MODERN BIOGEOCHEMISTRY: SECOND EDITION Phần 4 pdf

Trace metal fluxes, g/ha/yrMean content in plant species, ppb by dry weight In livingplantorganisms In deadorganicmatter In net annualproduction Airborneinput∗,g/ha/yrTrace metal ∗The ai

124 CHAPTER 5

For the calculation of critical loads of various pollutants on the human and tem health, the following working hypothesis has been considered This hypothesis isconnected to the assessment of sensitivity of various human physiological parameters

ecosys-to environmental biogeochemical facecosys-tors In this case interaction may be established

by statistical exploration of the dependence between loading and various types ofmorbidity The critical loads of sulfur and nitrogen at various ecosystems and theirexceedances during 1992–1996 were compared with human respiratory system mor-bidity both in the Crimea and the whole of Ukraine In Ukraine, the respiratory caseswere correlated with the exceedances of critical loads (Figures 9 and 10, Table 12)

NATURAL BIOGEOCHEMICAL PECULIARITIES

of the World’s ecosystem, we can suggest the existence of many peculiarities of geochemical cycling of various elements in natural terrestrial and aquatic ecosystemsand accordingly, different characteristic features of human and ecosystem exposure

bio-to various pollutants via direct or indirect impact

CHAPTER 6

ARCTIC AND TUNDRA CLIMATIC ZONE

1 GEOGRAPHICAL PECULIARITIES OF BIOGEOCHEMICAL CYCLING

AND POLLUTANT EXPOSURE

In the Northern Hemisphere the area of arctic and tundra landscapes with plantspecies’ ecosystems is 3,756,000 km2 In the Southern Hemisphere similar land-scapes are completely absent Most of these landscapes occur in northern Eurasia(Russia, Fennoscandia), Greenland, Alaska, and Canada

The climate conditions of Arctic and Tundra ecosystems are the main factor encing many peculiarities of biogeochemical cycling Because of the severity of theclimate the vegetation season is very short During the arctic summer the temporarymelted soil layer is less than 40–45 cm and the deeper layers of ground are perma-nently frozen These permanently frozen grounds are called permafrost The existence

influ-of permafrost mainly determines the qualitative and quantitative parameterization influ-ofbiogeochemical cycles of all elements We can say that the biological and biogeo-chemical cycles are restricted both temporally and spatially in Arctic ecosystems.The major restricting factor is the ocean Both continental coastal areas and areas

of islands are exposed to cold oceanic currents The Arctic oceanic basin is separatedfrom the warm influence of the currents from the Atlantic and Pacific Oceans owing

to the existence of both narrow channels like the Bering Strait and submarine ranges.The average precipitation is from 100–200 mm (North American areas) to 400 mm(Spitzbergen Island) and the average temperature of January is between −30◦C

and−38◦C (Figure 1).

The low precipitation and freezing water stage during 10–11 months per annumhave led to the development of arid polar and tundra landscapes The characteristicfeatures of these landscapes are the alkaline soil reaction (pH 7.5–8.0) and even theoccurrence of modern carbonate formations

1.1 Landscape and Vegetation Impacts

In accordance with the local maximum of precipitation and the relative low wintertemperatures, the most favorable climate conditions for biogeochemical processesand pollutant exposure are in the western part of Spitzbergen Island Three types oflandscapes with corresponding ecosystems are widespread (Dobrovolsky, 1994)

On the wide shore terraces of fjords and on the slopes of hills and low mountains,the Arctic Tundra ecosystems occur The mosses and lichens are predominant with

127

Pole

Canada USA

Iceland Greenland

Figure 1 Polar and Tundra ecosystem area in the Northern Hemisphere.

the twigs of willow (Salix polaris), varieties of rockfoils (Saxifraga oppositifoila,

S polaris, S caespitosa, etc.), dryad (Dryas octopetala), specimens of arctic poppies,

buttercups, cinquefoils, various tufted rushes (Juncus) and grasses In some areas the

vegetation forms a continuous covering and in others it is confined to depressionsenclosing cryogenic polygons The plant mat covers the soil surface Most soils areBrown Arctic Tundra soils having only A and C genetic horizons

The vegetation becomes sparse at the high plateau over 400–500 m above sea level(a.s.l.) The surface coverage is mainly less than 10% The short grown mosses arepredominant They occupy the depressions with shallow soil accumulation Lichens

ARCTIC AND TUNDRA CLIMATIC ZONE 129

Table 1 Chemical composition of different plant species in Spitzbergen island ecosystems (after Dobrovolsky, 1994).

