Heterotrophic and Autotrophic Soil Respiration under Simulated Dormancy Conditions

Soil CO₂ efflux is the primary carbon efflux from terrestrial ecosystems to the atmosphere. It is composed of autotrophic (plant) and heterotrophic (microbial) metabolic processes, and is usually quantified at the soil surface as a single source flux. Isotopic methods are a good tool for isolating heterotrophic and autotrophic respiration but are difficult to setup in situ and very costly. Thus, heterotrophic and autotrophic carbon efflux contributions to the gross soil carbon efflux are not well understood.

Accurate quantification of carbon fluxes is necessary to construct carbon budget models and accurately estimate ecosystem productivity. Soil CO₂ effluxes, as most ecosystem processes, are sensitive to seasonal and environmental change. It is understood that soil CO₂ effluxes are significantly lower during the winter of temperate ecosystems and assumed microorganisms dominate efflux origination.

In this paper, the authors hypothesized that heterotrophic contributions would be greater than autotrophic under simulated dormancy conditions. To test this hypothesis, they designed an experiment with the following treatments: combined autotrophic heterotrophic respiration, heterotrophic respiration, autotrophic respiration, no respiration, autotrophic respiration in vermiculite, and no respiration in vermiculite.

Engelmann spruce seedlings and soil substrates were placed in specially designed respiration chambers and soil CO₂ efflux measurements were taken four times over the course of a month. Soil microbial densities and root volumes were measured for each chamber after day thirty-three.

Seedling presence resulted in significantly higher soil CO₂ efflux rates for all soil substrates. Autotrophic respiration treatments were not representative of solely autotrophic soil CO₂ efflux due to soil microbial contamination of autoclaved soil substrates; however, the mean autotrophic contributions averaged less than 25% of the total soil CO₂ efflux. Soil microorganism communities were likely the primary contributor to soil CO₂ efflux in simulated dormant conditions, as treatments with the greatest proportions of microbial densities had the highest soil CO₂ efflux rates.

In conclusion, the findings suggest that fungal and bacterial soil communities are the major contributors to dormant season (simulated winter conditions) soil CO₂ efflux.

Article by Daniel Beverly and Scott Franklin, from USA.

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Image by UBC Micrometeorology, from Flickr-cc.