The effects of pH on carbon mineralization to CO2 and CH4 in peatlands across an ombrotrophic-minerotrophic gradient

Background/Question/Methods

Methane (CH₄)  is a major greenhouse gas, and wetlands are the largest natural source of CH₄.  Production and emissions of CH₄ vary greatly among different wetland types, and it is essential to understand fundamental controls over CH₄ production to be able to predict how future climate change will impact these fluxes.  Peatlands exist along an ombrotrophic (rain-fed only) to minerotrophic (with groundwater inputs) gradient within the landscape that reflect their hydrogeomorphic position in the landscape, and soil pH is one of several factors that co-vary along this gradient.  The efficiency of CH₄ production under anaerobic conditions varies greatly along this peatland gradient, but the underlying mechanisms that explain this variation are poorly known.  The objectives of this experiment were to determine to what extent that differences in soil pH among different types of peatlands along this hydrogeomorphic gradient control (i) total CH₄ and CO₂ production rates and (ii) the efficiency of CH₄ production vs. CO₂ production.  We adjusted the pH of peat slurries from an ombrotrophic bog, an intermediate fen, and a rich fen to 4 levels (3.5, 4.5, 5.5, and 6.5), followed by anaerobic incubation for 7 days. 

Results/Conclusions

Increasing pH significantly enhanced CO₂ production.  In the bog, CO₂ production increased 216%, 300%, and 544% when the pH was raised from 3.5 to 4.5, to 5.5, and to 6.5 respectively; the increase in the rich fen was 127%, 154%, and 209%, correspondingly.  Adjustment of pH significantly influenced CH₄ production, but the effects depended on time and peat types.  On the 2^nd day of incubation, the rates in the bog were not different among pH levels, but rates in the intermediate and rich fens were lower at pH 3.5 than those at higher pH.  On the 7^th day of incubation, CH₄ production rates in the bog at pH 3.5 were significantly lower those at  higher pH.  Likewise, the rates in the intermediate fen at pH 3.5 were significantly lower than those at higher pH.  pH effects on percentages of CH₄ production to total mineralized carbon (CH₄ + CO₂) were only significant in the rich fen.  At pH 3.5, CH₄ accounted for only 4.2% of total productions, which was significantly lower than those at pH 4.5 (11.7%), pH 5.5 (18.4%), and pH 6.5 (12.2%).  pH is an important environmental factor controlling carbon mineralization to CO₂ and CH₄ productions in peatlands.