Feeling the pulse: long-term studies are key to understanding carbon dynamics in bio-weathering

Besides rapid and deep greenhouse gas emission reductions, atmospheric Carbon Dioxide Removal strategies (CDRs) will be necessary to mitigate anthropogenic climate change. A promising CDR is Enhanced rock Weathering (EW), deployed on croplands. So far, most research focused on lab-based weathering and sequestration rates, but these might differ substantially in field settings, where biota may have a strong effect on EW processes (e.g., via bio-weathering). Additionally, these studies are mostly short-term, thus limiting our knowledge on the long-term effects of silicate addition on croplands. Here, we set up a mesocosm experiment to quantify the carbon (C) sequestration of EW and how this is affected by biota. The experiment consisted of soils applied with basalt and combinations of corn and/or earthworms. It spanned over two growing seasons, for a total duration of 16 months. We measured i) Soil CO2 Efflux (SCE) , ii) porewater and leachate water alkalinity, Dissolved Inorganic Carbon (DIC), pH and other elemental chemistry, and iii) soil pH and elemental chemistry in order to determine weathering rates. Our data shows that basalt had a different effect on SCE in the two growing seasons. In both seasons, the effect of basalt depended on plant presence, though in two different directions. In the first growing season, basalt increased emissions significantly compared to controls, but only when plants were present. In contrast, during the second growing season basalt significantly decreased SCE over time compared to controls, but only in absence of plants. Here, there was no effect of basalt on SCE whenever plants were present. These contrasting results suggest an initial stimulation of SCE (pulse) due to basalt application, especially prominent in the presence of plants and possibly involving changes in soil organic carbon dynamics. These findings highlight the need for long-term studies that outlast that initial pulse and elucidate the fate of organic carbon in order to accurately quantify the CDR potential of EW, as well as the role of biota therein.