A soil microbial community structural-functional index: the microscopy-based total/active/active fungal/bacterial (TA/AFB) biovolumes ratio

In most studies of fungal–bacterial communities in soils, single-value indices such as fumigation–extraction (FE) of microbe-derived organic carbon, measures of specific microbial cell chemical constituents, or activity-related measures have been used. These widely used single value indices, however, do not provide information on the physical structure of the filamentous fungal and bacterial community in a soil. The filamentous fungi, considered as indeterminate organisms, have a variable and changing hyphal network, most of which is devoid of cytoplasm. To meet this need for a direct integrated measure of the physical characteristics of the indeterminate fungi and their associated bacteria, a microscopy-based microbial biovolumes ratios approach is suggested. To provide this information, the total and active biovolumes of both the filamentous fungi and bacteria are assessed by microscopy. To normalize these responses, the ratio of total to active (TA) fungal plus bacterial biovolumes is divided by the ratio of the active fungal to bacterial biovolume (AFB), to yield the total/active/active fungal/bacterial (TA/AFB) biovolumes ratio. This approach has been used to analyze data from recently-cultivated early successional (ES) and uncultivated late successional (LS) sites at a shortgrass steppe of northeastern Colorado, where control plots were compared with those receiving mineral nitrogen amendments, using samples taken during the summer of 1995. The TA/AFB ratio index showed distinct and significant decreases in response to soil disturbance which reflected the decreased hyphal lengths present in these disturbed soils. These changes were not detected by the use of FE-based extractable carbon measurements. The TA/AFB ratio also showed significant positive correlations with indices of plant community development and mineral nitrogen, especially in the plots not amended with N. This TA/AFB ratios index should be able to provide information on the physical structure of the indeterminate filamentous fungi and associated soil bacteria for use in the assessment of soil quality, health and resiliency. © 2000 Published by Elsevier Science B.V. All rights reserved.