Disturbance Effects on Soil Carbonates and Implications for Soil Carbon Emissions.

Soil carbon emissions generally originate from the decomposition of organic materials. In addition, many soils contain inorganic C in the form of carbonate minerals. Carbonate weathering reactions can release carbon in gas (CO₂) form and are sensitive to changes in the soil’s chemical environment. Soil carbonates therefore have the potential to contribute to soil C emissions following disturbance, yet the reactivity of naturally-occurring carbonates under field conditions is largely unknown.

The objective of this study is to constrain soil carbonate dynamics following the conversion of previously idled agricultural land to a poplar plantation in northern Michigan. We conducted laboratory incubations to measure the sensitivity of the carbonate phase to perturbations. We also used isotopic methods to estimate the contribution of carbonates to soil C emissions in the field following land use change.

We found that carbonates were abundant in the soil profile despite the humid climate. Carbonate-C concentration averaged 2 g/kg in the topsoil and 15 to 25 g/kg in the deep soil. The persistence of measurable carbonate concentrations above the expected weathering front was likely due to biological translocation processes involving soil fauna. In a laboratory incubation of topsoil material, carbonates were found to contribute approximately 20% of total soil C emissions. For the deep soil incubation, the contribution of carbonates rose to 50%. Early field results suggest a detectable contribution of soil carbonates to total soil C emissions at the plot scale. There was a significant interaction between disturbance and season. In the spring, carbonate-derived CO₂ emissions were three times higher in newly established poplar plots than in control plots. In the summer, carbonate-CO₂ emissions in control plots rose to levels similar to disturbed plots. These results indicate that the soil carbonate pool is substantial and reactive, and that its dynamics are influenced by complex biogeochemical controls. This talk will further explore the interacting effects of disturbance, climate and biota on soil carbonates weathering and consequences for the ecosystem C stocks and greenhouse gas balance.