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

More documents

Recommendations

Info

466 H. Yao, R. Conrad / Soil Biology and Biochemistry 31 (1999) 463±473 50 mM N-2-hydroxyl-ethylpiperazin-N 0 -2-ethane sulfonate, pH 7), mixed and centrifuged at 14,000 rpm for 5 min. The supernatant was measured in a spectrophotometer (Hitachi U-1100 photometer) at 562 nm. Nitrate, nitrite and sulfate were determined after centrifugation of aliquots of the soil slurries and ®ltration through a 0.2-mm membrane ®lter (Sartorius, Minisart RC15). Part of the ®ltrate was stored frozen (208C) for fatty acid analysis. Total sulfate (absorbed and dissolved sulfate) was determined after 12 h extraction at room temperature using 5 ml phosphate mixture (15 mM Ca(H 2 PO 4 ) 2 and 8 mM H 3 PO 4 ) (Nelson, 1982). Nitrate, nitrite and sulfate were analyzed in a Sykam ion chromatographic system containing a Sykam (LCA, A09) column and a conductivity detector serially connected to a UV detector at 218 nm (Bak et al., 1991). Na 2 CO 3 /NaHCO 3 (3 mM/1.5 mM) was used as the eluent. Dissolved acetate was measured using a Sykam HPLC system equipped with an Aminex HPX-87G ion exclusion column and Fig. 2. Logarithmic plot of the increase of CH 4 during anoxic incubation of 16 di€erent rice ®eld soils.
H. Yao, R. Conrad / Soil Biology and Biochemistry 31 (1999) 463±473 467 a refractive index detector (Erma, CR Inc, ERC- 7512) (KrumboÈ ck and Conrad, 1991). Production rates of CO 2 and CH 4 were determined by linear regression of the partial pressures in the headspace against time for the appropriate phases of incubation. Gibbs free energies (DG) were calculated from the actual concentrations of reactants and products as described before (Conrad et al., 1986; Peters and Conrad, 1996) using the following reactions: 4H 2 …g†‡CO 2 …g† 4CH 4 …g†‡2H 2 O…l† DG 0 …308C† ˆ128:7 kJ CH 3 COO …aq†‡H ‡ …aq† 4CO 2 …g†‡CH 4 …g† DG 0 …308C† ˆ77:3 kJ: The standard Gibbs free energies (DG 0 ) were calculated from the standard Gibbs free energies of formation of the reactants and products which are tabulated in Thauer et al. (1977), and then corrected to a temperature of 308C using the Van 't Ho€ equation and tabulated values of the standard enthalpies of formation (Lange, 1979). 3. Results When slurries of rice ®eld soils were incubated under anoxic conditions, H 2 partial pressures immediately started to increase and reached relatively high values, in some soils >150 Pa, within 3±5 d (Fig. 1). Acetate usually behaved similarly, but the pattern was not as uniform as that of H 2 (Fig. 1). In some soils highest acetate concentrations were reached at a later time, usually around d 10±20, forming a further maximum. Accumulation of CH 4 (assessed on a linear scale) started at di€erent times depending on the soil tested (Fig. 1). In some soils, linear CH 4 production became apparent as early as 2±3 d after the beginning of incubation (soil no. 3, 8, 11 and 15), whereas in other soils (no. 13 and 14) more than 40 d passed. In many but not all cases, the beginning of CH 4 accumulation approximately coincided with the end of sulfate and iron reduction (Fig. 1). Eventually, CH 4 production became constant and H 2 partial pressures and acetate concentrations reached a constant value indicating steady state conditions. Plotting CH 4 accumulation on a logarithmic scale, however, revealed a di€erent picture during the early phase of CH 4 production (Fig. 2). At this scale it became obvious that CH 4 production started right at the beginning of anoxic incubation in all soils examined. In ®ve soils (no. 3, 8, 11, 15 and 16) this early CH 4 production steadily continued until the end of incubation and also became apparent on a linear scale after a few days (Fig. 1). In all the other soils, however, the early CH 4 production came to a halt 3 d later (Fig. 2). At this time, the CH 4 partial pressure was still low (about 10±100 Pa), since only little CH 4 (about Table 2 End of the halt phase of CH 4 production and of the phases of reduction of sulfate and iron together with the Gibbs free energies of H 2 /CO 2 - dependent methanogenesis and rates of total CH 4 production End of (d) DG (kJ mol 1 CH 4 ) CH 4 production (mmol g dw 1 d 1 ) soil halt phase sulfate reduction (70 5.8 30.5 ± ± ± 14 36 70 35 3 24.5 28.8 0.14 0.15 15 0 6 12 27.9 ± 29.8 0.72 0.13 16 0 14 14 22.4 ± 28.0 0.43 0.18 ± = not applicable, since either no halt phase or no steady state was observed.
Page 1 and 2: PERGAMON Soil Biology and Biochemis
Page 3: H. Yao, R. Conrad / Soil Biology an
Page 7 and 8: H. Yao, R. Conrad / Soil Biology an
Page 9 and 10: H. Yao, R. Conrad / Soil Biology an
Page 11: H. Yao, R. Conrad / Soil Biology an

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?