Content, ppm by dry plant weight

wide valley and they are bordered by sedges (Carex nordina, C rupestis), cottongrass (Eriophorum) and nappy plant species.

The ash of peat forming plant species contains a predominant amount of silicon

This element is particularly abundant in the Sphagnum, where its content achieves

36% by ash weight Iron and aluminum are the next abundant The first is accumulatedduring the peat formation process The accumulation of calcium and potash is morepronounced than sodium, and the sulfur content is also remarkable A large amount ofmechanically admixed mineral particles (40–80% by ash weight) is found in mosses.This is due to the deposition of fine dispersed mineral material from snowmeltingwaters and atmosphere dust deposition (Table 1)

1.2 Pollutant Exposure and Chemical Composition of Plants

Let us consider the influence of various exposure factors on the chemical composition

of plant species in the arctic islands It seems the most influential factor is the distancefrom the ocean shore For example, in arctic willow growing a few meters from thetide line, the content of Zn, Cu, Pb, and Ni was higher than that of the same plant

Table 2 The trace element composition of the Spitzbergen snowmelting water, ppb (Evseyev, 1988).

species growing about 1 km from the coast line and sheltered from the sea by a morainhill The coastal plants contain also more sea salt cations like Na, Ca, K, and Mg.The enrichment effect of the ocean is mainly related to the chemical composition

of aerosols, which determine the chemical composition of snow For the northern areas

of the Eurasian continent and the western areas of Spitzbergen Island we can estimatethe average values of sea salt deposition in snow from 3,000 to 5,000 kg/km2 Thepredominant chemical species in the snow water are chlorides (anions) and sodiumand calcium (cations) The content of trace elements (heavy metals) is negligible.Their origin is connected with long-range trans-boundary air pollution from industrialcenters of North America, Russia and Europe This was shown for the Greenlandglaciers, where the statistically significant growth of zinc and lead in the recentprobes in a comparison with ancient ice cores has been attributed to the environmentalpollution (Bashkin, 2002)

The role of air aerosols in the biogeochemical cycle of various nutrients in theArctic ecosystems has been studied in Spitzbergen Island The supply of oceanicaerosols is very important in these conditions since the interaction between plant rootsand soil or mineral substrates is depressed during a long part of the year According tothe monitoring data the following results are typical for the Spitzbergen snow meltingwater (Table 2)

For a comparison, the mobile forms of trace metals were extracted from the localgeological rocks, as water-soluble and 1.0 N HCl-soluble forms The results are shown

in Table 3

We can see that the content of trace metals in water extraction is very low Thismeans that the direct involvement of these metals in biogeochemical cycles is veryrestricted The significant increase of metal contents in acid-soluble form was shownonly for Fe, Mn and, partly, for Zn These data testify the importance of atmosphericdeposition for the Arctic ecosystems as a source of nutrients

The supply of sea salts and trace metals via precipitation appears to contribute tothe elevated content of water-soluble forms of alkaline and earth–alkaline elementsand trace metals in the uppermost soil layer

1.3 Influence of Soil on Pollutants Exposure

A high amount of various nutrients and trace metals is retained in peat and deadplant residues and thus temporarily eliminated from the biogeochemical cycles andpollutants exposure to human and ecosystem health The period of this eliminationdepends on the solubility of these metals It has been shown (Dobrovolsky, 1994) that

ARCTIC AND TUNDRA CLIMATIC ZONE 131

Table 3 Content of mobile forms of trace elements in rocks of Spitzbergen Island, number of rocks = 10 (after Dobrovolsky, 1994).

