The importance of humic reduction in anaerobic microbial carbon cycling in peatland soils

Background/Question/Methods

Wetlands store approximately one-third of the terrestrial soil carbon in their soils and these ecosystems are responsible for between 15% and 40% of the global flux of methane, a potent greenhouse gas.  Despite their importance in the global carbon cycle, a large fraction of anaerobic decomposition in many wetland soils cannot be explained by any measured microbial process.  A growing body of evidence suggests that humic substances can serve as organic terminal electron acceptors in microbial respiration, and that this process might account for the unexplained decomposition observed in wetland soils while competitively suppressing methane production in these systems.  We explored the importance of humic reduction in peatland soils along an ombrotrophic-minerotrophic gradient by comparing the electron shuttling capacity of soils subjected to biologically- and chemically-reducing conditions.  Rates of anaerobic carbon mineralization (as carbon dioxide and methane production) were measured in the same soils.  We hypothesized that: (1) changes in the electron shuttling capacity of the soil humic pool would be most dramatic in ombrotrophic (bog-like) peatlands which are characterized by high rates of unexplained decomposition, and (2) methane production in all soils would be suppressed until the soil humic pool was fully reduced.

Results/Conclusions

As we hypothesized, the humic pool in ombrotrophic (bog-like) soils became progressively reduced through time (as measured by a change in electron shutting capacity of these soils).  Rates of methane production in these soils increased only after the electron shuttling capacity of biologically-reduced soils reached the capacity measured in chemically-reduced soils, suggesting the humic pool has been fully reduced.  In contrast, there were small changes in the electron shutting dynamics of the soil humic pool in more minerotrophic (fen-like) soils which produced larger amounts of methane over the course of our experiment.  These results demonstrate that humic substances in wetland soils, especially ombrotrophic peatlands, can competitively suppress methane production by serving as alternative electron acceptors for anaerobic microbial respiration.  This suggests that humic substances may be key mediators of carbon storage and methane production in wetland ecosystems.