Silicon fractionation of fragipan and non-fragipan horizons in the Central United States

The silicon (Si) cycle is an important driver of the long-term climate cycle and Si is an important micronutrient for some plant species, but our understanding of Si partitioning into various soil fractions remains incomplete. The biogeochemical processes that result in the various Si fractions are poorly constrained in most soil environments, especially in soil horizons that are partially-cemented with Si, such as fragipans. Here, we investigate the fractionation of Si, iron (Fe), aluminum (Al), and Mn (manganese) in fragipans and compare these fractions to non-fragipan soils, including sandstone- and limestone-derived soils. We performed a sequential silicon fractionation scheme using water (4 h and 21 day), sodium pyrophosphate, and citrate dithionite extractions. On average, fragipan soils contained significantly greater extractable Si (6.08 ± 1.87, µ ± 1sd) compared to non-fragipans (2.17–3.55 ± 0.49–0.81). More Si was released from fragipan horizons following the 21 day water, sodium pyrophosphate and citrate dithionite extractions suggesting the presence of a silica pool not present in other soils. A significant relationship was observed between fragipan dithionite-extractable Si and Fe (r² = 0.64, y = 1.06x − 98.85), and the slope of this relationship was significantly greater when compared to other horizons. Water-extractable Si correlated with dithionite-extractable Si, which may indicate overlap between these fractions. A more rigorous fractionation scheme may be required to better differentiate these Si pools. These results suggest that additional Si sorbs to the surfaces of secondary, crystalline Fe oxides and at a potentially greater rate, which may have implications for fragipan genesis.