ALO docView - Contribution of nitrogen fixation to nitrogen nutrition in an alpine sedge community (Caricetum curvulae) (1988)

Oecologia (1988) 76:298-302

Oecologia

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Contribution of nitrogen fixation to nitrogen nutrition

in an alpine sedge Community {Caricetum curvulae)

H.-P. Holzmann and K. Haselwandter

Institut für Mikrobiologie, Universität Innsbruck, Technikerstrasse 25, A-6020 Innsbruck, Austria

Summary. In situ acetylene reduction assays (ÄRA) were

carried out over two growing seasons at 2550 m in the upper

alpine zone of the Tyrolean Central Alps of Austria. For

comparative purposes, some Fabaceae species introduced

into the upper alpine zone from lower elevation (2000 m)

were subjected to ÄRA. At the end of the growing season

the potted plants were transferred to the laboratory where

their acetylene reducing activities were measured again. In

situ nitrogenase activity is very low. The highest values were

found in association with Leucanthemopsis alpina and Ver-

onica bellidioides (150 and 217 nmol ethylene 24 h" 1 per

pot respectively). Higher levels of activity were detected

in pots transferred to the laboratory (maximum value

750 nmol ethylene 24 h~ 1 per pot; assay temperature about

12° C higher than in the field) and in the Fabaceae trans¬

ferred to the upper alpine zone (14xl0 3 nmol ethylene

24 h ~ l per pot of Trifolium badium and T. pallescens). Max¬

imum nitrogen input in the field is in the ränge of 8 mg m~ 2

a" 1 . Therefore, under natural circumstances biological ni- Study site

trogen fixation contributes only very small amounts of ni¬

trogen to this alpine Vegetation System, the process being

inhibited by low soil temperatures. Possible alternative

sources and patterns of N acquisition are discussed in rela-

tion to the overall nitrogen economics of plants of the upper

alpine zone.

Key words: Nitrogenase (acetylene reducing) activity - Mi-

crobial ecology - Soil - Plant analyses - Nitrogen economy

Biological nitrogen fixation has previously been investi-

gated in several nutritionally and climatically stressed

arctic, subarctic, and alpine environments. The results indi-

cate that in most cases biological nitrogen fixation is low,

and primarily due to the activity of cyanobacteria asso-

ciated with mosses or fungi (Blasco and Jordan 1976; Jor¬

dan etal. 1978; Henry and Svoboda 1986), or rhizobia in

symbiosis with plants (Labroue and Carles 1977; Tosca

and Labroue 1981). The estimated N inputs reached up

to 460 mg N m~ 2 a" 1 . Haselwandter et al. (1983) working

in the nival zone (3100-3200 m) of the Central Austrian

Alps showed that N 2 fixation yielded only small quantities

of N compared with those obtained from snow-melt water,

and that the latter source was sufficient to meet the require-

ments of nival zone plants during their short growing sea¬

son. At lower elevations with longer growing seasons plant

communities dominated by grasses and sedges are found

and it is possible that there, in the upper alpine zone

(2400-2700 m), biological nitrogen fixation may be more

important. In studies of a Carex curvula Community on

acid soils of the upper alpine zone Rehder and Schäfer

(1978) showed that rates of N mineralization were low.

Under these circumstances biological fixation could repre-

sent an important additional source of nitrogen for the

plants. In this study we aimed to determine the role of

nitrogen-fixing microorganisms in the nitrogen budget of

the upper alpine zone.

In addition to studies of the natural population of the

Caricetum curvulae, we also analysed the N 2 fixing activity

of some Fabaceae which were transferred into the Carice¬

tum curvulae for comparative purposes.

Materials and methods

Offprint requests to: H.-P. Holzmann

The Caricetum curvulae studied is located at Obergurgl

(46°52'N, 11°2'E) near to the summit of Hohe Mut (eleva¬

tion 2550 m) in the Tyrolean Central Alps of Austria. A

detailed description of the Vegetation is provided by Grab¬

herr et al. (1978). Thirteen plant species (with their sur-

rounding rhizosphere soil) characteristic of this association

were selected for the present study: the dominant sedge

Carex curvula, the subdominant grasses Festuca halleri, Or-

eochloa disticha, Avena versicolor, Luzula lutea, Poa alpina,

and the herbs Veronica bellidioides, Geum montanum, Ho-

mogyne alpina, Leucanthe mopsis alpina, Potentilla aurea,

Primula glutinosa, Silene acaulis, and Minuartia sedoides.

Soil temperatures were recorded continuously at 5 cm

soil depth in summer 1978 and 1979 by means of Pt resis-

tance thermometers. Temperature profiles are presented in

Fig. 1. For calculation of polynomial regression lines daily

means of maximum or minimum temperature were used.

While acetylene reduction assays (ARAs) were carried out

soil temperature (5 cm soil depth) was registered under mid-

day radiation conditions inside and outside the reaction

chamber using a maximum-minimum thermometer. From

these measurements we calculated a mean difference of

1.5° C between undisturbed soil (8.6° C + 4.1) and soil en-

closed in reaction chambers for ÄRA (10.1° C + 4.7).

Acetylene reduction assays

Nitrogenase activity in soils associated with plants of the

Caricetum curvulae was measured in situ during the growing