Supplementing enhanced weathering with organic amendments accelerates the net climate benefit in rangeland soils

Carbon dioxide removal (CDR) technology that combines negative greenhouse
gas emission pathways alongside substantial emissions reductions is needed
to limit climate change. Rock dust amendments to soils is an emerging CDR
technology that has the potential to decrease atmospheric CO2 via enhanced
rock weathering (ERW), although there is considerable uncertainty
regarding the efficacy of ERW with few multi-year field studies conducted
to date. We explored combining finely-ground metabasaltic rock dust with
other soil CDR technologies (compost, biochar) to stimulate carbon (C)
sequestration and support plant growth. A combined amendment of
 ground rock, compost and biochar observed the highest increases
in soil C stocks (15.3 ± 6.1 Mg C ha-1) over three years. All treatments
with compost applied alone or in combination increased soil C stocks by
2.0 ± 2.9 to 15.3 ± 6.1 Mg C ha-1.Ground rock only reduced rates of soil C
decreases relative to the control, although still soil C stocks still
decreased over time. . Continuous greenhouse gas fluxes showed that ground
rock amendments lowered nitrous oxide (N2O) emissions up to 11.0 ± 0.6 kg
CO2e ha-1 yr-1 and increased methane (CH4) consumption by -9.5 ± 3.5 to
-18.4 ± 4.4 kg CO2e ha-1 yr-1 (N2O and CH4 reduction after three years:
-0.06 ± 0.01 Mg CO2e ha-1). Combined compost, ground rock, and biochar
yielded the highest estimated net ecosystem benefit of -86.0 ± 7.2 Mg CO2e
ha-1 after three years. Net benefits (sum of relative changes in soil C
stocks, estimated enhanced weathering rates, and annual greenhouse gas
emissions) across all amendment treatments included slowing or reversing
background losses in soil C stocks (-16.3 ± 5.0 to -50.5 ± 5.3 Mg CO2e
ha-1). Benefits were dominated by increases in soil organic C stocks, both
directly from organic matter amendments and likely increases in
belowground plant C allocation. To determine an initial theoretical
potential, we scaled to 8% of California rangelands and found
supplementing enhanced weathering with compost and biochar could increase
the net CO2e benefit by 43 Mt CO2e y-1 relative to enhanced weathering
alone. The use of mixed inorganic and organic amendments significantly
increased estimated weathering rates and provided additional soil organic
C sequestration.