Top-down control of carbon sequestration: grazing affects microbial structure and function in salt marsh soils

Area of Science:

• Environmental Science
• Microbial Ecology
• Biogeochemistry

Background:

• Tidal wetlands are crucial carbon sinks, but research on land-use impacts, like grazing, on carbon sequestration is limited.
• Existing studies primarily focus on global-change factors (sea-level rise, temperature) affecting carbon cycling in wetlands.

Purpose of the Study:

• To investigate the mechanisms by which large herbivores influence organic matter decomposition in tidal wetlands.
• To understand direct animal-microbe and indirect animal-plant-microbe interactions in grazed versus ungrazed wetland soils.

Main Methods:

• Quantitative PCR (qPCR) to assess bacterial and fungal gene abundance.
• Fluorometric assays to measure microbial exo-enzyme activity.
• Field studies at two long-term experimental sites on the German North Sea coast.

Main Results:

• Grazing consistently increased the fungi-to-bacteria ratio by 38-42%, indicating a profound impact on microbial community structure.
• Grazing slowed microbial exo-enzyme activity, reducing carbon cycling enzyme activity by 28-40%, potentially through altered soil redox chemistry.
• A novel indirect interaction: herbivore control of vegetation structure influences allochthonous organic matter trapping and microbial community composition.

Conclusions:

• Livestock grazing significantly alters tidal wetland microbial ecology and biogeochemistry, affecting carbon turnover and sequestration rates.
• The findings highlight the importance of considering land use in predicting wetland carbon sequestration capacity.
• Further research is needed to distinguish between direct grazing effects and indirect, plant-mediated impacts on soil chemistry.