Investigating the impact of three nitrogen fertilisers on enhanced weathering outcomes with limestone

The weathering of limestone (CaCO₃) by carbonic acid (H₂CO₃) consumes atmospheric CO_2, and due to weathering kinetics that are orders of magnitude faster than silicate rocks, is increasingly seen as a promising enhanced weathering material (Hamilton et al., 2007; Holden et al., 2024). In agricultural settings, however, lime is typically applied to counteract the soil acidifying effects of nitrogenous fertilisers and is therefore thought of as a CO₂ source._. In brief, the nitrification of ammonium (NH₄⁺) generates protons (H⁺) that, when neutralised by CaCO_3, can result in direct CO₂ emissions (Raza et al., 2021). However, not all nitrogen fertilisers are made equal and the net CO₂ impact of lime differs according to fertiliser type and application rate. This study investigated the impact of three common fertiliser types on enhanced weathering outcomes in a 45-pot mesocosm experiment using homogenised loam soil planted with a spring barley crop. In order of most to least acidifying, the fertilisers investigated were protected urea (PU), calcium ammonium nitrate (CAN) and calcium nitrate (CN). In April 2024, lime was applied to ‘treatment’ pots at a rate of 5 tonnes hectare^-1 in combination with PU, CAN and CN at two application rates (75 kg N hectare^-1  and 150 kg N hectare^-1),  while lime-only and fertiliser-only pots were established as controls. Leachate samples were collected from the bottom of each pot at bi-weekly intervals over a period of 8-months and measured for pH, total alkalinity, cations and anions to track weathering and CO₂ uptake. Cation uptake in the barley crop and on soil exchange sites was measured at the end of the experiment to assess potential CO₂ loss pathways. Finally, the dry weight of grain, leaf and stem, from the harvested barley crop was measured to assess the agronomic impacts of different treatment combinations. Preliminary results indicate that while nitrogen application rate had a significant impact on crop yield, neither fertiliser type nor application rate had a significant impact on net CO₂ impact.