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NITROGEN AND PHOSPHORUS DISTRIBUTION iN AN ALASKAN TUSSOCI\ TUNDRA ECOSYSTEM: NATURAL PATIERNS AND IMPLICATIONS FOR DEVELOPMENT

F. STU}_RT CHAPIN III and KEITH VAN CLEVE Institute of Arctic Biology, Univenity ar Aluka, Fairb~nk!s, \luka

ABSTRACT

Man's imr.act on arctic ecos~ ~~ems can be best understood in the context of alterations in the nutrient cycling proce5$.es, At. Eagle Creek, Alaska, a reprl.'sent:~tive tussock tund!a community, the bulk of total nitrogen and phosphoruiJ is contained within the soil oreanic matter. Nitrogen and phosphorus contents and cation exchange capacity (CEC) are closely correlated with organic conlent. The carbon·to·nitrogen ratio decreases and CEC increases with i:lcreasing degree of d.:!composition, Disturban~e alters patterns and 1all~l\ of nutriE-nt cyclin(t. Compaction, increases nutrient avannbility and ptima11· pro· duction by increasing thaw depth and dt:~omposition rule. Removing the vegetative cover increases nutriel'kt a!Jailability in the same ways, but nutrients are gradually leached from the system. Removing the entire org1mic mat eliminates the bulk or the nutrient capital and nutrient retaining power or the site und renders the site unstable.

The concept of the tundra as a fragile ecosystem has been popularized in the last decade as a result of increasing exploration and development in the Arctic. Disturbances that in temperate climates would be quickly revegetated by secondary succession often cause massive erosion in the Arctic and are revegetated slowly if at all (Hok, 1969; Bliss and Wein, 1972a; Billing.q, 1973; Johnson and Van Cleve, 1976). Plant growth is slow in the Arctic, but the arctic flora periodicaliy experiences natural disturbances, such as front action (Hopkins and Sigafoos, 1951), intensive lemming grazing (Dennis, 1968), and fire (Wein and Bliss, 1973). Tundra l~kes periodically drain and present opportunities for recolonization, and many tundra species are effective colonizers of such habitats. We

738

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NITROGEN AND PHOSPHORUS IN ALASKAN TUNDRA ECOSYSTEivl 739

suggest that the fragility of the tundra is a consequence of system function and state variables related to climate more than of the inability of componf''lt species to invade disturbed sites.

In this paper we present data on nutrient distribution in the soil of a tussock tundra community and discuss why the physical distribution of nutrients and the processes controlling nutrient cycling render tundra particularly susceptible to disturbance.

METHODS

Eagle Creek (latitude: 65°10'N), a tussock tundra community 125 km northeast of Fairbanks, Alask~, is described by Wein and Bliss (1974). The site, which is dominated by Eriophorum vaginatum L. subsp. spissum, is representative of tussock tundra communities over broad expanses of northern Alaska, Canada, England, Scandinavia, and Siberia. Soil samples were collected from a profile extending from the top of an E. vaginatum tussock to pennafrost. Since there were no clearly distinguishable horizons within tJ1e organic soil, the soil wru; divided arbitrarily into 10-cm slices. The mean intertussock ground surface was taken to be 0 soil depth, and, from this reference point, soil depths were labeled as either positive (up into the tussock) or negative (down toward permafrost). Soils were oven-dried, ground with a mortar and pestle, anrl passed through a 2-mm sieve. Organic matter content was detennih~ by weight loss on ignition, nitrogen by Kjeldahl digestion, phosphorus iJy nitric-perchloric acid diges­tion, cation exchange capacity with an ;;ammonium acetate extraction (Jackson, 1958; Van Cleve and Viereck, 1972), and carb:n with a carbon-hydrogen-nitrogen analyzer.

To estimate the quantity of nutrients tied up in tussocks, we measured the height and diameter of two tussocks and then cut them \}ff at mean intertussock ground level and separated them into soil, E. vaginatum ro,ots, E. vagi1aatum rhizomes, E. vaginatum live leaves, live material of l'll other species, undecayed leaf litter, and partially d~cayed leaf htter. Height:; and diameters of 100 randomly chosen tussocks were then mt;asured. The ave!'tge weight of each plant and the se:il category for the two dissected tussocks were corrected for the difference in volume between the dissected tussocks and the average Eagle Creek tussock. Weight per unit area wru; calculated assuming five tussocks per square meter (Shaver, 1976). A!l materials from the dissected tussocks were analyzed for nitrogen, phosphorus, and organic content. For the purpose of this study, nutrient pools for all live and dead plant material except roots were lumped as tussock tops. R.oots and soil pools were lumped as soil.

