Biological Immobilization of Zinc and Manganese in Soil

A soil perfusion system was used to measure Zn (as Zn 2+ ) immobilization by soil microorganisms in Rubicon sand (Entic Haplorthods, pH 5.9). A 240‐ µM Zn solution (320 mL) was continuously perfused through 25 g of gamma‐irradiated (sterilized) or biologically active soil. At steady‐state, approximately 75.0% of the perfusate Zn became bound to the soil column by chemical and physical mechanisms under sterile conditions. The introduction of soil microorganisms and a source of C and energy to the sterile soil column resulted in an additional 20.5% reduction of Zn in the soil perfusate (significant difference at the 99% level) to a new steady‐state level of 10.8 µM after a 72‐hr period. Further perfusion did not remove additional Zn. The nutrient amendment alone did not contribute any significant immobilization of Zn. Thus, under conditions that are conducive to rapid growth, zymogenous soil microorganisms could significantly immobilize and reduce the level of the mobile Zn to a level below that attainable by chemical and physical factors in the soil alone. In contrast with results in Rubicon sand, microbial immobilization in finer‐textured soils (Brookston clay loam—Typic Argiaquolls, pH 7.5) and (Houghton muck soil—Typic Medisaprist, pH 6.0) was not detected. Biological immobilization of Mn in Rubicon sand was also highest when a combination of microbes and nutrients were added. With the use of the soil perfusion system, it is possible to quantitate the contribution of microorganisms to immobilization of Zn and Mn in Rubicon sand receiving utilizable organic materials.