Contribution of Soil Carbonates to CO2 Emissions Following Land Use Change

Abstract:

Land disturbance is known to contribute to carbon dioxide emission through accelerated decomposition of soil organic matter. Many soils also contain large stocks of potentially reactive inorganic C (C_i) in the form of carbonate minerals. Under strong acidification conditions, as can be generated by nitrification and other biogeochemical reactions, carbonate weathering has the potential to generate CO₂ emissions on a short time scale; yet little is known about the contribution of this process to disturbance-related C emissions. Information is particularly scarce for high-latitude alkaline soils.

The objective of this study is to determine the contribution of C_i to total greenhouse gas emissions from soil following a land disturbance event. The perturbation consisted of the conversion of long-idled agricultural land to a woody bioenergy plantation. We hypothesized that this land use change would intensify acidification processes and increase the release of C_i. We tested this hypothesis by monitoring greenhouse gas emissions during the establishment of a poplar plantation on soils developed from limestone-derived glacial drift in northern Michigan, USA. We used ¹³C natural abundance to estimate the contribution of C_i to soil C emissions.

We found that carbonates were abundant in the soil profile despite the leaching associated with the humid climate. Carbonate-C concentration averaged 2 g/kg in the topsoil and 15 to 25 g/kg in the deep subsoil. The persistence of measurable carbonate concentrations above the expected weathering front was likely due to biological translocation processes. Using a Bayesian isotopic mixing model accounting for endmember uncertainties, we detected a significant contribution of C_i to total soil CO₂ emissions at the plot scale. There was a significant interaction between disturbance and season. In the spring, C_i emissions were three times higher in newly established poplar plots than in control plots. In the summer, C_i emissions in control plots rose to levels similar to disturbed plots. The disturbance also affected the partition of C_i between the gas (CO₂) and dissolved (bicarbonate) phase. Altogether, these results indicate that the soil carbonate pool is substantial and reactive, and makes a measurable contribution to disturbance-induced CO₂ emissions.