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740 CHAPIN AND VAN CLEVE

All vegetation was scraped from a portion of the site with a bulldozer in early May 1970. The organic mat beneath the vegetation was left intact. Thermistor probes were installed in mid-June in the disturbed area and in the adjacent control, a.'ld soil temperatures were recorded at hourly intervals for the remainder of the growing season. Thaw depth was recorded at time .nf maximum thaw. The depth of subsidence was: measured 5 years after the disturbance.

RESULTS

Tundra soils show a distinct vertical stratification of biological, chemical, and physical properties. At Eagle Creek, an int-erior Alaskan tussock tundra site, there is a distinct separation between mineral and organic soils (Table 1}, ~'1'~b as occut-s in most tundra sites. The organic holizons have the h.1ghest concentrations of total nitrogen and phosphorus (Fig. 1) and the highest cation t;xchange capacity (CEC); CEC is significantly correlated with soil organic content (P <0.05).

Even within th~ organic h~rizon there is a distinct gradient in soil propert:es which depends primarily on the state of decomposition. The organic matter at Jle soil surface is fibric, consisting largely of recognizable plant fragments. The soil grades with depth to organic material that is more thoroughly broken down. The depth progres­sion in state of decomposition is paralleled by a decrease in carbon-to-nitrogen ratio (Table 1). The mineral soil h9.s a low i percentage of organic matter, but, because of the high bulk density i of the mineral layer, this horizon contains as much or more organic

TABLE 1

PROFILE OF SOIL FROM TUSSOCK TUNDRA, EAGLECREEKJALAS~~

Soil depth,• ~oil type Orl{anie matter,t Carbon·to·

em ~ dey weiaht nitroa~n ratiot

10 to 20 Fibric 96.2 ± 0.1 53.9 ± 2.5

0 to 10 1 92.5 ± 0.7 38.8 ± 1.5

-10 to 0 89.0;;: 0.7 23.3 ± 0.9

-20 to -10 Hemic 81.2 ± 1.5 17.3 ± 0.5

-30 to -20 Mineral 13.5 ± 1.0 25.4 ± 1.9

•Positive depth values clenote distance up into the tussock; neaative depth values denote d~pth beneath the tuuoek.

tn • 1 0; ±standard error •

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NITR·OGEN AND PHOSPHORUS IN ALASKAN TUNDRA ECOSYSTEM 741

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Fig. 1 Total phoaphorua <•-> and nitrogen (O .. u) contents of a aoil profile from a tullOCk of Eriophorum vaginatum, Eagle Creek, AlukL Zero soU depth is the mean lntertussock ll'OUnd aurface. Politive dept!\ valuea denote diatan~ up into the tuuock and ne8ative valua, deQ~ bene~th it. Each value 1• an averaae of 10 determination~, lhown with atandard error {I).

matter per unit volume than the organic horizons (Fig. 2). The organic fraction within the mineral soil is primarily colloidal and consists of dissolved material leached from above.

The live biomass, including roots, constitutes less th'an 1% of the organic matt8r, nitrogen, and phosphorus of the organic horizons of the system, i.e., to a depth of 20 em beneath mean intertussock ground surface (Table 2).

'fhe vegetation and litter associated with the tops of tussocks constitute only 1% of the organic matter and 0.4% of the nitrogen and phosphorus in the ecosystem if the permafrost surface is defined as the bottom of the ecosystem (Table 3). The tussocks as a unit comprise only 4.4% of the organic matter of the ecosystem. The organic material to a depth of 10 em beneath the tussocks contains 25% of the nitrogen and phosphorus capiial of the site. The organic horizons as a unit comprise 7 4 and 68% of the nitrogen and phosphorus capital of the site, respectively.

Disturbance was observed at two levels of intensity in upland tussock tundra h1 the Eagle Creek area. Where tundra was compacted by multiple passes of small tracked vehicles, there was a siight depression in the soil surface and increased flowering by E .

