The possible ecological effects of silver iodide accumulation in soil which might result from weather modification were examined by use of a 2.5 year old randomized treatment plot in a semi-arid grassland to define possible threshhold silver levels where changes in decomposer parameters might first be detected. After three growing seasons, silver iodide presence at levels above those which could be expected from weather modification appears to be related to decreased soil oxygen uptake, carbon dioxide evolution, and bacterial glucose mineralization activities. The threshhold for possible observation of decreased mineralization, the most sensitive assay evaluated to date, is in the range of 1–2 μ g−1 silver. In the range of 0–0.6 μ g−1 accumulated silver, no significant changes in this parameter were observed in relation to imposed silver. Silver from silver nitrate did not show equivalent effects. A significant increase in silver-reducing microorganisms was noted only in the high-level silver nitrate treated soils. Analysis of silver distribution between soils and roots in treated plots suggests concentration in the plant root zone, with lesser silver levels in grasses from silver iodide than from silver nitrate. Silver levels present in soils, and which might accumulate as a result of weather modification, appear to be at least 1–2 magnitudes below those where first silver iodide effects on decomposer functions might be seen. A model of silver iodide ecological effects on decomposer function is presented, in which control and treated soils may have equivalent apparent biological activities at a specific time after silver imposition in spite of changed ecosystem functions.

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