The functional significance of the microbial biomass in organic and conventionally managed soils

In order to achieve sustainability in managed ecosystems we must understand management impacts on soil processes and clarify the regulatory role of the microbial community on these processes. Crop rotation and organic management practices are thought to have positive impacts on the microbial biomass; however, the specific impacts of crop rotation organic management on soil microbial ecology are largely unknown. The effect of organic management on soil microbial ecology was investigated using soils collected from the Rodale Institute Research Center's long-term Farming Systems Trial (FST) experiment. The FST, begun in 1981, included a manured and a cover cropped organic rotation and a conventionally managed grain based rotation. Soil respiration rates and¹³C-isotope fate in a companion study suggest that the biomass characteristics of the FST treatment soils were different in November 1991. However, direct measurement of the microbial community at this time using Phospholipid Fatty Acid Analysis (PLFA) did not identify statistically significant treatment based differences in soil biomass characteristics. Variability among the PLFA profiles of treatment replicates was as great as variability between farming systems. Treatment based trends were observed among selected PLFAs, particularly those present in large amounts, that were consistent with indirect biomass and biomass-dependent measures. Overall, PLFA profiles, soil respiration rates and¹³C-cycling suggested that the organic cover cropped soil had the Largest and most heterogeneous microbial population while the biomass of the organic-manure amended soil was the least heterogeneous, and the most metabolically active. Present address: University of Illinois, 11025. Goodwin ave. Urbana, IL 61801, USA