Trace metal content, ppb

the soluble forms of such metals as iron and zinc accounted for about 70% and 50%

of the total contents of these metals in solution, correspondingly, in the upper peatlayer with living plants In the underlying peat layer, the percentage of soluble formstended to decrease A similar tendency was recorded for soluble forms of carbon: onleaching from upper to lower peat layer, the concentration of soluble form decreasestwice as much in the terrace and still greater, in waterlogged depression

Electrodialysis of the soluble forms of iron has revealed the predominance ofelectroneutral forms A similar distribution has been shown for carbon The hypothesisthat the organic iron-containing complexes are responsible for water-soluble forms

of iron in polar peat ecosystems seems logical Amongst the soluble zinc forms, thepercentage of electroneutral forms is somewhat lower that that of charged forms, withthe anions present in a larger amount in the upper peat layer

However, only the smallest part of soluble metals is involved in the biologicalcycle Most of these are either lost to water runoff, or retained in the peat organicmatter The latter is the source of gradual remobilization but the whole mineralizationmay last up to 50 years or even more The total accumulated retained amount of macro-

or trace metals in organic matter of peat is tens and hundreds of time higher than theconcentration of annually released soluble forms, which are available for plants

2 BIOGEOCHEMICAL CYCLES AND EXPOSURE ASSESSMENT

IN POLAR ZONES

2.1 Biogeochemical Cycles

The different metal uptake by plants is accompanied by a different involvement ofthese trace metals and macronutrients in the biogeochemical cycles A comparison of

Table 4 Airborne input of various trace metals in the

Spitzbergen island ecosystems, mg/yr per 100 mm of

precipitation (after Dobrovolsky, 1994).

the metal concentrations in plant tissues and the metal concentrations in the aqueousextracts from soil-forming geological rocks shows that iron and manganese are themost actively absorbed by plants The plant to soil metal ratio can be an indicator of

this absorption These values for Fe and Mn are in a range of n× 102to n× 103 This

ratio is about n × 101for Zn, Cu, and Ni It is noteworthy that the high concentrations

of iron and manganese tend to even increase in the dead organic matter of peat.The systematic removal of elements by runoff and the reimmobilization fromsolution by organic matter are continuously counterbalanced by the new input ofchemical species, which maintain both biological and biogeochemical cycles Themain sources of water-soluble elements are oceanic aerosols deposited on the landsurface and the weathering of rocks The airborne input of the trace metals may beranked as follows for the Spitzbergen island ecosystems (Table 4)

We can compare these values with those characterizing the fluxes of trace metals

in biogeochemical cycles The biological productivity of the Polar Tundra ecosystemgrown on the low terrace in the region of Barentsberg, Spitzbergen Island, is shown

in Table 5

To be noted for comparison, the annual growth increase for arctic willow (Salex

arctica) in Cornwallis Island in the Canadian Arctic Archipelago, 75◦N, is a mere0.03 ton/ha (Warren, 1957) The corresponded trace metal fluxes are shown in Table 6

2.2 Exposure to Airborne and Ground Pollutants

We can see that for iron and manganese the annual fluxes of trace metals are anorder of magnitude higher than airborne input For copper this input is sufficient tosupply the annual uptake, and for zinc is even in excess All these trace metals areessential elements and their input with deposition can be considered as positive for

Table 5 The biological productivity of the Polar Tundra

Low Terrace ecosystem.

ARCTIC AND TUNDRA CLIMATIC ZONE 133

Table 6 Fluxes of trace metals in the Spitzbergen Island ecosystems (after Dobrovolsky, 1994).

