/ 5 pages
Contribution of nitrogen fixation to nitroge... (1988)
Search


299

2 T„,„ 1978 4 T„,„ 1979

JULY

AUGUST

SEPTEMBER

Fig. 1. Soil temperature profile (at 5 cm soil depth) for the growing
seasons 1978 and 1979

Table 1. Maximum values for nitrogenase activity (nmol ethylene
production per pot 24 h" 1 ) of intact Systems under laboratory and
field conditions

Field

Plant species 3

Laboratory

Naturally occurring:


Carex curvula

525

Festuca halleri

24

Avena versicolor

<1

Veronica bellidioides

135

Geum montanum

180

Homogyne alpina

105

Leucanthe mopsis alpina

750

Potentilla aurea

150

Silene acaulis

<1

Minuartia sedoides

360

Transplanted:


Trifolium badium

-

T. pallescens

-

Lotus alpinus

-

217

150

75

14xlO 3
14xl0 3
12xlO 3

nomenclature follows Flora Europaea

seasons of 1979 (27-29 August, 18-20 September, 21 Oc-
tober) and 1980 (29-31 July, 14-16 August, 6-7 and 27-
28 September) using the acetylene reduction assay. Cores
containing soil and plants of various dominant species of
the Caricetum curvulae were excavated and transferred in¬
tact to plastic pots (13 cm diameter, 11 cm high) 4 weeks
before the first ÄRA in 1979. The pots were sunk in the
ground and exposed to the natural environment for the
duration of experimental period.

For the ÄRA clear plastic domes (plant propagator
No 312, Stewart Plastics, Ltd., Croydon, England) were
sealed onto the rims of all pots using adhesive tape. The
drainage holes of the pots were plugged with rubber stop-
pers. A syringe was inserted through the Stoppers to with-
draw 10% by volume of the air from each pot. This volume
was replaced by injecting acetylene into the pots. Immedi-
ately after injecting acetylene, gas samples were taken from
the sealed chambers using evacuated 5 ml vacutainer tubes
(B-D Vacutainer, 4507F, Becton, Dickinson-France S.A.
38043 Grenoble, France) fitted with double needles (B-D
Vacutainer needle no 05741-11). Following this initial sam-
pling the sealed Systems were incubated in the presence of
acetylene for up to 48 h, further sampling of the gas within
the pots being carried out at intervals during the incubation.
Pots receiving no acetylene were included as controls as
recommended by Postgate (1972) in order to determine nat¬
ural levels of ethylene production. Vacutainers containing
the sampled gas were transported to the laboratory where
they were stored at 4° C prior to analysis by the Standard
GC method.

Transfer experiments

The Fabaceae Trifolium badium, T. pallescens and Lotus
alpinus, which occur naturally at altitudes below the Carice¬
tum curvulae zone, were carefully excavated from their habi-
tat at 2000 m, potted and transferred to the Caricetum cur¬
vulae. They were then subjected to the ÄRA procedure as
described above.

When the in situ analyses were completed at the end

of the 1980 growing season, all pots except those with Faba¬
ceae plants were transferred to the laboratory where further
ARAs were carried out at higher temperatures (22°-25° C).

Soil and plant analyses

Soil cores with representative groups of individual plant
species were collected at three different locations during
the growing season of 1980 and were transported in cool
boxes to the laboratory for analysis. Water content, pH,
nitrate, and ammonium levels of the soil were determined.
Nitrate was extracted with distilled water and determined
by the phenol disulfonic acid method. Exchangeable ammo¬
nium was extracted with 2N HC1, distilled in the presence
of MgO and determined by the Nessler procedure. All these
procedures followed Allen et al. (1974). Organic nitrogen
of soil and plant materials was determined by a modified
semimicro-Kjeldahl method (Bremner 1960). After soil
samples had been air-dried, sieved and ground to a fine
powder, 200 mg portions of each sample were digested at
350° C on a metal block heater. Prior to analysis plant
material was freeze-dried for 24 h and ground to a fine
powder in a mortar mill. 25 mg of each sample were di¬
gested as described above.

Results

Nitrogenase activity of intact plant-soil Systems
in situ

A total of about 600 samples were analysed for ethylene
content. The results are summarized in Table 1. Only 5%
of all samples contained ethylene. The highest values were
obtained in pots of L. alpina and V. bellidioides where levels
of ethylene production reached 150 and 217 nmol 24 h" 1
per pot respectively. In contrast, pots containing F. halleri,
A. versicolor, S. acaulis, or M. sedoides produced only traces
of ethylene (< 10 nmol 24 h" 1 per pot). Using the maxi-
mum values for L. alpina and V. bellidioides and extrapolat-
ing from the rate per pot to a square meter, and a conver-