  Enhanced Rock Weathering (ERW) is a promising scalable and cost-effective
Carbon Dioxide Removal (CDR) strategy with significant environmental and
agronomic co-benefits. A major barrier to large-scale implementation of ERW is
a robust Monitoring, Reporting, and Verification (MRV) framework. To
successfully quantify the amount of carbon dioxide removed by ERW, MRV must be
accurate, precise, and cost-effective. Here, we outline a mass-balance-based
method where analysis of the chemical composition of soil samples is used to
track in-situ silicate rock weathering. We show that signal-to-noise issues of
in-situ soil analysis can be mitigated by using isotope-dilution mass
spectrometry to reduce analytical error. We implement a proof-of-concept
experiment demonstrating the method in controlled mesocosms. In our experiment,
basalt rock feedstock is added to soil columns containing the cereal crop
Sorghum bicolor at a rate equivalent to 50 t ha$^{-1}$. Using our approach, we
calculate rock weathering corresponding to an average initial CDR value of 1.44
+/- 0.27 tCO$_2$eq ha$^{-1}$ from our experiments after 235 days, within error
of an independent estimate calculated using conventional elemental budgeting of
reaction products. Our method provides a robust time-integrated estimate of
initial CDR, to feed into models that track and validate large-scale carbon
removal through ERW.
