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What are they doing in there? A closer look at microbes in the soil in varying topography & cropping systems By Sara L. Scheiding, Sarah K. Hargreaves, Elizabeth M. Bach, Ryan J. Williams, Kirsten S. Hofmockel Department of Ecology, Evolution, & Organismal Biology, Iowa State University Sampled from Landscape Biomass Project at Uthe Research and Demonstration Farm in Boone County, IA on 1 June 2011 From each plot, 10 soil cores (2.2 cm wide x 15 cm deep) were taken randomly and combined for a plot-level total soil sample. Measured gravimetric water content and soil pH and subsampled 1g. of soil into Nalgene bottles. Determined enzyme activity by performing fluorometric enzyme assay protocol. Why do we care? Research Hypotheses Methods and Materials Uthe Research and Demonstration Farm Discussion Results I would like to thank the ISU REU program and the NSF for giving me this amazing opportunity this summer, as well as Dr. Kirsten Hofmockel for having me in her lab. I would like also to thank Sarah Hargreaves, Elizabeth Bach, and Ryan Williams for having so much patience with me this summer and teaching me so much more about soil and Acknowledgments Enzyme activity will be higher under switchgrass and sorghum due to the use of a cover crop (sorghum) and the use of a perennial system (switchgrass). Enzyme activity will be higher at landscape positions with the greatest soil moisture. Enzyme activity will increase downslope possibly due to erosion of carbon and other nutrients from higher landscape positions. The U.S. Energy Independence and Security Act of 2007 increased interest in the consequences of mass production of biofuel cropping systems such as switchgrass, corn, and sorghum. Benefits related to the soil microbial community includes reduced greenhouse gas emissions, increased nutrient retention, and increased carbon storage. Getting a better sense of how the microbial communities interact with cropping systems across a topographic gradient can assist in management practices. Enzymes vs. Landscape Enzyme activity was greatest at the backslope position. BX was most influenced by landscape position (P = 0.002), and was 29% greater at the backslope compared to the toeslope and 2 times greater than the summit. BG activity was marginally significant (P = 0.06), with 17% greater activity at the backslope Enzymes vs. Soil Moisture BG, CB, and BX had positive relationships with gravimetric water content. Soil water content explained 23-32% of the variation. No significant differences when comparing the landscape positions (P = 0.3541) and cropping systems (P = 0.3637) to soil moisture. Enzymes vs. Cropping System No significant differences were detected among the cropping systems examined The average BG activity among all of the cropping systems was 5 times greater than CB activity and 10 times greater than BX activity. Enzyme activity may indicate greater carbon availability at the backslope. It could be that cellulose is more readily available to the microbes than hemicellulose or pectin. The lack of an observable crop effect may be due to our early sampling date. Greater root production under switchgrass in comparison to corn later in the growing season may influence enzyme activity. Our results support the hypothesis that soil moisture limits the production of hydrolyzing enzymes. We expect carbon to limit enzyme activity later in the season depending on root growth across cropping systems. Due to erosion downslope of the summit we expected activity to be highest at the toeslope. Our results suggest that water and nutrient availability may peak at the backslope rather than the toeslope. Therefore, hydrolyzing enzyme activity may be a poor indicator of expected erosion at our site. Our results suggest that carbon cycling occurs at a faster rate at the backslope independent of cropping system early in the growing season. This suggests that there may be a smaller opportunity for carbon sequestration at the backslope under agricultural land-use. Conversely, greater microbial activity could lead to
