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 indi ces such as fumigation-extraction (FE) of microbe-derived organic carbon, m easures of specific microbial cell chemical constituents, or activity-relat ed measures have been used. These widely used single value indices, however , do not provide information on the physical structure of the filamentous f ungal and bacterial community in a soil. The filamentous fungi, considered as indeterminate organisms, have a variable and changing hyphal network, mo st of which is devoid of cytoplasm. To meet this need for a direct integrat ed measure of the physical characteristics of the indeterminate fungi and t heir associated bacteria, a microscopy-based microbial biovolumes ratios ap proach is suggested. To provide this information, the total and active biov olumes of both the filamentous fungi and bacteria are assessed by microscop y. To normalize these responses, the ratio of total to active (TA) fungal p lus bacterial biovolumes is divided by the ratio of the active fungal to ba cterial 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 successi onal (LS) sites at a shortgrass steppe of northeastern Colorado, where cont rol plots were compared with those receiving mineral nitrogen amendments, u sing samples taken during the summer of 1995. The TA/AFB ratio index showed distinct and significant decreases in response to soil disturbance which r eflected the decreased hyphal lengths present in these disturbed soils. The se changes were not detected by the use of FE-based extractable carbon meas urements. The TA/AFB ratio also showed significant positive correlations wi th 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 filamen tous fungi and associated soil bacteria for use in the assessment of soil q uality, health and resiliency. (C) 2000 Published by Elsevier Science B.V. All rights reserved.