Formation of secondary soil minerals following additions of exogenous primary mineral weathering products across a gradient of iron-to-aluminum ratios

Abstract

Iron- (Fe-) and aluminum- (Al-) containing secondary soil minerals and their composition affect and reflect many biogeochemical processes in soils, including organic carbon (OC) storage and nutrient retention. The secondary mineralogy of any particular place is generally regulated by the factors of soil formation but is particularly constrained by the mineralogy of the parent material. However, surficial soil amendments may alter the factors that influence soil formation and, by extension, the secondary minerals that form. This may be particularly true following the application of easily weatherable silicate rock or mineral powder, a practice that is increasingly common and often referred to as enhanced rock weathering (ERW). While the intention is to capture atmospheric carbon dioxide, which is consumed during the weathering process, less is known about the potential fate of primary mineral weathering products such as the secondary mineral assemblages. To better understand potential changes to soil mineralogical pools after ERW additions, we conducted an experimental soil column study using simulated ERW product solutions that varied in Fe and Al contents, the most common metals in most secondary soil minerals. The simulated ERW solution was derived from Fe- and Al-rich silicate mineral dissolution and adjusted to different Fe:Al ratios with similar ionic strengths using additional salts. To simulate the migration of these weathering products into and through the soil profile, these ERW solutions were added periodically over 15 weeks, resulting in a cumulative addition of two column pore volumes. Column leachate was continuously captured and aggregated weekly for analysis of Fe and Al concentrations. After 15 weeks, the columns were destructively sampled and reactive soil Fe and Al pools were determined using selective dissolutions. Relative to pre-treatment soils, column soils generally accumulated semi-crystalline and organically-bound Fe and Al. Semi-crystalline and organically-bound Fe decreased with increasing Fe in the treatment, while crystalline Fe increased, suggesting that crystalline Fe may have formed through transformations of semi-crystalline and organically-bound forms. In contrast, semi-crystalline and organically-bound Al increased with Al-rich additions, while crystalline Al was unaffected by Fe:Al ratios. Further, the most Al-rich treatment was the only group to have significantly more Fe and Al in the organically-bound pool (p < 0.10) compared to the control. We suggest that Fe-rich weathering products may favor the formation of crystalline secondary minerals while Al-rich solutions may favor semi-crystalline secondary minerals and the precipitation of organic complexes. The rapid accumulation of Fe- and Al-containing secondary minerals demonstrates the potential for surface amendments like ERW to shift secondary mineralogy to novel assemblages via the introduction of exogenous mineral weathering products. This is a currently underrecognized secondary effect of ERW with implications for soil biogeochemical function, including long-term soil OC storage.