Yao und Conrad - 1999 - Thermodynamics of methane production in different

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Yao und Conrad - 1999 - Thermodynamics of methane production in different

464

H. Yao, R. Conrad / Soil Biology and Biochemistry 31 (1999) 463±473

(Fetzer and Conrad, 1993). Roy et al. (1997) observed

that Italian rice ®eld soil immediately started to produce

trace amounts of CH 4 after the soil was ¯ooded.

Inhibition experiments suggested that this early CH 4

was produced by H 2 /CO 2 -utilizing methanogens

which apparently were not inhibited either by the

high redox potential or by the presence of inorganic

oxidants such as Fe(III) and sulfate at this early

stage of ¯ooding.

To better understand the chain of events that ®nally

leads to vigorous CH 4 production, we investigated 16

di€erent rice ®eld soils which were incubated as anoxic

slurries. We measured the concentrations of reactants

and products of the H 2 and acetate-dependent methanogenesis

and calculated the thermodynamic conditions

for these reactions after onset of anoxic

conditions.

2. Materials and methods

Soil samples were obtained in autumn or winter, 3±4

months after rice harvest. The soil was broken into

lumps of 3±4 cm dia, air dried and stored in darkness

at 48C until experiments were performed. The soils

have been described by Yao et al. (1998). The main

characteristics are given in Table 1. The aerobic storage

of dried soil has been shown to have no signi®cant

e€ect on soil methane production capacity

(Mayer and Conrad, 1990). For experiments, soil

samples were pulverized and passed through stainless

steel sieves to obtain soil particles between 0.1 and

1 mm dia. Ten grams of soil and 10 ml of distilled

water were put into a 120-ml serum bottle. The distilled

water was previously bubbled with N 2 for 30 min

to drive out all dissolved O 2 . The serum bottles were

closed with butyl rubber stoppers and the headspace

was ¯ushed with N 2 at a rate of 300 ml min 1 for at

least 20 min. The incubation temperature was

3020.38C. All measurements were carried out in triplicate.

One set of bottles (n = 3) was used to follow the

partial pressures of CH 4 , CO 2 and H 2 . Gas samples

were taken from the headspace of the bottles, after vigorous

shaking by hand for about 30 s, using a gastight

pressure-lock syringe which had been ¯ushed

with N 2 before each sampling. Analysis of CH 4 and

CO 2 was performed on a Shimadzu GC 8A gc

equipped with an FID and a catalytic converter

(Chrompack, nickat methanizer). The separation column

was a 80-cm long Porapak Q 60±80 mesh column

operated at a temperature of 508C. Analysis of H 2 was

performed on a Trace Analytical RGD2 HgO±Hg

vapor conversion detector. Hydrogen was separated

from other gases using a molecular sieve 5A column

(80±100 mesh, 70 cm length) at 608C (Conrad et al.,

1987).

Another set of bottles (n = 3) with soil slurries was

used for determination of dissolved compounds. The

soil slurries were repeatedly sampled (1±2 ml) by a

syringe, after heavy shaking of the bottles by hand.

The pH and E h of the soil slurries were measured

with a pH-meter and a Pt-electrode (Wissenschaftlich-

Technische WerkstaÈ tten GmbH, PH-539). E h readings

were corrected by a reference electrode (210 mV). Iron

was measured by taking subsamples of 300 ml which

were then added to 5 ml of 0.5 N HCl and kept for

more than 2 h at room temperature. For Fe(II) determination,

50±100 ml of the HCl suspension was added

to 1 ml Ferrozin solution (0.1% w/w Ferrozin in

Table 1

Characteristics of the soils including the amounts of reducible inorganic electron acceptors

Soil Origin Initial

pH

Initial

E h

Organic

carbon (%)

Total

nitrogen (%)

Nitrate

(mmol g dw 1 )

Fe(III)

(mmol g dw 1 )

Sulfate

(mmol g dw 1 )

1 Zhenjiang 7.7 460 1.04 0.07 0.16 146 0.85

2 Changchun 6.0 510 1.68 0.14 0 170 0.48

3 Guangzhou 5.1 340 1.85 0.13 0 89 0.84

4 Beiyuan 7.4 360 1.34 0.09 1.41 87 5.18

5 Jurong 6.3 460 1.15 0.10 0 196 1.16

6 Shenyaong 6.7 450 1.35 0.07 0.01 163 1.00

7 Qinghe 7.6 390 0.95 0.08 0.54 76 0.51

8 Buggalon 5.9 465 1.97 0.16 0.01 153 1.20

9 Luisiana 5.1 455 1.65 0.16 0.02 420 1.11

10 Maahas 6.2 460 2.14 0.17 0 311 1.73

11 Pila 6.8 400 2.62 0.30 1.66 202 1.16

12 Gapan 6.0 505 1.51 0.12 0.01 277 0.37

13 Urdaneta 6.7 430 1.07 0.07 0.32 153 0.28

14 Maligaya 5.8 480 1.39 0.11 0 205 1.54

15 Pavia 6.1 440 0.81 0.07 0.45 86 0.45

16 Vercelli 6.0 480 1.55 0.14 3.69 166 2.07

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