may transport N bound in

  • Figure 1 Concentrations and amounts of N in decomposing litter plotted versuslitter mass loss. (A) Scots pine needle litter. (B) Silver birch leaf litter.

    160 BJORN BERG AND RYSZARD LASKOWSKIdiVerent compounds into the litter. The distance over which the transporta-tion of N takes place from the surroun dings into the litter probab ly is mostly

    in the order of millimeters or centimeters but may take place over distances

    of more than one meter.

    It has been possible to construct a conceptual model for the dynamics of N

    in decomposing litter and a similar approach may be applied also to P and S,

    since these nutrients appear together in defined ratios, for example, in

    proteins and nucleic acids in the decomposing microorganisms, thus creating

  • Figure 2 Four types of Scots pine needle litter originating from a nitrogenfertilization experiment were incubated simultaneously in a nutrientpoor Scots pineforest. The initial N concentration is of importance for whether an N release takesplace or not.

    NITROGEN DYNAMICS IN DECOMPOSING LITTER 161rather constant ratios in the decomposing litter as decomposition proceeds

    (Se ction IV and Fig. 9, Chapt er 4). Duri ng litter decomposi tion, the dyn a-

    mics of the amounts of N may be divided into three diVerent steps or phases.We may also see three cases of possible N dynamics (Fig. 3). In the first

    case, there is a short leaching of N followed by a net uptake and a net

    release (Fig. 3A). In another case, there may be a net uptake followed by a

    net release (Fig. 3B), and in a third case, only a net release is observed

    (Fig. 3C). Thus, all three phases are not always present and not always

    clearly distinguished. These will be presented more in detail.B. The Leaching PhaseNewly fallen litter becomes invaded by microorganismsa process which

    can take considerable time. Berg and Soderstrom (1979) found that the

    ingrown total (live plus dead) fungal mycelium in Scots pine needle litter

    reached a maximum first after approximately one year. Even in the early

    stages of this microbial invasion, the decomposition process starts. There

    is a very early period after litter fall, however, when litter mass loss and

  • Figure 3 Three separate phases may be distinguished for the change in amount oflitter N over time. Not all of them are always seen in practical experiments, though.For example, the accumulation phase could be missing, especially in litter with highN concentrations. (A) A leaching phase (I) is followed by an accumulation (II) and arelease phase (III). (B) An accumulation (phase II) is followed by a release (phaseIII). (C) Only a release is seen (phase III or phase I phase III).

    162 BJORN BERG AND RYSZARD LASKOWSKInutrient release are not caused by microbial decomposition. This was

    first demonstrated as a shortterm leaching using distilled water. Nykvist(1959) demonstrated the leaching of N from whole leaves of common

    ash and found that about 15% of their N could be physically leached

    (Table 1).

    A rapid release of initially leachable N in litter constitutes this first phase

    of N dynamics (Fig. 3). Leachable, in this case, means extractable by water

    from whole litter. In its simplest form, studies on leachable N mean that, for

    example, a weighed amount of leaf litter may be allowed to soak in water

    for a certain time, maybe 1 to 24 h, and afterwards the water is analyzed

    for total N. A sequence of such short leaching events, sometimes studied in

    the presence of an inhibitor for microbial growth, will leach out what is

    possible to extract from a whole needle or a leaf. When litter decomposes on

    the ground, this leaching phase is rather short (Fig. 3A). In the case shown

    in Fig. 3C, leaching may take place but is not distinguished from the

    general release.

    There are relatively few studies on leaching of substances from litter. Some

    results for N are compiled in Table 1. For nitrogen, leaching has been

    determined in laboratory studies on whole litter or milled samples and for

    whole litter in the field. Nykvist (1963) compared such leaching of soluble

    components from whole litter to that from milled samples and found

    the latter to be higher to a varying degree, which also may be valid for N

    (Table 1). We thus have two valuesone for the actual leaching from whole

    litter and one for a maximum leaching, where the latter stands for potentially

    leachable substance, which is the same as the concept watersoluble sub-stance (see Chapter 4). From the leaching data so far presented, it appears

    possible that the shortterm leaching of whole litter in the laboratory could

  • Table 1 Leaching of nitrogen from some leaf and needle litter species (laboratorymeasurements)

    Litter type Total N (%) Leached N (% of total litter N) Reference

    Black alder 2.1 13 (1)Common ash 1.1 15 (2)Common ash 0.86 18 (1)Willow sp. 0.94 25 (1)Downy birch 0.91 13 (1)Trembling aspen 0.82 34 (1)Mountain ash 0.71 42 (1)European maple 0.51 40 (1)Scots pine 0.38 34 (3)Scots pine 0.36 15 (1)Scots pine 0.49 9 (1)Scots pine 0.73 2 (3)Scots pine (green) 1.3 ca 6 (3)Scots pine (green) 1.8 < 1 (3)

    References: (1) Bogatyrev et al. (1983), (2) Nykvist (1959), (3) B. Berg, unpublished.

    NITROGEN DYNAMICS IN DECOMPOSING LITTER 163give lower values than those found in nature. Berg and Staaf (1981) found in

    field experiments that there was an initial release (leaching) of 10% of the N

    content of Scots pine needles versus about 2 to 4% for the same needle litter

    in the laboratory.

    Some factors of importance for N leaching can be distinguished. Litter

    structure (seen as litter species) thus appears important, although only

    recognized as a diVerence among litter species rather than by specific physi-cal properties. So far, we lack a systematic explanation regarding the litter

    properties versus leaching but leaching of both organic substances and N

    appears higher for deciduous leaves than for needle litter (Table 1). It may

    also be seen that leaching of N from one species, in our case, Scots pine

    needles, in laboratory measurements was not in proportion to the initial N

    levels in spite of the wide range from 3.6 to 18 mg g1.A possible factor which determines the amount leached in the field would

    be rainfall and the movement of water, more intensive water movements

    promoting high leaching. Another factor may be freezethaw cycles, in

    which the freezing followed by thawing breaks tissue and cell structures

    and causes a release of N and other nutrients. Bogatyrev et al. (1983) showed

    that after all leachable substances h...

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