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Lal. Carbon Footprints 2026, 5, 10 Page 9 of 20

(b) Weathering of silicate minerals and formation of secondary carbonates (Eq. 2) is called the Urey reaction.
Refers to the chemical process of silicate weathering. It is a long-term geological C cycle where CO from the
2
atmosphere is removed from the air and reacts with water and silicate minerals (such as calcium silicate
called wollastonite).

The reaction

CaSiO (s) + CO (g) → Ca (aq) + 2HCO (aq) + SiO (s) (2)
2+
-
2
3
2
3
leads to the formation of dissolved bicarbonates which are transported to the ocean and sequestered in
carbonate rocks with long MRT [51,52] . In this reaction, some CO is released back to the atmosphere. Thus, the
2
Urey reaction regulates Earth’s climate over a long period of millions of years.
(c) Water (H O) reacts with CO in soil, forming weak carbonic acid (H CO ), which dissolves calcium
2
2
3
2
carbonate (CaCO ) and magnesium carbonate (MgCO) in rocks (limestone/dolomite), releasing calcium,
3
magnesium, and bicarbonate ions. Bicarbonates are soluble and can be leached into the groundwater. Details
of these reactions are given in Eqs. 1 and 2.
It is reported that application rate of basalt powder can be as much as 74 Mg/ha which may lead to SIC
sequestration of 1.13 Mg CO /ha over the course of six months . However, Honvault et al. also observed
[53]
3
that co-application of biochar (12 Mg/ha) with basalt did not significantly increase C sequestration
potential . The lack of increase in C sequestration by biochar may be attributed to the so-called priming
[53]
effect , which must be taken into consideration. Rock weathering in relation to SIC sequestration needs
[54]
additional research, especially on the co-sequestration of SOC and SIC under enhanced weathering and how
to minimize losses of SOC [55,56] .

Other methods of reducing emissions and sequestering C in soil include limiting deforestation and striving
for net-zero emissions. However, there is some concern about the idea of a net-zero target in
agro-ecosystems. It is also argued that the current mechanisms suggested to achieve net zero emissions may
be economic disincentives or financial penalties for emitters . Thus, there is a need for identification and
[57]
implementation of policies that discourage the use of fossil fuels and encourage the adoption of BMPs to
harness the C sequestration potential of land-based C sinks.

CO-BENEFITS OF SOIL/TERRESTRIAL CARBON SEQUESTRATION AND TRADEOFFS
Sequestration of C in terrestrial ecosystems (soil and biomass) can generate several co-benefits [Table 2] over
and above creating the drawdown of atmospheric CO . Indeed, C sequestration in soil is a win-win option
2
with numerous co-benefits. Site-specific information about the strategies to harness these benefits
(economic, agronomic, ecological, aesthetic) needs to be identified and pertinent program(s) implemented.
The economic co-benefits depend on the unit price of C credit, which at present varies widely. For example,
Kragt et al. observed that respondents to a survey were willing to pay $1.13 per credit, but as much as $19/ha
if the area under native vegetation on farmland can be increased . Yet the societal value of C was US$ 35 in
[58]
2010 , but may be as much as $50/C credit in 2025.
[69]
Improvement in soil functions and the attendant increase in agronomic yield is another co-benefit. Iizumi
and Wagai observed the importance of increasing SOC content in top 30 cm layer to enhance drought
[59]
resilience, and stated that improvements in soil functions can occur by increase in SOC stock (i.e., pH, cation
exchange capacity (CEC), availability of macro and micro-nutrients) and the attendant enhancement of soil