Influence of Living Plant Roots and Mycorrhizal Hyphae on Soil Hydraulic Properties

The interrelationships between vegetation, soil, and water are fundamental in evaluating the projected impacts of global climate change. Many predictive models require soil hydraulic parameters as inputs. As most hydraulic parameter datasets are for repacked soil, the influence of vegetation on hydraulic parameters is not thoroughly understood. Living roots and mycorrhizal fungi cause physicochemical alterations in soils. Quantifying how vegetation influences soil hydraulic parameters is necessary to more accurately simulate soil water dynamics in climate models.

Laboratory experiments were conducted to test if the presence of roots and roots inoculated with mycorrhizal fungi have a significant effect on the saturated and unsaturated hydraulic conductivity, and water retention properties of two soils with contrasting textures: Flint sand and Hamblen silt loam soil. Cores were seeded with Switchgrass (Panicum virgatum) and grown in a greenhouse over three separate growth periods. Serendipita indica was injected as liquid inoculant into designated mycorrhizal cores. In both soil types, the presence of roots with mycorrhizal fungi increased total biomass.

Saturated hydraulic conductivity measurements were obtained with a soil permeameter using the constant head method. Analysis of variance (ANOVA) found that saturated hydraulic conductivity was reduced (due to pore clogging) by the presence of plant roots when grown under nutrient-deficient conditions in comparison to bare soil. In contrast, no significant differences were found between treatments for unsaturated hydraulic conductivity curve parameters obtained using the evaporation method. Soil water retention curves were also obtained using the evaporation method, and supplemented at the dry end for the Hamblen silt loam by water activity meter data. Retention curve parameters were obtained by fitting the van Genuchten equation to the resulting measurements. ANOVA indicated the presence of roots changed the shape of the water retention curve in two ways: (i) by increasing water content at saturation, and (ii) and by reducing the slope of the curve. These changes suggested roots created additional porosity and broadened the pore size distribution. The presence of mycorrhizal fungi further accentuated these effects.

Future research should investigate the effect of root-mycorrhizal interactions on soil hydraulic parameters for more soil types, plant-fungal associates, and time periods.