Klug, M.J. and H.P. Collins
Presented at the All Scientist Meeting (1996-07-16 to 1996-07-17 )
Agricultural ecosystems are distinguished by their lack of structural complexity and diversity in plant communities that results from limited crop rotation and soil disturbance from annual cultivation. Disturbance in agroecosystems clearly affects productivity and plant diversity, with highly managed (high chemical and tillage inputs) systems having low plant diversity low high crop productivity. The frequency and timing of disturbance affects both the composition of the plant community and the distribution, retention, and utilization of nutrients within the system. The diversity of soil microbial communities may also be reduced in response to less diverse organic matter inputs and frequent, large scale soil disturbance.Our understanding of soil communities within agroecosystems is more advanced with respect to the flow of C and N through various soil organic matter pools than the structure of the microbial community carrying them out. The importance of understanding the relationship between microbial community structure and function, within agroecosystems, has been long touted as a useful measure to evaluate how agricultural management (crop rotations, tillage, ag-chemicals) or environmental perturbations (drought) influence soil microbial processes such as, residue decomposition and nutrient cycling. There currently is an effort to use descriptions of the structure and function of the soil microbial community as potential predictors of “soil quality” or sustainability since changes in microbial energetics or diversity of disturbed systems can be detected long before changes in soil characteristics are observed.“Function” (biochemical potential) of soil microbial communities within agroecosystems has historically been defined at the process level across various scales of resolution with respect to the activity of the entire microbial community. Microbial biomass has been correlated to C- or N-mineralization potentials, rates of residue decomposition and nutrient cycling. However, recent evidence suggests that although the size of the biomass is important in driving soil processes the functions of that biomass may be more critical to the rate and efficiency of biogeochemical processes such as N-cycling. Although the major functions of the soil biota (eg. residue decomposition, nitrification) appear unchanged in cropped soils, several studies have shown that soil disturbance related to tillage breaks down micro-habitats, lowers diversity and the ability to maintain resiliency of function. A dual approach using both FAME and community-based Biolog lends itself to linking the structure of microbial communities to their function or biochemical potential.Soil samples from three of the management regimes at the KBS LTER site were examined for divergences in microbial community structure and function. Treatments were: a) conventional tilled corn-soybean-wheat (Zea mayes-Glycine max-Triticum aestivum) rotation (CSW); b) a successional plant community developing after abandonment in 1988 NS); and c) a plant community that has never been cultivated (NT). Each of these treatment alters the controls over agronomic productivity by manipulating organism-level interactions. A disturbance gradient exists through varying physical (e.g., tillage and frequency) and chemical (e.g., fertilizers, pesticides) characteristics in the treatments. The cultivated treatment represents the greatest disturbance while the never-tilled grassland the least. Differences in microbial community structure and function of soil samples from each treatment were determined by comparing the profiles of fatty acids (FAME) associated with lipids extracted from soils and carbon oxidation profiles of whole soil microbial communities using the GN micro-titer plate system of Biolog Inc. (Hayward, CA). The diversity of extracted FAMEs and of carbon compounds utilized provides an index of change in the structural and functional diversity of microorganisms present.Microbial biomass increases rapidly in the conventional tilled plot following tillage and residue incorporation, early in the cropping cycle, and then declines with time. In the never-tilled systems, microbial biomass remains high throughout the growing season. Comparisons of extracted lipids from in situ soil microbial communities indicate a high heterogeneity in FAME profiles among samples within each treatment. This heterogeneity most likely results from spatial differences in resources and abiotic conditions. (Figure 1) illustrates the results of a principle components analysis for the 1992 and 1993 sampling years using a subset of the total fatty acids extracted. These fatty acids have been identified as “bacterial markers” making them useful to describe differences among treatments. Profiles from the conventional tilled treatments were 73 and 66% similar to the FAME’s extracted from the successional and never tilled treatments, respectively. The major difference between years was a higher percentage of branched chain fatty acids recovered in the 1992 than 1993 samples. These fatty acids are common to gram positive bacteria. Lower recovery suggests a shift in the abundance of bacterial populations between years.BIOLOG substrate oxidation profiles suggest that the heterotrophic potential is very similar among these treatments. There is however an indication that there is a greater heterogeneity of function within the successional plant community than either the cultivated or never-tilled treatments, as reflected in FAME profiles. FAME and Biolog are effective tools for initially characterizing differences in the dominant members of microbial communities associated with varying agricultural practices. Future studies are planned to identify and then demonstrate which selective pressures associated with changes in agricultural management drive the shifts in microbial populations and energetics.
Back to meeting | Show |