Soil cation storage as a key control on the timescales of carbon dioxide removal through enhanced weathering


Significant interest and capital are currently being channeled into techniques for durable
carbon dioxide removal (CDR) from Earth’s atmosphere. A particular class of these approaches
(referred to as enhanced weathering (EW)) seeks to modify the surface alkalinity budget to
durably store CO2 as dissolved inorganic carbon species. Here, we use SCEPTER (a reaction-
transport code designed to simulate EW in managed lands) to evaluate the throughput and
storage timescales of anthropogenic alkalinity in agricultural soils. Through a series of alkalinity
flux simulations, we explore the main controls on cation storage and export from surface soils in
key U.S. agricultural regions. We find that lag times between alkalinity modification and climate-
relevant CDR can span anywhere from years to many decades, with background soil cation
exchange capacity, agronomic target pH, and fluid infiltration all impacting the timescales of CDR
relative to the timing of alkalinity input. There may be scope for optimization of weathering-driven
lkalinity transport through variation in land management practice. However, there are tradeoffs
with total CDR, optimal nutrient use efficiencies, and soil nitrous oxide (N2O) fluxes that
complicate attempts to perform robust time-resolved analysis of the net radiative impacts of CDR
through EW in agricultural systems. Although CDR lag times will be more of an issue in some
regions than others, these results have significant implications for the technoeconomics of EW and
the integration of EW into voluntary carbon markets, as there may often be a large temporal
disconnect between deployment of EW and climate-relevant CDR.

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