Influence of tall fescue endophyte infection on structural stability as quantified by high energy moisture characteristic in a range of soils

Abstract

Plant microbial symbionts could play a significant role on aggregate stability, an important index for soil physical quality. So far there is little information available linking the presence of endophyte in the aboveground parts of plants, with aggregate stability of rhizosphere soil. We hypothesized that the presence of Epichloë coenophiala in the aboveground parts of tall fescue (Festuca arundinacea Schreb.=Schedonorus arundinaceus and Lolium arundinaceum) may alter the structural properties of rhizospheric soil. A greenhouse pot experiment was performed to evaluate the evidence for a relationship between endophyte–tall fescue symbiosis and rhizosphere aggregate stability for a range of soils. Traditional methods for measuring aggregate stability would not be able to distinguish the impacts of these biotic factors because they usually rely on rapid immersion of aggregates in water which is very disruptive. In this study, the aggregate stability was quantified by high energy moisture characteristic (HEMC) because HEMC is capable to accurately characterize small differences in aggregate stability among the treatments and soils. Clonally propagated tillers of tall fescue (genotype 75C) naturally infected with E. coenophiala (E+) and endophyte-free ones (E−) were used. Plants were grown under optimal water, light and temperature conditions. After eight months, when plants reached the maximum vegetative growth, intact soil aggregates adherent to plant roots (i.e. rhizosphere soil) and those free of root (i.e. bulk soil) were collected for HEMC determination. The HEMCs of slow and fast wetted aggregates were determined at high matric suctions of 0 to 50hPa. HEMC data was modeled using a modified van Genuchten function. Then, various indices including volume of drainable pores (VDP), relative VDP (RVDP), VDP ratio (VDPR), modal suction (hmodal), relative hmodal (Rhmodal), stability index (SI), relative SI (RSI), stability ratio (SR), relative SR (RSR), slope at the inflection point (Si), and inflection point slope ratio (SiR) were derived to characterize soil structural stability. Endophyte presence was correlated with increased soil organic carbon (SOC) pools especially for the fine-textured soils. Changes in the SOC storage of rhizosphere soil altered the structural stability and increased the SR. The effect of endophyte infection on SR of the rhizosphere soil was also more evident in medium- and fine-textured soils due to physical protection of SOC. Lower values of hmodal and higher values of VDP of fast wetted aggregates revealed more stable macropores and greater aggregate stability in the E+ rhizosphere soil vs. E− one. VDPR, SR and SiR values of rhizosphere soil were greater in E+ plants associated soils. Fine-textured soils had lower values of SiR because of their greater values of Si in the slow wetting treatment. Plant root presence could improve the soil structure compared to the bulk soil due to exudation of cementing materials; values greater than 1 for RVDP, RSI and RSR indicate that plant root enhanced the structural stability of rhizosphere soil. In contrast to stability ratio, the effects of Epichloë endophyte–tall fescue symbiosis on relative structural stability indices such as RVDP, Rhmodal, RSI, and RSR were more pronounced in coarse- and medium-textured soils. Overall, endophyte–tall fescue symbiosis could increase aggregate stability of rhizosphere soil and thus its physical quality.