Trace metal fluxes, g/ha/yrMean content

in plant

species, ppb by

dry weight

In livingplantorganisms

In deadorganicmatter

In net annualproduction

Airborneinput∗,g/ha/yrTrace metal

∗The airborne input was calculated per 300 and 400 mm per year in accordance with annual precipitation

rates in the western Spitzbergen coast and trace metal rates shown in Table 4.

the ecosystem’s behavior The excessive deposition input of lead is rather dangerousowing to the unknown physiological and biogeochemical role of this element in plantmetabolism However, the significant amounts of lead can be immobilized in deadorganic matter and excluded from biological turnover

The other output from watershed and slope landscapes positions is related to thesurface and subsurface runoff of trace metals The ecosystems of waterlogged glacialvalleys, geochemically subordinate to the above mentioned landscape, can receivewith surface runoff an additional amount of various chemical species This results in3–4-fold increase of plant productivity in comparison with elevated landscapes and

in corresponding increase of all biogeochemical fluxes of elements, which are shown

in Table 6 For instance, the accumulation of trace metals in dead peat organic matter

of waterlogged valley was assessed as the follows: Fe, n× 101kg/ha, Mn, 1–2 kg/ha,

Zn, 0.1–0.3 kg/ha, Cu, Pb, Ni, n× 10–2kg/ha

3 BIOGEOCHEMICAL CYCLES AND EXPOSURE ASSESSMENT

IN TUNDRA ZONESThe tundra zone and corresponding tundra ecosystems occupy the northernmost strip

of the continental area of Eurasia and North America bathed by the seas of the Arcticbasin The climate conditions of the tundra zone provide for a higher productivity

of ecosystems and higher activity of biogeochemical cycles of various elements ascompared with the Arctic ecosystems The mosses, lichens, and herbaceous plantspecies are predominant in the northern part of the Tundra ecosystems and shrubs areprevalent in the southern part

The edaphic microflora is diversified, and the microbial community is more merous than that of the arctic soils The bacterial population varies from 0.5× 106

nu-to 3.5× 106specimens per gram in topsoil horizon

3.1 Plant Uptake of Pollutants

The ash contents of the total trace elements and nitrogen are similar in Tundra tem biomass The highest concentrations,>0.1% by dry ash weight, are typical for

ecosys-Ca, K, Mg, P, and Si We can note the increase of iron, aluminum and silicon contents

in the underground parts of any plants

The uptake of trace metals depends on both from the plant species and metal Suchelements as titanium, zirconium, yttrium, and gallium are poorly absorbed owing

to their minor physiological role in plant metabolism Rockfoils (Genus Saxifraga) and mosses (genus Bryophyta) are especially sensitive to alternations of trace metal

concentrations The bryophytes are capable of sustaining higher concentrations ofsome trace metals as compared to vascular plants (Shacklette, 1962) Some species

of mosses can accumulate enormous amounts of trace elements and can serve asindicators of copper metal ore deposits with elevated copper contents

3.2 Tundra Soils and Exposure to Pollutants

The Acidic Brown Tundra soils (Distric Regosol) are formed under the conditions ofthe free drainage commonly encountered in slopes and the watershed relief positions.The characteristic features of these soils are related to the accumulation of non-decomposed plant residues and the built-up peat layers Below the peat horizon thesoil profile differentiation is indistinct In the thin indistinct humus horizon underlyingthe peat layer, the humus content is from 1 to 2.5% with predominance of solublefulvic acids This presents an acid reaction of soils, with values of soil pH<5.0.

The acid geochemical conditions facilitate the migration of many trace elements,phosphorus, nitrogen and many earth–alkaline metals The migration of chemicalspecies is mainly in the form of Me–organic or P–organic complexes This facilitatesthe exposure of humans and ecosystems to different pollutants

The deficiency of oxygen is very common in lowland plains with an impededdrainage This is favorable to the formation of Gley Tundra soils (Gelic Regosol) with

a grey gleyic horizon This horizon includes the gray and rusty spot-like inclusions

of precipitated gels of Fe3 +oxides These oxides are the geochemical barriers in the

pollutants biogeochemical cycles and they can retard significant amount of variouschemical species

3.3 Exposure to Pollutants and Productivity of Tundra Ecosystems

The biomass of Tundra ecosystems gradually increases from 4–7 ton/ha for moss–lichen tundra to 28–29 ton/ha by dry weight for low-bush tundra In the northerntundra, the plant biomass and dead organic matter are eventually shared Southwardsthis percentage tends to diminish, and low-bush living biomass is smaller than deadplant remains mass A typical feature of the Tundra ecosystems plant species is theprevalence of underground matter (roots) up to 70–80% of the total biomass

ARCTIC AND TUNDRA CLIMATIC ZONE 135

Table 7 The partition of Tundra ecosystem biomass, ton/ha (after Rodin and Bazilevich, 1976).

