Iron in a Soil Chronosequence: Biologic Influence On Mass Balance, Nodule Formation and Mottling.

The evolution of Fe distribution with landform exposure time was studied in a marine terrace chronosequence northwest of Santa Cruz, California. The abundance of soil Fe increases with terrace age on the five terraces studied (65 to 226 Ka). All soils but the youngest are prominently mottled and contain Fe nodules of biologic origin.

 Mass change calculations for Fe, indicate that not only is Fe concentrated near the surface but, it is also depleted at depths >1.5m in the older soils. Unlike other mineral nutrients, Fe is comparatively insoluble in aerobic soil solutions. We propose that plant roots and symbiotic fungi (mycorrhizae) have moved Fe from the deep regolith to the soil. The Fe content of the current grassland vegetation was measured and yearly biomass Fe uptake calculated. The accumulation of plant-utilized Fe released from decay of above ground biomass multiplied over the age of the terrace is roughly equivalent to the soil Fe content.

 Plants that use the strategy I Fe uptake process fractionate light Fe (Guelke and Von Blankenburg, ES&T, p1896; 2007). To test the biolifting hypothesis, Fe isotope ratios were determined for bulk soil samples from several soil depths of 4 terraces. The shallow soils generally have increasingly lighter δ^56/54Fe with terrace age. The δ ^56/54Fe values at 10cm soil depth are (youngest to oldest) 0.546, 0.628 0.381 and 0.182. The deep sample ratios are between the values of the known source rocks for these sediments. However, grasses utilize the strategy II process of Fe uptake which does not isotopically fractionate Fe. The Fe isotope composition of these soils is indicative of a forest or chaparral ecosystem existing on the terraces previous to the present grassland.