Consequences of elevated CO2 on soil acidification, cation depletion, and inorganic carbon: A column-based experimental investigation

Abstract

The anthropogenic release of atmospheric CO2 affects soil nutrient cycling through varied biogeochemical processes. Cation depletion from exchange sites, carbonate mineral dissolution, and subsequent changes in organic and inorganic carbon flux are growing concerns for soil health and sustainability. Particularly, the effects of acid deposition on soil pH and soil health under changing global carbon flux is poorly understood. To estimate the magnitude and effects of pH change across different soil profiles, the depth-wise response of soil cations, pH, and soil inorganic carbon (SIC) to varying CO2 levels was investigated using column-leachate experiments. Six soils, representing acidic, neutral, and alkaline pH, were exposed to different CO2 enrichments (100%, 10%, 1%, and Control) of water percolate for 30 days. Analysis of the column leachates revealed that the base cations (Ca, Mg, Na) were higher initially than on day 30, while Si leaching increased with time. The pH in leachates tended to attain an equilibrium state with time under CO2 saturation, indicating the buffering capacity of alkaline and neutral soils. Evaluation of the soil before and after the experiments showed that the solid SIC, such as carbonate minerals and any dissolved SIC components (e.g., carbonate, bicarbonate ions) entrained in the pore space increased, whereas the potential cation exchange capacity (PCEC) decreased in all soils. This trend was especially prominent for alkaline soils at 100% CO2 enrichment. Over the short-term study, the SIC showed minimal response to varying CO2 enrichment conditions in neutral and acidic soils. The altered cation concentrations in the leachates suggest that under long-term exposure to elevated CO2, a deficiency of cationic essential plant nutrients such as Ca2+, Mg2+, and K+ is likely, especially with low buffering capacity and primary mineral content. In contrast, an increase in nutrient solubility and elemental toxicity (e.g., Al3+ Mn) can be observed in the short term. The initial soils with greater ECEC experienced an increased SIC leaching in the shorter term and suggested potential deposition in deeper layers. The resultant SIC loss might be amplified in arid to semi-arid soils with low SIC and affect soil health.