Plant living Dead organic Annual net Annual litterfall

The average mass distribution of Tundra ecosystems is as shown in Table 7.The biogeochemical turnover of nitrogen is about 50 kg/ha per year A similarvalue was shown for the turnover of total mineral elements, 47 kg/ha/yr The relevantvalues for various trace and macroelements are shown in Table 8

Table 8 Annual fluxes of chemical species in the Low-Bush Moss Tundra ecosystem (after Dobrovolsky, 1994).

Chemical species Chemical species symbol Plant uptake fluxes, kg/ha/yr

Total uptake of ash elements by vegetation, kg/ha/yr 47

The flux of chemical elements per unit area in tundra ecosystems is not tional to the plant uptake Presumably, some elements, like Zn and Cu, are taken upselectively, whereas other trace elements, like Ti, Zr, V, or Y, are absorbed passively,depending on their content in the environmental media.

propor-Finally, this determines the actual and potential exposure of living organisms topollutants’ impact

CHAPTER 7

BOREAL AND SUB-BOREAL CLIMATIC ZONE

The Boreal and Sub-Boreal Forest ecosystems represent the forests of cold and perate climate These ecosystems occupy an extended zone in the northern part of theNorthern Hemisphere The total area is 16.8× 106km2, or 11.2% from the wholeWorld’s territory

tem-The general scheme of biological and biogeochemical cycling in Forest tems is shown in Figure 1

ecosys-1 BIOGEOCHEMICAL CYCLING OF ELEMENTS AND POLLUTANTS

EXPOSURE IN FOREST ECOSYSTEMSThe plant species biomass of Boreal and Sub-Boreal Forest ecosystems accumulates

a significant part of living matter of the whole planet This value is about 700 ×

106 tons of dry weight The biomass per unit area of different Forest ecosystemsvaries from 100 to 300 ton/ha and even 400 ton/ha in the Eastern European OakForest ecosystems The annual net primary productivity, NPP, varies from 4.5 to 9.0ton/ha (Table 1)

The overall biomass accumulated in Forest ecosystems per unit area is 20–50 timeshigher than the annual productivity This means that various chemical species areretained during long periods in plant biomass thus being excluded from biologi-cal or biogeochemical cycling The duration of biological cycles may be from 1.5

to>25 years for Broad–Leaved Sub-Boreal Forest and Coniferous North Taiga Forest

ecosystems, correspondingly The slow turnover rates are connected with both a lence of aboveground biomass and slow mineralization of plant litterfall on the soilsurface

preva-The microbial activity in forest soils is much more intense in comparison withTundra ecosystems Fungi, bacteria and actinomycetes play a significant role in degra-dation of carbohydrates of forest litterfall (Box 1)

137

Figure 1 Schematic illustration of the biogeochemical cycling processes in Forest ecosystems (Nihlgard et al., 1994).

Box 1 Microbial regulation in Forest ecosystems (after Nihlgard et al., 1994; Fenchel et al., 1998)

The regulation of biogeochemical cycles by microbial populations is of most directimportance in the cycling of N, S, P, and C Most of the ecosystem pool of theseelements resides as organic forms in forest floor and mineral soil compartments Theseorganic complexes are subjected to microbial transformations, which regulate nitrate,sulfate and phosphate ions dynamics and availability In turn, this influences indirectly

Table 1 Net primary productivity of Forest ecosystems, ton/ha.