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Fi~r. 2 t;ation excbanae capacity (CEC) (.__....)and or1anic content (0--0) per unit volume of a aoil profile from a tuiiOCk of Eriophorum vaginatum, Eagle Creek, Aluka. Each 'VIllue ia t.be averaae of 10 det.enninatlona, ahown with atan ... ard error (1). Positive depth valuea denote diatanc:e up h1to the tussock and neeative value~r d,ptb beneath it.

vaginatum. Along slopes of high-centered polygons and near roadside ditches, where there was greater groundwater movement, E. vagina-

tum also flowered profusely. When tussocks and vegetation were removed but the lower

portion of the organic mat was left intact, the1e were deeper thaw, warmer soil temperatures, and -60-cm subsidence of the general ground surface (Table 4). Seedlings of E. vaginatum became estab· lishcd on this plot as soon as there was a local seed source. These seedlings grew to several hund:ed tillers within 2 years. Most tussocks in the disturbed site suffered heavy winter grazing by rodents. Virtually all overwintering greeil leaves were clipped off, and abundant plant litter and rodent feces covered the ground. In contrast, tussocks in adjacent undisturbed tundra showed little evidence of grazing.

Movement of water over the ground surface was observed in the disturbed area after snowmelt and after periods of heavy precipita· tion, particularly in the first 2 to 3 years after disturbance. The

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TABLE 2 . . , . '

TOTAL CARBON, NITROGEN, AND PHOSPHORUS TO A DEPTH OF 20 em IN VARIOUS 2l

·.· ECOSYSTEMS AND PERCENTAGES OF EACH ELEMENT IN VEGETATION =i :n 0

Total quantity .;n Cl

J system," r,/m'l. Percent in veeetation, m z

,, . Phos· f'boa·

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I Ecosystem Latitude Sit.e Carbon Nitrogen pborus C;ubon Nitrogen phorus Ref~rer.cc; 0

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I Tussock tundra 65° Eagle Creek, 18,767 964 48.7 (•·.6 0.4 0.3 This study :1:

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Alac;ka . l'n I c' · ,-

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Wet meadow 71° Barrow, 1!1,7SQ 972 63.4 1.7 1.0 1.5 Chapin an\.3 !Ft. J, Bars~!.-1,•;;;., J.: 0 t

tundra Alaska un~tl;'.:Jt~shed data :n I Subalpine heath 47° Washington

c en

1 residucJ soil 13~379 <)79 2.2 0.5 Gr~er, 1973 z lJrganic soil 32,096 1,276 2.7 0.0 )>

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Red alder 64a Failibanks, .'1.276 154 H6 76.l 3e.o 3.3 Van Cleve, Viei'eck, I

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I I Van Clewe and Viereck, ~ l· I )>• ll ;·. 1972 z

Ren alder 46° c.~.;cades, Wash. 19,52•t 701 4.fJ ". 9.e Cole, Tur.ner, -t I (35 years)

and Bledsoe, 1977 c z Douglas fir forest 46° Cascad~. Wa.:n 12,276 160 16fl 73.7 20.4 4 .. 0 Cole.,, Gessel, and e

{35 yf'ars) Dice, 1967

:n )>

Gras.sland Bokhari and Singh,1975 m

Pawnee 40° CoJ,or:.Hiu 140 5.5 n, 0

('~~ttonwood 43° South Dakota 275 4.0 (/)

-< ll!sage 36° Oklahoma 261 2.1 en

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O~k forest 35° Okl<nh•>ma 12.884 250 37.7 75.0 4:l.3 25.2 Johnson and Risser, m 1974 ~

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TABLE 3

NITROGEN AND PHOSPHORUS CONTENT OF EACH HORIZON AND EXTEN'f OF NUTRIENT LOSS IF HORIZON IS REMOVED FROM SITE DURING DISTURBANCE*

Organic matter Nitrogen Phosphorus Organic matter, Nitroge!'l, Phosphorus, loss, loss, lo:>S,

Horizon . g/m2 g/m2 g/m2 %of total %of total %of total

Tussock topst 602 6 0.3 1.0 0.4 0.4 Tussock soil:f: 1,378 12 0.8 3.4 1.4 1.5 0 to-10 em 15,800 324 16.7 30.2 26.0 24.7 -10 to-20cm 21,300 624 30.9 66.5 73.6 67.6 -20 to-30cm 19,700 349 23.3 100.0 100.0 100.0

ToU:Al 58,780 1,315 72.0

*We assume that removal by disturbance entaHs loss of the given horizon pius aU horizons above it. Calculations ignore new nutrient capital made available by melting of permafr()l)t.

tAU live and dead material in and above the rhizome zone· of a tus.oock. :f:Soil bt!twt!en bottom of rhiwmes and 0 soil depth (mean intertussock ground surface), includes E.

vaginatum roots. Loss of material with this horizon igm>res loss of intertussock vegetation, which was not measured.

