Recovery from Acid Deposition Increases Streamwater Carbon Export Potential in a Silicate-Treated Forested Watershed

Enhanced Mineral Weathering (EMW) is a land-based Carbon Dioxide Removal (CDR) strategy to accelerate delivery of inorganic carbon and alkalinity to the oceans. It requires research and verification in the form of long-term field trials, but has a range of co-benefits for agriculture and forestry. The first catchment-scale EMW experiment was initiated in 1999 when an 11.8-ha watershed in the Hubbard Brook Experimental Forest (HBEF), New Hampshire, was treated with 3.5 t ha^-1 of the calcium silicate mineral wollastonite (CaSiO₃) in an effort to reverse the effects of acid deposition on ecosystem function. Over the next 15 years, the treatment substantially increased cumulative streamwater calcium and bicarbonate fluxes relative to an untreated reference watershed. However, bicarbonate fluxes were considerably lower than calcium fluxes (on a molar charge basis) due to the presence of organic acids, transient nitrate spikes and particularly sulfate originating from acid deposition. Because only carbonic acid weathering consumes CO₂, weathering by sulfuric, organic and nitric acids occurs at the expense of CDR. Forward modelling suggests that decreased acid deposition has created more favorable conditions for EMW, but interactions with the nitrogen cycle highlight the need for nitrate mitigation strategies if EMW is deployed on fertilized croplands or forests subjected to enhanced nitrogen deposition. Finally, we consider the implications of our results for marine carbon cycling and storage.