Coniferous north Coniferous and mixed Broad–leaved

BOREAL AND SUB-BOREAL CLIMATIC ZONE 139

Figure 2 A tentative model illustrating decomposition interactions (Nihlgard et al., 1994).

the migration of other solutes though maintenance of ionic balances of solutions Forquantification of the role of microbes in forest biogeochemical processes, models likethat shown in Figure 2 should be applied

1.1 Nitrogen Cycle and Exposure Pathways

Since nitrogen is a nutrient, which limits the productivity of almost all Boreal and Boreal Forest ecosystems, its biogeochemical cycling is relatively well understood atpresent The major N transformations and fluxes are shown in Figure 3

Sub-Figure 3 The general nitrogen model for illustrating the biogeochemical cycling in Forest ecosystems Explanations for the fluxes: 1, ammonia volatilization; 2, forest fertilization; 3,

N2-fixation; 4, denitrification; 5, nitrate respiration; 6, nitrification; 7, immobilization; 8, mineralization; 9, assimilatory and dissimilatory nitrate reduction to ammonium; 10, leaching;

11, plant uptake; 12, deposition N input; 13, residue composition, exudation; 14, soil erosion;

15, ammonium fixation and release by clay minerals; 16, biomass combustion; 17, forest harvesting; 18, litterfall (Bashkin, 2002).

Processes of dinitrogen fixation, mineralization, immobilization, and nitrificationhave received the most attention, but there is a paucity of information on denitrifica-tion in forest ecosystems The status and fluxes of nitrogen are strongly regulated byrates of N mineralization and immobilization The rates of mineralization are greatlyenhanced after clearcutting The influence of clear cutting has been demonstrated inthe experiments at Habbard Brook Experimental Forest and in Coweeta (see Likens

et al., 1977) Over a three-year period after clearcutting a hardwood forest in HabbardBrook, forest floor organic matter decreased by 10.8 ton/ha, soil organic matter de-clined by 18.9 ton/ha and net N loss from the soil was estimated to be 472 kg/hawith an increased export of inorganic N in the stream waters of 337 kg/ha Signifi-cant alterations of N fluxes have been monitored also at Coweeta In the first 3 yearsafter clear cut and logging, soil N mineralization increased by 25% and nitrificationincreased by 200% However, only a small fraction of this mineralized nitrogen wasexported from the ecosystem The retention was owed partly to rapid revegetationand high rates of nitrogen uptake and partly due to microbial immobilization.When nitrogen input owed to mineralization and atmospheric deposition exceedsthe demand of both the vegetation and the microbes in undisturbed maturated forest

BOREAL AND SUB-BOREAL CLIMATIC ZONE 141ecosystems, the phenomenon of nitrogen saturation takes place (Gunderson andBashkin, 1994) This phenomenon is accomplished by nitrogen leaching from theforest ecosystems These nitrogen losses are highly variable, but generally sites inNorth America and Northern Scandinavia show N loss rates of<1.4 kg/ha/yr, whereas

sites in southern Scandinavia and Central Europe exhibit loss rates often>7 kg/ha/yr.

Generally, N leaching from undisturbed forest ecosystems starts when the N tion rates are higher than 10 kg/ha/yr

deposi-Fixation of molecular nitrogen, N2, to ammonia in forest ecosystems can occur

on and/or in a variety of forest substrates including plant canopy and stems, epiphiticplants compartments, wood, litter, soil and roots A recent review of the magnitude of

N inputs to forest ecosystems indicates that non-symbiotic fixation ranges from<1

to 5 kg/ha/yr and symbiotic fixation ranges from about 10 to 160 kg/ha/yr in earlysuccessional ecosystems where N2-fixing species are present

Denitrification, a dissimilatory pathway of nitrate reduction (see Section 3.3 also)into nitrogen oxides, N2O, and dinitrogen, N2, is performed by a wide variety ofmicroorganisms in the forest ecosystems Measurable rates of N2O production havebeen observed in many forest soils The values from 2.1 to 4.0 kg/ha/yr are typical forforest soils in various places of Boreal and Sub-Boreal Forest ecosystems Allin situ

studies (field monitoring) of denitrification in forest soils have shown large spatialand temporal variability in response to varying soils characteristics such as acidity,temperature, moisture, oxygen, ambient nitrate and available carbon

