Potential importance of bacteria and fungi in nitrate assimilation in soil

Soil microorganisms can use a wide range of N compounds but are thought to prefer NH4 . Nevertheless, N isotope dilution studies have shown that microbial immobilization of NO3 can be an important process in many soils, particularly relatively undisturbed soils. Our objective was to develop a method for measuring NO3 immobilization potential so that the relative contributions of bacteria and fungi could be determined. We modified and optimized a soil slurry method that included amendments of KNO3, glucose, and methionine sulfoximine (an inhibitor of N assimilation) in the presence of two protein synthesis inhibitors: chloramphenicol, which inhibits bacteria, or cycloheximide, which inhibits fungi. By adding N-labeled KNO3, we were able to measure gross rates of NO3 production (i.e., gross nitrification) and consumption (i.e., gross NO3 immobilization). We found that bacteria, not fungi, had the greatest potential for assimilating, or immobilizing, NO3 in these soils. This is consistent with their growth habit and distribution in the heterogeneous soil matrix. r 2007 Elsevier Ltd. All rights reserved.