(note: Determining the cumulative effects of silver iodide cloud seeding becomes crucial towards understanding our changing environment and possible health effects. The following is an updated posting of an excerpt from a study on the effects of silver iodide on soils and plants. The PDF and the abstract are both available)
Ecological Ramifications of Silver Iodide Nucleating Agent Accumulation
in a Semi-Arid Grassland Environment
by D.A. Klein and E.M. Molise Department of Microbiology, Colorado State University, Fort Collins 80523 (Manuscript received 28 August 1974, in revised form 20 January 1975)
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 treament plot in a semi-arid grassland to define possible threshold silver levels.
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.
A major question regrading the short- and long-term effects of weather modification activities concerns the potential effects of silver iodide, used as a nucleating agent in most programs, on soil and [water] processes.
Although work with other agents is being considered, the major seeding agent is silver iodide
This preference is based on several factors: the long term use of this seeding agent, its high nucleating efficiency, and the ease of delivery into varied atmospheric systems by use of a wide range of ground-based and airborne procedures.
Concern for the potential ecological effects of AgI [silver iodide] on biological systems is based on the well known anti-microbial activity of silver nitrate, used widely in topical treatments
The existance of microbially-mediated silver transformations suggests that in any specific situation or environment, added silver should be allowed to equilibrate until the normal, distribution of silver forms [in that particular ecosystem].
The strongest relationships were observed between silver added orginally in the [silver] iodide form, and the decreased ability of the soil micorflora to oxidize glucose to carbon dioxide.
These preliminary considerations suggest that the relationship between decreased glucose mineralization and increased copper and iron levels may reflect a secondary soil response to AgI [silver iodide] imposition.
In a further analysis of silver localization within the soil-grass community, an examination of silver levels in treated intact soils, and in separated soil and roots was complete.
The highest relative silver concentration appears to be associated with the plant roots.
In additional analyses of above-ground vegetation, grasses had higher relative silver concentrations than cactus.
It is of interest to note that silver originally added in iodide or nitrate forms still shows differences in soil responses after three growing seasons have been completed.
Thus one may not be able to assume that silver added to soils in differing forms will eventually equilibrate to a similar series of compounds.
The observation of a possible increased percentage of silver-reducing microorganisms in soils, in response to higher silver nitrate additions, suggests that some of the soil microflora may have the ability to shift their enzymatic capabilities to respond to the presence of this metal. This could also occur with silver iodide.
Silver accumulation above a certain level will result in measurable decrease in whole soil carbon dioxide evolution, and oxygen uptake, and of bacterial glucose mineralization.