SOIL IRON IN NODULES AND RETICULATE MOTTLING: PROCESSES OF FORMATION IN THE GREEN MOUNTAIN SOIL AND BEYOND

The Green Mountain (GM) soils of California were first named and described by Don L. Johnson, these soils occur on the Channel Islands and along coastal California from San Diego to Mendocino. The GM soils contain iron and manganese rich nodules in the top meter and have a distinct argillic horizon with reticulate mottling. Here we present further evidence and data on nodule formation and the nature of reticulate mottling in the argillic horizon. California has a north to south precipitation gradient. Iron nodules are very abundant in soils of southern California (up to 70 Wt %, San Diego County), and occur in lower abundances to the north (~6 Wt %, Santa Cruz County). In addition, nodules on several marine terrace chronosequences show an increase in nodule abundance with soil age. We hypothesize that nodule abundance is governed by amount of precipitation and the age of the soil. Several processes for nodule formation have been proposed: disaggregation and ferruginization of an argillic or spodic horizon, precipitation and coalescence around organic matter or mineral grains, and precipitation by fungi. Detailed microscopic work on nodules from a soil chronosequence near Santa Cruz , California show microbial evidence for nodule formation. Extracted DNA from Santa Cruz nodules amplified with PCR suggests several fungal phylotypes reside in the nodules. D.L. Johnson presented the hypothesis that the Green Mountain Soil is a relict soil, formed under coniferous vegetation. Detailed analyses of color separated fractions from reticulated horizons in the oldest soils of the Santa Cruz chronosequence show differences in Fe, Al, & C content, as well as specific surface area, grain size, and d^56/54Fe values. The soils near Santa Cruz which have reticulate mottling are 90 ka and older. These soils have formed through glacial cycles of cooler wetter climate. They are currently occupied by coastal prairie ecosystems, but have experienced other ecosystems through their soil development history and are therefore polygenetic.