Stabilisation of soil organic matter with rock dust partially counteracted by plants
Soil application of Ca- and Mg-rich silicates can capture and store atmospheric carbon dioxide as inorganic carbon but could also have the potential to stabilise soil organic matter (SOM). Synergies between these two processes have not been investigated. Here, we apply finely ground silicate rock mining residues (basalt and granite blend) to a loamy sand in a pot trial at a rate of 4% (equivalent to 50 t ha-1) and investigate the effects of a wheat plant and two watering regimes on soil carbon sequestration. Rock dust addition increased soil pH, electric conductivity and soil-exchangeable Ca and Mg contents, as expected for weathering, but decreased exchangeable levels of micronutrients Mn and Zn, likely related to soil pH. Importantly, it increased mineral-associated organic matter by 22% due to the supply of secondary minerals and associated sites for SOM sorption. Additionally, in the non-planted treatments, rock supply of Ca and Mg increased soil microaggregation that subsequently stabilised labile particulate organic matter as organic matter occluded in aggregates by 46%. Plants, however, reduced soil exchangeable Mg and Ca contents and hence counteracted the silicate rock effect on microaggregates and carbon within. We attribute this cation loss to plant exudates released to solubilise micronutrients and hence neutralise plant deficiencies. The effect of enhanced silicate rock weathering on SOM stabilisation could substantially boost its carbon sequestration potential when pH and micronutrient effects are considered.