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NITROGEN .t~NO PHOSPHORUS IN ALASKAN TUNDRA ECOSYSTEM 745

TABLE 4

PHYSICAL PROPERTIES OF SOILS IN CONTROL AND ADJACENT TUSSOCK TUNDRA F·ROM WHICH

VEGETATION WAS REMOVED Bl.r£ ORGANIC MAT LEFT INTACT

Property Con·trol Disturbed

Soil thaw depth,* em 38.6 ± 0.7 56.9 ± 0.4

Soil tempetaturet 0 at 5 em, C 3.7 9.8

Subsidence;~ ~m 0 -so ·----~·--

*On August 21 in year of disturbance (n • 100}. tJuiy mean in yeax- of disturbance. :j:Depth below mean intertussock ground surface after 5 years.

undisturbed tussock tundra downslope from the bladed area was characterized by highly productive tussocks of E. vaginatum which flowered profusely.

DISCUSSIO·N

Nutrients limit primary prodr.ction in every tundra community that has been examined (Warren Wilson, 1957; Schultz, 1964; GGodman and :Perkins, 1968; Haag, 1974; Chapin, Van Cleve, and Tieszen, 1975). In each of these studies nitro-gen and/or phosphorus were shown to be particule~ly limiting to· plant growth. The phosphorus capital of tundra areas tends to be low, but the total nitrogen present in tundra systems is substantial ('l'able 2). It is primarily the slow rate of nutrient cycUng rather that" a small total nutrient quantity in the system which limits the availal 1'ty of these elements to plants. ·

Marks and Bormann (1972) pointed out the importance of biomass as a reservoir that retains nutrients within \the system. In tht! tundra, however, biomass is relatively small. It is primarily the dead organic matter in the soil which Ferves the nutrient·retaining function (Table 2) by (1) structurally binding a large percentage of the nutrients, (2) providing exchange sites for cations that move through the soil during runoff, and (3) physically preventing thermokarst development (soil thermal erosion).

The impnrtance of soil organic matter as a nutrient reservoir has implication:; for man's manipulations of the tundra. Human distur-

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746 CHAPIN AND VAN CLEVE

bance is nov occurring in the Arct.ic on progressively larger scale.•;. Calculations from nitrogen balance studies carried out at BarrO\·:. Ala3ka (Barsdate and Alexe.nder, 1975), suggest that the nitrogen contained in the top 20 em of soil organic matter at this site would require 12,500 years to replace at current fixation rates. It is not known how rates of fixation on a disturbed mineral soil would differ from CUj,Tently observed rates. In undisturbed tundra blue-green algae associated with moss are responsible for the bulk of the fixation (Alexander and Schell, 1973). These would be removed if the veg~retion were stripped off. Although phosphorus derives primarily from parent material rather than from gaseous input, removal of the organic mat would deplete the r.ystem of the bulk of the circulating phosphorus pool. This study a.11d that by Van Cleve (1977) show that the bulk of the phosphorus canital in a variety of tundra and taiga sites in northern Alaska is a.ssoc!ated with the organic rather than the mineral horizons. Moreover, rates of weat~.1ering of parent material that might release more phos'(lhorus into active circulation are extremely slow (Hill and Tedrow, 1961). It is not known to what extent the nonexchangeable inorga...nic phosphorus fraction contained in deeper mineral horizons would enter active circulation if organic phosphorus pool5 were stripped from the tundra.

To fully understand the implications of disturbance, we must not view problems simply as engineering or agronomic issues but must consider them within the context of nutrient cycling. The impact of disturbance in the Arctic extends beyond the immediate removal of nutrient capital. Three levels of disturbance will be examined here. The initial effect on the system is similar in all cases, but the outcome differs dramatically depending on the relative changes in different componenta of the nutrient cycle.

