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EVl,.LUATl():~ m ECOLOCICAL [FfEeT:::: OF SILVE~ WIJIDE. SEEDI};G AGE~TJ./ Do::alci A. Klein~/ ana ?irh~.c;" Sokol Accumulatio~ o( silver iodide Tram weather modification will result in nc overt toxicity effects. but maY re"ult in a slight delay in biological p.o~esses. 1/ In Steinhoff. H.~J., and J.D. Ives (Eds). 197fi. Eccl03ical impacts of snowpack Mountains, Colo.ado. San Juan Ecolc~y PTC"jt>,::t, Final Report. Colorado State ~/ Present address: Dept. of ~icrobiology. Color~do State Univ., ~t. Collins, Co. ABSTRACT The possible effects of silver iodide seeding <<f.;cnt accumulation in su.face soils of the San Juan area were investigated by use of laboratory studies in- volving Arthrobacter - silver interactions, treatment of soils ....ith varied forms dnd levels of silver, by examination of silver gradients surrounding seeding generator sites, and by use of treatment plots in spruce, aspen, and subalpine meadow communities located in the San Juan snow aug~entation area. Lab- oratory studies have shown that on a short-term basis, silver iodide or seeding generator burn mixtures at hiKher than field concentration show only slight effects on microbial gro....th or enzyme function, while free silver ions, even at part-per-billion levels, can cause marked changes in microbial activities. These effects were lessened in the presence of soil or complexing ions. However, under field conditions on a 3 year basis. silver iodide appears to have a great- er potential for altering soil decomposer functions chan silver from silver ion. The presence of silver from silver iodide - sodium iodide complexes, again at higher than background levels, in the San Juan target area led to a gradual increase in soil organic matter content in spruce and aspen surface soils. No such incr~ase was observed in the subalpine meadow plot. Silver from silver nitrate tended to concentrate more in upper plant portions than silver from silver iodide, although silver from silver iodide was more closely associated with the plant roots. These effects were only observed using silver levels ID-IOOX expected field accumulation levels. Based on the available literature and experiments carried out to date, the accumulation of silver iodide seeding agent does not Appear to present an ecological threat, although con- tinual monitoring of test systems should be carried out to extend the time period over which this conclu- sion can be tested. OBJECTIVES AND HYPOTHESES I. To monitor soil microbial activities in the San Juan area, to determine if possible silver accre- tion is influencing soil microb1al processes. To analyze treatment plot s011s to which various silver additons have been made in aspen, spruce, and grass communities. To measure microbial responses to silver gradients present in the vicinity of the Pagosa Spr1ngs gen- erator site No. Zi. and the emerald ~wuntain site. To determine the existence ot poss1ble sub-lethal effects ot varled silver leVel-S ana lUIllll:o UII ~icrobial processes, 1nclud1ng ettects on m1crob1- al viability and capabi11t1es to transtorm SUb- strates requir1ng enzymatlc adaptation ana enef&y- production processes. To determine if soil microbes are capable of transforming silver present as silver iodlde t~ additional silver forms, and to determine phYS10- logical mechanisms which may result in silver transformatIons. To determine if assay procedures for varied silver forms in soil CRn be developed. 2. 3. 4. 5, 6, These objectives have been surr~arized in a series of hypotheses: ."~Hypothesis 1. Hypothesis II. Laboratory studies can give an indica- tion of the degree of possible aimin- uation of microbial activity which I might .esult from silver accumul."tian. :Hypoth~Sis III. Soil bacteria can cause a change in silver iodide to forms more easily taken up by plants. This modification of silver form may make it necessary to carry out experiments under field- related conditions as much as possible. SUMMARY -II"" 'ys,'s s,iJ!c t/ons 0";'.. rr~ _ 1 Based on ....ork carried out on the possible ecological effects of silver iodide seeding agent accumulation in soils of the San Juan area, the following conclusions can be drawn: ]. The accumulation of silver iodide nucleatinR agentf presents go overt-danger to humans, plants or ani- mals, bas~d on available literature and work car- ried cut under this study for a 3 vear oeriod. Silver ion CEn ca~se decreased gro~th and enzYme induction in a soil Arthrobacter species at l-lQ ppb free silver ion. The effect observed is depen- dent on other anions and cations present. Soil decreases the activity of free silver ions. Silver from silver iodide or hurn mixtures does not show such eltects on mlcroOlal growrn or en- zymatic activity. Generally 10-100 ppm ot Agl torm silver is required to observe effects on growtn, viab111ty, and enzyme 1nduct10n. Free silver 10n t!om silver iodide dissociation at 5-6 ppb can cause' observable effects on micro- organisms. To achieve this silver level 100-200 ppm of silver as silver iodide are re- quired. Microorganisms can cause reduction of free silver ion or silver iodide to metallic silver. Amine- type materials may be responsible for this reduc- tion. Silver iodide added at 1000 ppm shows no immediate effects on soil resDiration processes. Attempts to establish respiration indices for test soils across the impact area were not successful due to short-term variability of soil-moisture an~ temperature. Silver accumulation in the vicinity of two seeding enerator sites did not cause decreases in m1cro- bial respiration. Silver tended to accumu ate and remain in the surlace soil zone in sp1re ot ex- tended water percOlation through the zone. A subalpine meadow test plot installed for 3 years using silver 10d1de and silver nitrate at I, lU, 100 ppm surface soil levels did not show changes in decomposer functions ....hich could be related to this treatment. Silver from silver iodide moved to upper plant por- tions to a lesser extent than silver from silver nitrate. Silver from silver iodide was more closely associ- ated with plant roots than silver from silver ion. After 3 years, silver from these two differing original forms have not equilibrated to forms which will give similar activities. In spruce and aspen plots, imposition with silver iodide tends to be related to increased organic matter retention in the test systems. No such relationships were observed ....ith equivalent treat- ments using silver from silver nitrate (silver ion) . 2. 3. 4, 5. 6. 7. 8. 9. 10. 11, 12. I i I \ 13. augmentation Univ. Pub!., 80523, ' in the San Juan Fort Collins. ::.'k'_