Microbial extracellular polysaccharide production and aggregate stability controlled by switchgrass (Panicum virgatum) root biomass and soil water potential

Authors: SHER, YONATAN - University Of California; BAKER, NAMEER - University Of California; HERMAN, DON - University Of California; FOSSUM, CHRISTINA - University Of California; Hale, Lauren; ZHANG, XING-XU - Lanzhou University; NUCCIO, ERIN - Lawrence Livermore National Laboratory; SAHA, MALAY - Noble Research Institute; ZHOU, JIZHONG - University Of Oklahoma; PETT-RIDGE, JENNIFER - Lawrence Livermore National Laboratory; FIRESTONE, MARY - University Of California

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/3/2020
Publication Date: 2/7/2020
Publication URL: https://handle.nal.usda.gov/10113/6840647
Citation: Sher, Y., Baker, N.R., Herman, D., Fossum, C., Hale, L.E., Zhang, X., Nuccio, E., Saha, M., Zhou, J., Pett-Ridge, J., Firestone, M. 2020. Microbial extracellular polysaccharide production and aggregate stability controlled by switchgrass (Panicum virgatum) root biomass and soil water potential. Soil Biology and Biochemistry. 143. https://doi.org/10.1016/j.soilbio.2020.107742.
DOI: https://doi.org/10.1016/j.soilbio.2020.107742

Interpretive Summary: Cultivation of deeply-rooted, perennial, biofuel crops inputs carbon into marginal soils. But, it is unclear if and how the introduced carbon promotes soil fertility in poorly structured soils or sequesters carbon long-term. This research investigated microbe-plant interactions that potentially promote stable soil carbon; microbial production of extracellular polysaccharides (EPS) and increased aggregate stability. 13C labeled carbon dioxide was used to trace switchgrass-photosynthesized carbon to microbially produced soil polysaccharides in a greenhouse study. Samples from long-term field studies under switchgrass cultivation corroborated the greenhouse findings that switchgrass roots promote microbial EPS production, which can improve aggregate stability. These results signify a potential mechanism by which deeply-rooted perennial crops impact soil carbon storage.

Technical Abstract: Deep-rooting perennial grasses are promising feedstock for biofuel production, especially in marginal soils lacking organic material, nutrients, and/or that experience significant water stress. Perennial grass roots can alter surrounding soil conditions and influence microbial activities, particularly the production of extracellular polymeric substances composed primarily of extracellular polysaccharides (EPS). These polymers can alleviate cellular moisture and nutrient stress, and enhance soil characteristics through improved water retention and aggregate stability, the latter of which may in turn enhance carbon persistence. In this study we used a CO2 tracer greenhouse experiment to examine the effect of switchgrass cultivation on the production and origin of EPS in a marginal soil with five fertilization/water treatments (control, +N, +NP, +P, low water). Soils with both added nitrogen and phosphorus had the highest root biomass, EPS and percentage of water-stable soil aggregates. Multiple linear regression analyses revealed root biomass was the most important determinant for soil EPS production potentially by controlling carbon supply and diurnal changes in soil water potential. Path analysis highlighted the role of soil water potential were and EPS on with water-stable soil aggregates, indicating that EPS concentration and soil aggregation have similar drivers in this soil. High mannose content confirmed the microbial origin of EPS. 13CO2 labeling indicated that 0.18% of newly fixed plant carbon was incorporated into EPS. Analysis of field samples suggests that EPS is significantly enhanced under long-term switchgrass cultivation. Our results demonstrate that switchgrass cultivation can promote microbial production of EPS, providing a mechanism to enhance sustainability of marginal soils.