Karst Soil Patch Heterogeneity with Gravels Promotes Plant Root Development and Nutrient Utilization Associated with Arbuscular Mycorrhizal Fungi

Arbuscular mycorrhizal (AM) fungi associated with plant roots play an essential role in the belowground ecological process in karst habitats with high spatial and substrate heterogeneity. However, the effects of AM fungi on root morphology and nutrient uptake under different soil patch sizes and gravel content in karst habitats are still unclear. A controlled experiment was conducted using a square device divided into 16 grid patches. This experiment had three treatments, including the mycorrhizal fungal treatment inoculated with (M+) or without Glomus etunicatum Becker & Gerd (M−), the patch heterogeneity treatment through the homogeneous patch (Homo), heterogeneity-large patch (Hetl) and heterogeneity-small patch (Hets), and substrate heterogeneity treatment through the gravel-free substrate (GF), gravel-low substrate (GL), and gravel-high substrate (GH). Root traits and nutrients of Bidens pilosa L were analyzed, and the result showed the AM fungi significantly increased the dry weight, length, surface area, average diameter, volume, tips, branching points, and N, P, and K acquisitions of B. pilosa roots, but significantly decreased the specific root length. The Hets with soil and gravel increased the dry weight, length, surface area, tips, branching points, and N, P, and K acquisitions of B. pilosa roots compared with Hetl regulated by AM fungi. The GL and GH treatments also increased the dry weight, length, surface area, tips, branching points, and N, P, and K acquisitions of B. pilosa roots compared with GF regulated by AM fungi. These results indicate that the B. pilosa roots’ nutritional acquisition benefits were higher in Hets mixed with gravel for its root morphological development regulated by AM fungi in karst soil. In conclusion, we suggest that soil patch heterogeneity with gravels promotes root morphological development and nutrient utilization to karst plants associated with arbuscular mycorrhizal fungi.