Soil compaction reversed the effect of arbuscular mycorrhizal fungi on soil hydraulic properties

Abstract

Arbuscular mycorrhizal fungi (AMF) typically provide a wide range of nutritional benefits to their host plants, and their role in plant water uptake, although still controversial, is often cited as one of the hallmarks of this symbiosis. Less attention has been dedicated to other effects relating to water dynamics that the presence of AMF in soils may have. Evidence that AMF can affect soil hydraulic properties is only beginning to emerge. In one of our recent experiments with dwarf tomato plants, we serendipitously found that the arbuscular mycorrhizal fungus (Rhizophagus irregularis 'PH5') can slightly but significantly reduce water holding capacity (WHC) of the substrate (a sand-zeolite-soil mixture). This was further investigated in a subsequent experiment, but there we found exactly the opposite effect as mycorrhizal substrate retained more water than did the non-mycorrhizal substrate. Because the same substrate was used and other conditions were mostly comparable in the two experiments, we explain the contrasting results by different substrate compaction, most likely caused by different pot shapes. It seems that in compacted substrates, AMF may have no effect upon or even decrease the substrates' WHC. On the other hand, the AMF hyphae interweaving the pores of less compacted substrates may increase the capillary movement of water throughout such substrates and cause slightly more water to remain in the pores after the free water has drained. We believe that this phenomenon is worthy of mycorrhizologists' attention and merits further investigation as to the role of AMF in soil hydraulic properties.

Keywords: Arbuscular mycorrhizal fungi; Irrigation; Pot shape; Sand–zeolite–soil mixture; Tomato; Water holding capacity.

Fig. 1

Water holding capacity (WHC) of 1 kg of dry substrate – sand-zeolite-soil mixture, 9:9:2 (v: v:v) – observed in Experiment A, conducted in pots with tapered walls, and Experiment B, conducted in pots of cylindrical shape. The effects of the experimental factors – presence of arbuscular mycorrhizal fungi (AMF) and saturation cycle (Cycle) – are shown according to repeated measures ANOVA. Different letters indicate significant differences in WHC between the saturation cycles according to Tukey’s HSD test. For the individual saturation cycles for which a t-test identified a significant effect of Rhizophagus irregularis, the boxplots and superimposed points are separated by color, with blue representing mycorrhizal (M) and red control (NM) treatments; asterisks indicate a level of significance according to the t-test (* 0.01 ≤ p < 0.05, ** 0.001 ≤ p < 0.01). Center lines of the boxplots indicate the medians, box limits represent the 25th and 75th percentiles and whiskers extend to 1.5 times the interquartile range. Diamond shape points indicate treatment means. In cases of non-significant difference, M and NM treatments are plotted together in grey and the mean in yellow. Horizontal variation within each category (n = 50 per treatment) is determined randomly to separate the points

Fig. 2

Correlation between the water holding capacity (WHC) of 1 kg of dry substrate in the 7th saturation cycle and the morphological characteristics of the roots in Exp-A. Because the t-test did not reveal significant differences between the root characteristics of mycorrhizal and control plants, both treatments are plotted together, not distinguished by color. Linear regression lines are shown together with R² and p values