Climate change mitigation? Interactions between bio-weathering and soil organic carbon dynamics

There are two main processes acting as carbon sinks for CO₂ on land: Silicate weathering and photosynthesis. The former creates bicarbonates which can be stored in soils or leached out and stored in the oceans, the latter leads to the formation of plant tissue which can be stored as soil organic matter. Soils are thus at the foundation of both of these land sinks and could therefore play a crucial role in tackling climate change. Either via enhanced silicate weathering (acceleration of the naturally extremely slow weathering process by finely grinding silicate rock and applying it on soils) or via improved management of soil organic carbon stocks.

However, there are still knowledge gaps regarding both, the natural weathering process and, more notably, the enhanced weathering process:

• Does the presence of plants (further) accelerate the weathering rates?
• Does an active weathering process increase soil organic carbon (SOC) stability?
• Is there an interaction effect between the weathering process, plants and SOC?

Addressing these questions is crucial to evaluate effectiveness and safety of enhanced silicate weathering. For example, increased weathering rates due to the presence of plants, could increase the climate change mitigation effect, while increased organic matter decomposition could drastically reduce the mitigation potential of enhanced silicate weathering.

In an enhanced weathering experiment we investigated weathering rates, mineralogical changes, inorganic and organic carbon dynamics and greenhouse gas emissions. First findings provide evidence of bio-weathering; soil planted with maize displayed higher dissolved inorganic carbon and alkalinity concentrations as compared to unplanted soil. These findings are also supported by mineralogical analyses which revealed more pronounced changes in the mineral composition of planted treatments. So far, we have not only found compelling evidence of bio-weathering but we have also identified a potential connection between bio-weathering and SOC dynamics. Soil analyses showed that there was no clear change in SOC contents in planted pots. Unplanted pots, however, displayed an increase in SOC contents when silicates were added.