Switchgrass Roots Alter Soil Microbiome Exo-Polysaccharide (EPS) Production in Marginal Soils - ISRR 2018 10th Symposium for the International Society of Root Research

Yonatan Sher ¹ Nameer Baker ¹ Kateryna Zhalnina ² Lauren Hale ³ Christina Fossum ¹ Malay Saha ⁴ Trent Northen ² Jizhong Zhou ³ Jennifer Pett-Ridge ⁵ Mary Firestone ¹

¹Department of Environmental Science Policy and Management, University of California-Berkeley, USA
²Environmental Genomics and Systems Biology, Lawrence Berkeley National Lab, USA
³Department of Botany and Microbiology, University of Oklahoma, USA
⁴Forage Improvement Division, The Samuel Roberts Noble Foundation, USA
⁵Physical and Life Science Directorate, Lawrence Livermore National Laboratory, USA

Cultivation of biofuel crops, such as Switchgrass (Panicum virgatum) on marginal lands can reduce pressure on agricultural land and potentially enhance marginal soils health, by increasing soil organic carbon content. One mechanism that may mediate an increase in soil organic carbon content is the production of exopolysaccharides (EPS) by soil microbes. Soil microorganisms are believed to produce EPS to ameliorate harsh environmental conditions, such as water stress. EPS can also affect soil structure, by binding soil particles together. We examined the factors regulating EPS production by growing switchgrass (clonal plants) in a large-scale greenhouse experiment. Ninety 1m-deep plexiglass mesocosms were packed with field soil from central Oklahoma, reconstructing horizons and bulk densities found in the field. Five treatments were applied: control treatment (no fertilization), +P treatment with added rock phosphate, +N treatment with added slow-release urea, +NP treatment with added phosphate and urea, and -W treatment that received half the water of the control and not fertilized. After 20 weeks, mesocosms were destructively harvested. EPS production was determined with a cation exchange resin extraction followed by carbohydrate quantification. Results show that, in the surface horizon, +NP treatment had significantly higher concentrations of EPS and more stable aggregates, as a measure for changes in soil structure, than in other treatments. Monosaccharides composition analysis of EPS found high amounts of Mannose and low amounts of Xylose and Arabinose, providing additional evidence that detected polysaccharides are derived from a microbial source. Multiple regression analysis showed that the main factors controlling EPS production are root biomass and soil water content. Results of PLFA (phospholipid fatty acid) analysis, to estimate microbial biomass, will be presented as well. This study suggests that fertilized switchgrass grown in marginal soil can potentially increase soil organic carbon through microbial EPS production, which also increases aggregate stability.