Improved net carbon budgets in the US Midwest through direct measured impacts of enhanced weathering

Abstract

Terrestrial enhanced weathering (EW) through the application of Mg- or Ca-rich rock dust to soil is a negative emission technology with the potential to address impacts of climate change. The effectiveness of EW was tested over 4 years by spreading ground basalt (50 t ha^-1 year^-1 ) on maize/soybean and miscanthus cropping systems in the Midwest US. The major elements of the carbon budget were quantified through measurements of eddy covariance, soil carbon flux, and biomass. The movement of Mg and Ca to deep soil, released by weathering, balanced by a corresponding alkalinity flux, was used to measure the drawdown of CO₂ , where the release of cations from basalt was measured as the ratio of rare earth elements to base cations in the applied rock dust and in the surface soil. Basalt application stimulated peak biomass and net primary production in both cropping systems and caused a small but significant stimulation of soil respiration. Net ecosystem carbon balance (NECB) was strongly negative for maize/soybean (-199 to -453 g C m^-2 year^-1 ) indicating this system was losing carbon to the atmosphere. Average EW (102 g C m^-2 year^-1 ) offset carbon loss in the maize/soybean by 23%-42%. NECB of miscanthus was positive (63-129 g C m^-2 year^-1 ), indicating carbon gain in the system, and EW greatly increased inorganic carbon storage by an additional 234 g C m^-2 year^-1 . Our analysis indicates a co-deployment of a perennial biofuel crop (miscanthus) with EW leads to major wins-increased harvested yields of 29%-42% with additional carbon dioxide removal (CDR) of 8.6 t CO₂ ha^-1 year^-1 . EW applied to maize/soybean drives a CDR of 3.7 t CO₂ ha^-1 year^-1 , which partially offsets well-established carbon losses from soil from this crop rotation. EW applied in the US Midwest creates measurable improvements to the carbon budgets perennial bioenergy crops and conventional row crops.

Keywords: agriculture; bioenergy crop; carbon budget; carbon dioxide removal; eddy covariance; enhanced weathering; net ecosystem carbon balance; net primary production; soil respiration.