Can Phytolith Concentrations Indicate That Wind Erodes Drained Peatlands?

Surface elevations of organic soils in peatlands in the Sacramento-San Joaquin Delta, California, have dropped by as much as fifteen meters since they were drained for agricultural use in the late 19th and early 20th centuries. In this and analogous areas, peat loss is commonly attributed to microbial oxidation of the organic material; while wind erosion is generally considered to be a minor factor. However, in the soil science literature it is widely accepted that organic soils are highly susceptible to wind erosion. The goal of this study is to investigate the competing roles of wind erosion and oxidation in the loss of peat in the Sacramento-San Joaquin Delta by using natural phytoliths found in these peat soils. Phytoliths are biogenic opal; unlike organic matter they are not removed by oxidation. Because of their size, mostly between 2-50 microns, phytoliths are very susceptible to aeolian removal and hence are potentially a good proxy for wind erosion. In this study we sampled peat from drained farmlands and nearby undrained wetlands of the Sacramento-San Joaquin Delta. Samples were treated using conventional phytolith extraction methods. Phytolith concentrations were determined by adding a known quantity (lycopodium spore tablet) of tracer to the phytolith extract (coarse silt fraction >20 microns) and counting the tracer along with the phytoliths. Compared to the phytolith concentrations in undrained peat, the phytolith concentrations should substantially increase in the drained soils as the organic matter oxidizes, unless wind is also eroding the peat. Preliminary results demonstrate an increase in phytolith concentrations for the portion of soil profile (top 45 cm) that has been mixed by agricultural equipment of the drained peat (mean=3.6x104 phytoliths/cm3) compared to the undrained wetland peat (mean=2.3x104 phytoliths/cm3.) However, this higher phytolith concentration in the drained peat is well below the expected phytolith concentration (3.4x105 phytoliths/cm3) if there had been no removal of phytoliths, suggesting that wind erosion is a contributor to the degradation of peat drained for farming.