Abstract:
Rising global temperatures, increasing atmospheric CO2 conditions, combined with the persistent presence of cadmium (Cd) in agricultural soils threaten soil functionality, microbial diversity, and key ecosystem processes, including greenhouse gas emissions. While elevated atmospheric CO₂ concentrations and warming can change soil pH, mineral solubility, and microbial activity, the interactions between these factors and soil Cd remain poorly understood. Previous research suggests that Cd can negatively affect soil microbial communities and their nutrient cycling. How these processes may evolve under predicted future climatic conditions and whether multiple environmental stressors, such as climatic parameters and Cd contamination, act synergistically, antagonistically, or additively has not yet been investigated.
The overarching hypothesis of this project was that elevated temperatures and atmospheric CO2, as expected with climate change, combined with increased soil Cd concentrations, reduce greenhouse gas emissions more than either stressor alone. It is hypothesized that the reduced emissions are due to an increased fraction of Cd in porewater, which at elevated temperature and CO2 lead to more pronounced Cd toxicity, reducing microbial activity.
By incubating soil under controlled future climatic scenarios combined with detailed microbial and conducting geochemical analyses, and with a particular focus on Cd bioavailability, this thesis represents one of the first more detailed attempts to unravel how soil Cd coupled to future climatic conditions jointly shape soil processes. It identifies whether rising temperature or atmospheric CO₂ plays a dominant role in modulating soil Cd dynamics and explores the response of microbial communities in terms of greenhouse gas emissions and potential microbial adaption to combined stressors.
Simulated future climatic conditions, characterized by elevated temperature and atmospheric CO₂, fundamentally increased the mobility of cadmium in agricultural soils by up to 40%. in soil with pH below 7. This mobilization was mainly caused by enhanced ammonium oxidation and a resulting drop in porewater pH of up to 0.2 units. Thereby, temperature emerged as the primary driver of increased Cd bioavailability, while elevated atmospheric CO₂ on its own has a limited, more selective influence on nitrogen transformations. At moderate Cd levels (<1 µg L⁻¹), microbial growth and nutrient turnover was stimulated, increasing CO₂ and N₂O emissions. However, once porewater Cd surpassed ~1 µg L⁻¹, antimicrobial toxicity manifested, limiting carbon and nitrogen cycling, and thus suppressing greenhouse gas outputs. Intriguingly, the simultaneous presence of doubled CO₂ and elevated temperature does not merely add up their individual effects on Cd dynamics but can counterbalance each other, producing antagonistic outcomes.
This interplay between climate parameters and Cd availability also altered soil nutrient cycling pathways. Elevated temperature accelerates carbon and nitrogen mineralization, ammonium oxidation, and N₂O generation, whereas elevated atmospheric CO₂ enhanced nitrogen fixation, expanding the ammonium pool. Additional Cd added complexity: abiotically formed Cd N ion pairs disrupted nitrogen cycling processes, ultimately affecting N2O production. Initially, high Cd concentrations or combined stressors inhibited respiration and denitrification, but over extended timescales, the soil microbial community adapted. This adaptation restores or modifies biogeochemical functions, but does not fully restore them, though often at the cost of diminished biomass, reduced diversity, and potentially greater vulnerability to future perturbations.
These findings highlight that multiple environmental factors, rising temperature, elevated CO₂, and persistent Cd contamination, interact in nuanced, often unpredictable ways. When combined, these stressors do not act independently, but lead to synergistic, antagonistic, and non-linear responses in soil function and greenhouse gas fluxes. By integrating future climatic conditions, metal availability, and detailed microbial-geochemical perspectives, this work provides a holistic view of how complex stresses shape oxic soils. While microbial communities can adapt and maintain some level of ecosystem functionality, their altered state underscores the need for proactive management and mitigation strategies to maintain soil health and resilience in a rapidly changing environment.
Soil resilience