Influence of arbuscular mycorrhizal fungi on soil structure and soil water characteristics of vertisols

The influence of inoculation with arbuscular mycorrhizal fungi (AM fungi) on soil water characteristics of fast and slowly wetted vertisol samples was studied. Vertisols characteristically have a low stability to wetting, and the disruption of their larger pores when they swell leads to reduced water infiltration and thereby to runoff. The degree of aggregate breakdown determines the ability of the soil to drain. A vertisol was used in this pot experiment with four treatments: T1: Pasteurized soil, T3: Pasteurized soil, with plants, T4: Inoculated, pasteurized soil, with plants. T5: Unpasteurized soil, with plants. A treatment using inoculated, pasteurized soil (T2) was included in a related study (Bearden and Petersen, 2000) comparing aggregate stability, and the present study follows the same numbering to aid in comparison of experiments. After fast, disruptive wetting, the soil inoculated with AM fungi (T4) was found to have a lower soil water content than did the soils from the other treatments at matric potentials lower than -3.92 kPa. This indicates greater drainage from pores smaller than 75 μm for the soil inoculated with AM fungi, and the greater drainage appears to be directly related to a characteristic pore range between 67 and 75 μm. The soil without plants (T1), when wetted fast, had a lower soil water content at matric potentials higher than -3.92 kPa than soils from the other treatments, which indicates less pore volume due to pores larger than 75 μm in the treatment without plants. The pore indexes, calculated as the ratio between the slope of the fast and the slope of the slowly-wetted water characteristics, generally had the highest values for the soil inoculated with AM fungi (T4) from matric potential 0.00 to -0.29 kPa. In this matric potential range, the pore indexes were less than one. The unpasteurized soil with naturally present AM fungi (T5) generally had the highest pore indexes from matric potential -0.49 to -3.92 kPa, and the pore indexes in this matric potential range were above one. These results indicate the smallest loss of very large pores in the soil inoculated with AM fungi (T4) and the largest gain of smaller sized pores in the unpasteurized soil (T5). This suggests that the resistance to breakdown of the largest pores is related to the presence of roots, and that the gain of groups of smaller pores is related to the presence of hyphae.