Thus, from the viewpoint of environmental risk assessment (critical loads) themost important exposure pathways are nitrate leaching and denitrification, which areboth very sensitive to anthropogenic pollution These links of biogeochemical nitrogencycle should be firstly quantitatively parameterized to assessing environmental risk

1.2 Sulfur Cycle and Exposure Pathways

Both inorganic and organic transformations are important in the sulfur cycle in est ecosystems The major sulfur pools and transformation processes are shown inFigure 4

For-Similarly to N, most S pools are found in organic form in forest floor and soilhumus However, unlike nitrogen, there are important abiotic processes, especiallysulfate sorption processes, which play a critical role in regulating sulfate dynamics

in forest ecosystems An example of this type of exposure pathway was shown inthe Habbard Brook whole-tree harvesting experiment, where the decrease in sulfateoutput from the watershed was attributed to sulfate adsorption, which was enhanced

by soil acidification from nitrification (see above)

Biological exposure pathway of sulfur movement in soils of forest ecosystems isrelated to microbial transformation of sulfolipids Back conversion of sulfate-S intoorganic matter immobilizes the anion and potentially reduces soil cation leaching.Processes of sulfur mineralization and incorporation proceed rapidly in response toseveral factors, including temperature, moisture, and exogenous sulfate availability

in soils and water

Figure 4 A model illustrating sulfur biogeochemical cycle in forest ecosystems (Bashkin, 2002).

1.3 Phosphorus Cycle and Exposure Pathways

The internal and external regulation of the phosphorus biogeochemical cycle in Forestecosystems is tightly coupled to soil development and the change of phosphorus poolsfrom the predominance of primary inorganic phosphorus (e.g., apatitte) to that oforganic-P, secondary-P, mineral-P and occluded-P as is illustrated in Figure 5.The organic phosphorus in forest soils is derived mainly by microbial synthesisand the accumulation of plant and animal residues plays a subordinate role Much

of the organic-P occurs in ester linkages (up to 60%) with lesser amounts in otherforms The leaching losses of P are ranged from as little as 7 g/ha/yr at HabbardBrook to up to 500 g/ha/yr for a glacier outwash in New Zealand Loss rates generallyare greatest in young, base-rich soils and lowest in acidic soils (pH< 5.0) with high

content of sesquioxides, which may fix phosphates Thus, soils at intermediate stages

of development have the highest availability of phosphorus, which is partly regulated

by microbial mineralization processes and open for pollutants’ exposure

1.4 Carbon Cycle and Exposure Pathways

The carbon biogeochemical cycle in Forest ecosystems is shown in Figure 6 Thiscycle is open and exposure to anthropogenic loading will definitely be accompanied

by transformation of many cycle links Global and regional climate change is theonly example, the eutrophication of surface waters is the second one, however inthe latest case the carbon cycle is coupled with those of nitrogen, phosphorus and

BOREAL AND SUB-BOREAL CLIMATIC ZONE 143

Figure 5 Biogeochemical transformations of phosphorus in forest soils: (a) the long-term relative development of different phosphate fractions in soils of the forest ecosystems; (b) the relative contents of P fractions in different soil types Also, the idealized carbon to organic-P rates (C:P o ) are illustrated (Nihlgard et al., 1994).

some microelements like Fe and Mo Accordingly, exposure to various anthropogenicpollutants is changing the carbon cycle and v/v the knowledge of the most sensitivelinks of C biogeochemical cycle will allow the regulation of the environmental riskassessment and risk management on a different scale

Obviously the fates of N, S and P are tightly coupled not only with each other, butalso with C dynamics of soils (Bashkin, 2002) For example, it has been suggestedthat the leaching of dissolved organic species of nitrogen sulfur and phosphoruscontributes to the accumulation of these elements in mineral soils This leaching

of organics is an important component of soil formation of Spodosols, which arecommon especially in Northern Coniferous Forest ecosystems The ratio betweentotal nitrogen and total carbon, the C:N ratio, is widely applied to predict microbialmineralization–immobilization of nitrogen in soils (Figure 7)

