Strong‐Acid Weathering in a Carbonate‐Siliciclastic Cropland Diminishes Removal of CO ₂ From the Atmosphere Without Becoming a CO ₂ Source

Abstract

One strategy to remove carbon dioxide (CO ₂ ) from the atmosphere is to accelerate weathering in croplands by adding powdered rock. Both silicate and carbonate minerals can be effective as amendments, but strong acids from rain and fertilizer diminish CO ₂ removal and can promote degassing of CO ₂ from carbonate minerals (geogenic CO ₂ ). In addition, denitrification of fertilizer inputs releases dissolved inorganic carbon that could promote re‐precipitation of calcite. To investigate these processes, we used synoptic sampling and analysis of chemistry, dual nitrate isotopes, and solute loads in waters from a 168 km ² humid‐temperate agricultural catchment on siliciclastic and carbonate lithologies receiving sulfuric‐and nitric‐acid inputs. We explored (a) whether strong‐acid dissolution of carbonate minerals became a source of atmospheric CO ₂ , (b) whether strong acids diminished CO ₂ ‐driven weathering, and (c) whether calcite precipitation (with possible CO ₂ degassing) accompanied denitrification. We saw no evidence of carbonate weathering as a source of CO ₂ in the atmosphere. Carbonate weathering was not a CO ₂ source mainly because alkalinity remained high and pH stayed above six except in silicate‐dominated terrain. Weathering removed CO ₂ from the atmosphere: 2.3 ± 2.0 mmol km ⁻²  s ⁻¹ for silicate‐rich uplands and 21 ± 9 mmol km ⁻²  s ⁻¹ for carbonate‐rich lowlands. This drawdown was 22% and 12% less, respectively, than a non‐acidified counterfactual. The silicate‐versus‐carbonate difference in diminishment of CO ₂ drawdown reflects slow silicate versus fast carbonate dissolution. Removal of nitrate by denitrification and biotic uptake ameliorated some of the effects of strong acids but may also have driven calcite re‐precipitation downstream.