When the tundra is compacted by a few passes of tracked vehicles, there is a thinner insulating layer of vegetation and litter, and the thermal conductivity of the organic horizons incre~es. This leads to warmel soil and deeper thaw (Fig. 3) (Challinor and Gersper, 1975; Miller, 1976). Melting of ice from formerly frozen ground leads to minor subsidence, such as that noted at Eagle Creek, and to a tendency for influx of groundwater from surrounding areas. Unless the compacted trail parallels slope contours, groundwater tends to move downhill along the zone of compaction, thus adding more heat to the system and further increasing thaw depth. These purely physical changes in the soil environment enhance nutrient uptake by plants, especially deep·l·ooted species like E. uaginatum or E. angusti{olium, by (1) adding formerly untapped frozen soils to the zone of root penetration (Challinor and Ger~per, 1975), (2)

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NITROGEN AND PHOSPHORUS IN ALASKAN TUNDRA ECOSYSTEM 747

cm.1PACTION

l NITROGEN A~t 0 PHOSPHORUS AVAILABILITY

Fii. 3 Changes in nutrient cycling resulting from compaction of tuss.>ek tundra hy tracked vc~hicles. Arrows between box~• indicate &

causal relationship, and arrows within boxes indicate change in size of the compartment.

increasing the rate of nutrient movement past the roots, and (3) increasing nutrient uptake through higher soil temperature (Chapin, 1974). Unless the vegetation is severely damaged, plants and microorganisms are probably effective in absorbing the available nutrients so that le9.ching losses are minimal.

Compaction also enhances the biotic components of nutrient cycling. Warmer soils exhibit higher rates of decomposition, releasiug nitrogen and phosphorus from the large organically bound pools. As carbon is respired and lost, nitrogen and phosphorus become more concentrated within the organic matter. Increased decomposition also leads to an increase in CEC (Van Cleve and Noonan, 1975; Kononova, 1966), which further increases the capacity of the organic horizon to retain cations such as NH~, Ca2 +,and K+. The overall effect is to release organically bound nutrients and to retain them on the larger cation exchange complex in a form readily available to plants. These trends in .soil properties are seen in the undisturbed soil profile at Eagle Creek, where more thoroughly decomposed soil horizons hliw-e higher CEC, lower carbon-to-nitrogen ratios and higher percentages of nitrogen and phosphorus. Increased nutrient availabil­ity leads to incre:lSed production and flowering, especially in E. vcgincztum (Bliss and Wein, 1972b). The higher nutrient content of the vegetation can attract sr:!all mammals that clip the vegetation and

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t NUTRIENT RETENTION

CHAPIN AND VAN CLEVE

VEGETATION REMOVAL

l DECREASEDl INSULATION AND ALBEDO

'

NITROGEN AND PHOSPHORUS AVAILABILITY

NUTRIENT LOSS BY EROSION

SUBSIDENCE

lGROUNDWATERJ INPUT ··1

Fig. 4 Effect of. vesetation removal on nutrient cycling in tus.soek tundra. Arrow• between boxes indicate a causal relatioruihip, and arrows within boxes indicate change in size of the compa"'tment. Thicknesa of arrow indicates .. magnitude o~ effect. Successful revegetation by exotic apeciea depends on many factors. {See discussion in the text and in Van Cleve (1977).}

deposit feces and urine, ru; seen, e.g., in the rapidly growing seedling tussocks in the disturbed site at Eagle Creek. Since feces and urine contain nitrogen and phosphorus in readily available form, this further accelerates the cycling of nutrients and increases primary production. Whether the enhanced productivity of the tundra system above its normal state is desirable depends on subjective criteria.

If the vegetation is removed but the organic mat is left inta~t. Lhe nutrient cycle experiences the same type of alterations descr:bed for compaction, but biotic and abiotic aspects of the cycle assume different imvortance, so that tl1e overall consequence is quite different (Fig. 4). The positive feedback loop by which the warmer soil resulting from removal of vegetative insulation leads to d~eper

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NITROGEN AND PHOSPHORUS IN ALASKAN TUNDRA ECOSYSTEM 749

thaw (which, in turnt leads to subsidence and increased groundwater input) was obser;~::d at Eagle Creek and elsewhere (Haag and Bliss, 1973). Another result is greater nitrogen and phosphorus availability than if thaw depth were not increased. Likewise, soil temperature is increased more than it would be if the ground were simply compacted because of the decreased albedo and loss of vegetative insulation. If soil moisture remains adequate, thi~ greatly increases decomposition rate and the biotic phase of the nutrient cycle (Van Cleve, 1977). The overall effect of the increased nutrient availability is a gradual leaching in the increased groundwater flow. This was quite evident at Eagle Creek, resulting in greatly increased productiv­ity of E. vaginatum in undisturbed tundra downslope from the disturbance.

