System Settings
All corn was grown in 121-liter containers. We chose to use large containers relative to those used in other enhanced rock weathering (ERW) mesocosm experiments (e.g., ~55-liter (Amann et al., 2020), and ~7.8-liter (Buckingham et al., 2022)) given that small column experiments may lead to differences in soil and system behavior that can alter ERW rates and therefore may alter NH3 fluxes. In each container, we planted 8 corn seeds at standard agricultural densities, (8-12in apart), and thinned to 4 stalks after 2 weeks if necessary, selecting the smaller stalk to be removed in all cases. We applied fertilizer at a high rate of 216 lbs N/acre because many farms in major corn producing states apply nitrogen fertilizer in excess (Xia et al., 2021). Each container received fertilizer via injection of urea-ammonium-nitrate fertilizer at least 5 cm away from the seeds to minimize risk of seed burning.
We also maintained other factors than soil pH (via basalt amendments) including soil moisture, soil texture, crop type, fertilizer timing and application rate, soil organic carbon content, temperature, and upper soil modification (tilling practice). We kept these variables as constant as possible with the Argus Prime automation system between containers within each iteration of the experiment except for soil pH, which was adjusted via the basalt amendments. The system was programed to mimic average July conditions in Spring Grove, Illinois, USA. We chose to aim for 60% saturated soil moisture – which was in our case, a volumetric water content (VWC) of 25. We maintained consistent soil moisture via automated irrigation lines dispensing reverse osmosis water (Argus Prime system). A porous drainage medium was placed at the bottom of each container. We ended the experiment approximately 4 weeks after fertilizer addition.
We sourced waste basalt dust from the East Haven Trap Rock Quarry and prepared it by passing it through a 1mm sieve. The basalt applications were introduced to the system by homogenization in the upper 10 cm of soil to simulate light disc tillage. Control containers received the same tilling practice without any basalt application.