Typically high C:N ratios (>60) for forest plant organic matter have a major

impact on the nitrogen cycle A number of empirical and theoretical analyses haveestablished a strong linkage between nitrogen mineralization, assimilation and organicmatter decomposition In particular, C:N ratios >30 decrease mineralization and

increase assimilation instead, with the balance between two processes dependent

on the nitrogen content in microbial biomass The latter parameter sets the minimum

Figure 6 Carbon biogeochemical cycle in the hypothec forest ecosystem (Schulze, 2000).

BOREAL AND SUB-BOREAL CLIMATIC ZONE 145

Figure 7 Relationship between net mineralization and immobilization of nitrogen as a function

of substrate C:N ratio and microbial assimilation efficiency (F) for three different biomass nitrogen levels equivalent to C:N ratios of 4–8–12 (after Bashkin, 1987; Fenchel et al., 1998).

nitrogen requirement for biosynthesis per unit amount of substrate metabolized symbiotic nitrogen fixation can ameliorate nitrogen limitation, and to some extent highC:N ratios may be a determinant of soil microbial diversity

Non-However, the microbial activity is depressed during long and severe wintertime,and this leads to an accumulation of semi-mineralizable plant residues on the soilsurface With the increasing duration of cold season from south to north, the mass

of these half-destroyed remains enlarges from 15 ton/ha of dry organic matter inBroad–Leaved Sub-Boreal Forest ecosystems to 80–85 ton/ha in Northern Taiga For-est ecosystems

In the Northern Forest ecosystems, the relative content of chemical species in deadorganic matter of forest litterfall is higher than that in living biomass In Mixed and De-ciduous Forest ecosystems, this is true for the total mass of chemical species, howeversome elements are more abundant in living biomass Thus, the general biogeochem-ical feature of biological turnover in forest ecosystems is the prolonged retention ofmany chemical species in dead organic matter and exclusion from cycling (Table 2).The data of Table 3 provide a general characteristic of trace element fluxes inBoreal and Sub-Boreal Forest ecosystems

2 GEOGRAPHICAL PECULIARITIES OF BIOGEOCHEMICAL CYCLING

AND POLLUTANT EXPOSURE

2.1 North American Forest Ecosystems

In the USA, two focal points for biogeochemical research have been the forest ment ecosystems at Hubbard Brook Experimental forest in the White Mountains of

catch-Table 2 Averaged fluxes and pools of biological cycling in Forest ecosystems (after Rodin and Bazilevich, 1965; Dobrovolsky, 1994).

EcosystemsNorthern Taiga Southern Taiga Sub-Boreal Southern TaigaFluxes and pools Spruce Forest Spruce Forest Oak Forest Sphagnum SwampPools, ton/ha

Annual fluxes, ton/ha/yr

de-K and P, are expected to lower the productivity when occurring in the root zone Mostnutrients available for circulation in the temperate forested ecosystems are found inthe tree layer or in the accumulated organic mater of soil layer This is especiallytrue for the most important macronutrients (C, N, P, K, Ca, Mg and S) Nitrogen isalmost completely bound to the organic matter and when it is mineralized it is eitherleached as nitrate or assimilated and immobilized by organisms in the soil Includingthe humus horizon, the soil organic matter contains the largest pool of nitrogen in theBoreal Forest ecosystems (Figure 8)

For phosphorus and potassium this pool of organic matter is also of importance,but in the Boreal Forest ecosystems, a relatively higher amount is in the living biomass.The long-term soil development proceeds towards a lower rate of weathering in theroot zone and relatively higher amounts in biogeochemical fluxes (Nihlgard et al.,1994)

Thus, one should mention that the quantitative estimates of various links of geochemical cycles of elements and their interactions are of crucial importancefor environmental risk assessment for the given ecosystems under anthropogenicpressure