Restoration of a "normal" nutrient cycle in an organic disturbed site depends on successful plant establishment, either from rhizomes remaining in the organic mat or from seedling establishment. Seedling establishment, in turn, depends cc:osiderably on seed source and soil moisture since the unvegetated organic horizon can dry out exce~CJivel~. Some native species, such as E. vagina tum, appear to be considerably .10re successful than exotics in establishing such sites (F. S. Cha1 .t and M. C. Chapin, unpublished data). The rapid vegetative growth of such seedlings is good indirect evidence of greatly increased nutrient availability in the site.

If the entire organic mat is removed, as has traditionally been the case in arctic disturbances, the nutrient cycle is distorted in quite a diffe!"e·tt fashion.(Fig. 5). The positive feedback loop of deeper thaw leading to uoundwater input dominates the cycle and leads to loss of nutrients by erosion (Hok, 1969). The organic matter present in the mineral soil is primarily dissolved and colloidal in nature and is readily leached. Warmer soils lead to increased decomposition of the organil! matter that remains, resulting in further availability of nitrogen and phosphorus. If there are no seedlings to exploit this nutrient flush, the nutrients may ~ quickly leached from the system because of the low exchange capacity of the mineral component of thP system. The rate of leaching d~pends on the nutrient in question and is great.ly affected by pH, etc. If surface stability can be attained, ·revegetation of mineral soil may depend heavily on supplementing the existing nutrient capital with fertilizers. Since these mineral soils generally ha.ve low clay mineral content [14% or less clay content (Van Cl~ve, 1977)] and are poor at retaining nutrients, fertilizer addition to mir.eral surfac~s may be inefficient and can lead to eutrophication of streams and to a continued requirement for fertilizers on the disturbed sites (Van Cleve, 1977 ). Such a mineral

•

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750 CHAPIN AND VAN CLEVE

NO

NUTRIENT LOSS BY EROSION

_____ _, I

NITROGEN AND ?HOSPHORUS LOSS BY EROSION

REVEGETATION

NUTRIENT RETENTION

NUTRIENT RETENTION

Fig. 5 Effect of removinl the oraanic mat on nutrient cyclin1 in tuuock tundra. The upper half of the diaaram lhowa cilanaes :n nutrient cyclina, and the lower half ahowa key factors infi\!encinl Leve1etation and auhlequent effecta on nuttient cycling. T~lickneu of arrow• indicatea macnitude of effect or probabUlty of occunence. Duhed linea indicate uncertain resulta (aee dilcu•ion in the text),

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NITROGEN AND PHOSPHORUS IN ALASKAN TUNDRA ECOSYSTEM 751

system can return to steady state in the tundra only by (1) losing the bulk of its remaining nutrients so that nutrient losses are small, or (2) building up an organic mat capable of retainin~ nutrients within the system, a process that may require centuries. 'fhe problems associ­ated With revegetation of taiga and alpine and C0.1Stal tundra sites depend considerably on soil moisture, soil ice content, and species composition of the surrounding community and are discussed elsewhere (Vnn Cleve, 1977).

The change from a nutrient-conserving, tight nutrient cycle to cycles that ar1~ progressively morE1leaky as disturbance becomes more severe may bo characteristic of temperate as well as arctic systems. In the Arctic, however, the soil has a small finite ability to supplement the e>tisting J:AUtrl.ent !:.'1.1pply because the environment severely limits the rate at which parent material and atmospheric sources can be tapped. Hence, the consequences of severe disturbance in the Arctic are particularly serious. These fa~ts should be carefully c0!'::!dered when 1oanagement criteria are established in the developing tundra regions of the world.

ACKNOWLEDGMENTS

We gratefully acknowledge the financial support of the Alyeska Pipeline Service Company, the U. S. Army Cold Regions Research and Engineering Laboratory, and the U. S. Army Research Office.

REFERENCES

Alexander, V., and D. M. Sch~'l, 1973, SeMOnal and Spatial Variation in Nitroeen Fixation in the Barrow, Aluka, Tundra, Arct. Alp. Res., 5: 77·38.

Barsdate, R. J., end V. Alexander, 1975, The Nitrogen Bt-.!ance of Arctic Tundra: Pathways, Rates, and Environmental lmplicatione, J. Environ. Qual., 4: 111·117.

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752 CHAPIN Ai-40 VAN CLEVE

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York.

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NITROGEN AND PHOSPHORUS IN ALASKAN TUNDRA. ECOSYSTEM 753

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