Supplementing Enhanced Weathering With Organic Amendments Accelerates the Net Climate Benefit of Soil Amendments in Rangeland Soils

Abstract Carbon dioxide (CO 2 ) removal (carbon dioxide removal (CDR)) that combines decreased greenhouse gas emissions with atmospheric CO 2 reduction is needed to limit climate change. Enhanced rock weathering (ERW) of ground silicate minerals is an emerging CDR technology with the potential to decrease atmospheric CO 2 . However, there are few multi‐year field studies and considerable uncertainty in field‐rates of ERW. We explored combining finely ground metabasaltic rock with other soil CDR technologies (compost and biochar amendments) to stimulate carbon (C) sequestration. The combined ground rock (GR), compost, and biochar amendment had the greatest increases in soil C stocks over 3 years (15.3 ± 4.8 Mg C ha −1 ). All other treatments slowed or reversed background C losses, with GR‐only treatments reducing rates of soil C loss relative to the control but still losing soil C over time. Ground rock amendments lowered nitrous oxide (N 2 O) emissions by 11.0 ± 0.6 kg CO 2 e ha −1 yr −1 and increased methane (CH 4 ) consumption by 9.5 ± 3.5 to 18.4 ± 4.4 kg CO 2 e ha −1 yr −1 ; while noteworthy, emissions reductions were an order of magnitude smaller than organic C sequestration with compost amendments. The combined amendment yielded the greatest estimated net ecosystem benefit (3 year relative changes in soil C, estimated ERW rates, and greenhouse gas emissions) of −86.0 ± 24.7 Mg CO 2 e ha −1 . Benefits were dominated by soil organic C gains, directly from organic amendments and indirectly from increased plant growth. Weathering rates were <10% of the theoretical potential. Combined ERW and organic amendments increased estimated weathering rates and stimulated soil organic C sequestration.