Switchgrass Roots and Reduced Soil Water Potentials Enhance Soil Extracellular Polysaccharide Content and the Frequency of Water-stable Aggregates

Deep-rooting perennial grasses are promising cellulosic feedstocks for biofuel production, especially in marginal soils that experience significant water stress. Perennial grass roots can alter surrounding soil conditions and influence microbial activities, particularly via production of extracellular polymeric substances composed primarily of extracellular polysaccharides (EPS). These polymers can alleviate cellular moisture and nutrient stress and enhance soil characteristics such as water retention and aggregate stability. Improved soil aggregation may in turn impact long-term carbon storage. In this study we used a ¹³CO₂ stable isotope labeling greenhouse experiment to examine the effect of switchgrass cultivation on the production and origin of EPS in a nutrient-poor sandy soil.

We grew ramets of a single switchgrass genotype for 18 weeks in three reconstituted field soil horizons, subjecting them to five treatments: added nitrogen, phosphorus, nitrogen plus phosphorus, 50% water, and controls. We then labeled plants either with ¹²CO₂ or ¹³CO₂ for 12 days before destructively harvesting plants and soil horizons. We assessed root biomass, soil chemistry, EPS content, the monosaccharide composition of EPS, and the amount of water-stable aggregates to determine how abiotic stresses altered the size and nature of EPS stocks and soil characteristics.

Soils with both added nitrogen and phosphorus had the most EPS, root biomass, and water-stable soil aggregates. Multiple linear regression analysis showed root biomass was the most important determinant for soil EPS production, potentially by controlling carbon supply and diurnal changes in soil water stress. Root biomass and soil water potential were also correlated with water-stable 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 soils obtained via deep coring indicates that EPS concentrations are significantly enhanced under long-term switchgrass cultivation, suggesting a mechanism by which deep-rooted perennial grass cultivation may positively affect soil aggregation in soils with low organic material.