Enhancing rhizosphere soil water retention in wheat through colonization with endophytic fungus Serendipita indica

Effect of S. indica colonization of wheat (Triticum aestivum) on water retention and physical quality of the rhizosphere soil was investigated in a greenhouse study. Colonized and uncolonized seedlings of four wheat cultivars (i.e., Roshan, Ghods, Pishtaz, and Kavir) were grown in rhizoboxes. Plants were harvested and intact samples were taken from the rhizosphere to measure soil organic carbon (SOC), hot-water soluble carbohydrates (HWSC), and soil water retention curve (SWRC). Water contents at field capacity (θ_FC) and permanent wilting point (θ_PWP), and plant available water (PAW) were obtained from the measured SWRC data. The integral energy (E_I(PAW)) and integral water storage (W_I(PAW)), two alternative indices of soil water availability and physical quality, were computed by integration of SWRCs over the PAW range with respect to water content and matric potential, respectively. Low E_I(PAW) and high W_I(PAW) indicate improved soil physical quality and better soil water availability to plant. The results showed that the SOC storage and HWSC of the wheat rhizosphere were approximately 146 and 83% greater than those of unplanted soils, respectively. Moreover, compared with the uncolonized counterparts, S. indica colonization significantly increased the SOC and HWSC by 76 and 134%, respectively. The presence of plant roots and S. indica colonization improved soil water retention by increasing SOC and HWSC and subsequently enhancing portion of structural porosity (i.e., pore space between the soil aggregates in the size range of 0.5–500 μm, mainly includingmacropores and mesopores). Despite having the same PAW, colonized soil samples had lower E_I(PAW) (≈ 133 J kg⁻¹) compared to uncolonized counterparts (≈ 153 J kg⁻¹). This finding implies that plant roots take up water from the colonized wheat-planted soils easier than from the uncolonized counterparts. Compared to uncolonized samples, an increase of 50% in S₁ (Dexter's index of soil physical quality) of the colonized Kavir cultivar [with the lowest root extension, SOC (i.e., 0.97 g kg⁻¹), and HWSC (i.e., 0.059 g kg⁻¹)] showed that S. indica can improve soil physical quality due to greater structural pores. Among the four wheat cultivars, the rhizosphere of Roshan (≈ 1.60 g kg⁻¹) and Ghods (1.61 g kg⁻¹) had the highest SOC values. Higher HWSC (i.e., 0.138 g kg⁻¹) and more extensive root distribution for Roshan, especially upon co-cultivation with S. indica, led to greater contribution of structural pores in water retention in the rhizosphere soil when compared with three other cultivars. To conclude, S. indica colonization not only in the presence of plant roots and by increasing SOC and HWSC improves the physical quality of the rhizosphere soil but also the fungus by itself enhances soil physical quality due to enhancing portion of structural porosity involving in water transition and storage.