Potential importance of bacteria and fungi in nitrate assimilation in soil

• Published in: Soil biology & biochemistry 2007-07, Vol.39 (7), p.1737-1743
• Main Authors: Myrold, David D., Posavatz, Nancy Ritchie
• Format: Article
• Language: English
• Subjects: 15N isotope dilution; Agronomy. Soil science and plant productions; Allylthiourea; antibiotics; assimilation (physiology); Bacteria; bioassays; Biochemistry and biology; Biological and medical sciences; Chemical, physicochemical, biochemical and biological properties; Chloramphenicol; Cycloheximide; forest soils; Fundamental and applied biological sciences. Psychology; Fungi; glucose; grassland soils; immobilization in soil; isotope dilution technique; Methionine sulfoximine; nitrate nitrogen; nitrification inhibitors; NO 3− assimilation; NO 3− immobilization; Physics, chemistry, biochemistry and biology of agricultural and forest soils; soil bacteria; soil fungi; Soil science
• ISSN: 0038-0717; 1879-3428
• DOI: 10.1016/j.soilbio.2007.01.033

Soil microorganisms can use a wide range of N compounds but are thought to prefer NH 4 +. Nevertheless, 15N isotope dilution studies have shown that microbial immobilization of NO 3 − can be an important process in many soils, particularly relatively undisturbed soils. Our objective was to develop a method for measuring NO 3 − 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 KNO 3, 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 15N-labeled KNO 3, we were able to measure gross rates of NO 3 − production (i.e., gross nitrification) and consumption (i.e., gross NO 3 − immobilization). We found that bacteria, not fungi, had the greatest potential for assimilating, or immobilizing, NO 3 − in these soils. This is consistent with their growth habit and distribution in the heterogeneous soil matrix.