Iron Mass Balance in a Chronosequence of Upland Soils: Surficial Accumulation Due to Biological Lifting of Mineral Iron

Iron mass balance was calculated for deep soils of a marine terrace chronosequence (5 terraces aged from 65 to 226 Ka) located northwest of Santa Cruz, California. Iron content in the top meter of the soils increased with terrace age. However, mass change calculations, indicated that not only was iron concentrated near the surface, it is also depleted deeper (>1.5m) in the regolith. The surficial concentration of iron cannot be fully accounted for by weathering and compaction of the soil profile or by the addition of iron content through eolian deposition to the soils. The terrace regoliths were generally unsaturated and aerobic, thus lateral movement of large amounts of dissolved reduced iron is unlikely.

We propose that plant roots and symbiotic fungi (mycorrhizae) have transported iron from deeper in the regolith to the shallow soil through the process of biolifting. Iron is a plant micronutrient; unlike other mineral nutrients, it is relatively insoluble in aerobic soil solutions. Iron content of the current grassland vegetation was measured and yearly biomass uptake of iron was calculated. When plants decay, the iron is released and forms oxides at shallow depths. The yearly cycling of plant utilized iron in the above ground biomass multiplied by the age of the terrace can account for the shallow iron accumulation in these soils. Preliminary iron isotope measurements indicate that isotopically light iron is enriched in the shallow soils by this process.