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-15 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-15-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. (c) 2007 Elsevier Ltd. All rights reserved.