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Feedback by the soil microbial community on the distribution of two weeds

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Title: Feedback by the soil microbial community on the distribution of two weeds
Author(s): Lou, Yi
Advisor(s): Yannarell, Anthony
Department / Program: Natural Res & Env Sci
Discipline: Natural Res & Env Sciences
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: M.S.
Genre: Masters
Subject(s): Soil microbial community weed sunflower ragweed multivariate data analysis
Abstract: Soil microbial communities may influence the composition and species abundance of plant communities through the phenomenon of “soil feedback,” which is the tendency of a plant species to cause physical, chemical and/or biological changes in the soil that are beneficial or detrimental to its own fitness or that of its competitors. In this study, I investigated the microbial communities associated with soil feedback to two agricultural weeds, common sunflower (Helianthus annuus) and giant ragweed (Ambrosia trifida). These two plants generally follow opposite gradients of abundance from west to east in the northern United States, and I hypothesized that soil feedback can partially explain this pattern. In each of six states, sunflower and ragweed were grown separately in local soil for two 10-weeks “soil training” phases in order to allow each plant species to modify the microbial communities. Next, sunflower and ragweed were grown separately in the soil trained by the same species and also in the soil trained by the other species, and a soil feedback score was calculated by comparing the biomass of the plants in these two treatments. Bacterial and fungal communities at the end of the experiment were assessed by automated ribosomal intergenic spacer analysis (ARISA). I evaluated the relative effects of three forces shaping microbial communities: 1) the source community, which was the initial microbial community present in the local soil of each state; 2) the influence of the training plant species (ragweed or sunflower) in the initial phase of the experiment; and 3) the influence of the plant species (ragweed and sunflower) in the final phase of the experiment. I also used multivariate modeling to identify the key microbial taxa that contributed to the feedback score calculated from the plant biomass. I found that the source community had the strongest effect on microbial community composition. This suggests that the plant - soil feedback largely depends on the starting microbial communities that the plant encounters. However, within each state, I could detect a significant influence of the plants on microbial community composition over the course of the experiment. Of the microbial taxa that responded to plant influence, only a few were identified by multivariate modeling as being significantly related to plant growth (i.e. soil feedback). Only 10% of the taxa in the total microbial community were needed to classify soil feedback as either net positive or net negative, and these same key taxa could predict the observed feedback scores for sunflower and ragweed (R2 = 0.80). Furthermore, some key taxa may be involved in interactions between ragweed and sunflower. Sunflower growth increased the abundance of bacteria that significantly affected ragweed growth. However, ragweed growth generally increased the abundance of bacteria that only affected ragweed. For each plant species, beneficial fungal taxa were more abundant in the soils of states where the plant received positive soil feedback, and harmful fungal taxa were more abundant in the soils of states where the plant received negative feedback. Another difference between bacteria and fungi is their apparent response to plant selection pressure. Bacterial community variability was reduced in treatments with large positive or large negative feedback scores (P=0.02), but the same was not true for fungi. Overall, my results show that ragweed and sunflower’s capacity to shape soil microbial communities is strongly dependent on the source community, which varies over the geographic range of these weeds. A minor fraction of the soil microbial community appears to be involved in soil feedback. Beneficial fungi for sunflower tend to be found in places where sunflower abundance is highest, while harmful fungi are found in places where sunflower abundance is lowest. The same is true for ragweed. However, soil bacteria appear to respond more strongly to plant selection and may be involved in interactions between sunflower and ragweed. A better understanding of the geographic variability of microbial communities, the key microbial taxa that respond to plant selection, and their feedback to plant growth, may lead to more effective management strategies for agricultural weeds.
Issue Date: 2012-02-06
Genre: thesis
URI: http://hdl.handle.net/2142/29745
Rights Information: Copyright 2011 Yi Lou
Date Available in IDEALS: 2012-02-06
Date Deposited: 2011-12
 

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