Forest canopy structural controls over throughfall affect soil microbial community structure in an epiphyte-laden maritime oak stand

Abstract:

Identifying spatiotemporal influences on soil microbial community (SMC) structure is critical to understanding of patterns in nutrient cycling and related ecological services. Since forest canopy structure alters the spatiotemporal patterning of precipitation water and solute supplies to soils (via the “throughfall” mechanism), is it possible changes in SMC structure variability could arise from modifications in canopy elements? Our study investigates this question by monitoring throughfall water and dissolved ion supply to soils beneath a continuum of canopy structure: from a large gap (0% cover) to heavy Tillandsia usneoides L. (Spanish moss) canopy (>90% cover). Throughfall water supply diminished with increasing canopy cover, yet increased washoff/leaching of Na⁺, Cl^-, PO₄^3-, and SO₄^2- from the canopy to the soils (p < 0.01). Presence of T. usneoides diminished throughfall NO₃^-, but enhanced NH₄⁺, concentrations supplied to subcanopy soils. The mineral soil horizon (0-10 cm) from canopy gaps, bare canopy, and T. usneoides-laden canopy significantly differed (p < 0.05) in soil chemistry parameters (pH, Ca²⁺, Mg²⁺, CEC). PCR-DGGE banding patterns beneath similar canopy covers (experiencing similar throughfall dynamics) also produced high similarities per ANalyses Of SIMilarity (ANO-SIM), and clustered together when analyzed by Nonmetric Multidimensional Scaling (NMDS). Correlation analysis of DGGE banding patterns, throughfall dynamics, and soil chemistry yielded significant correlations (p < 0.05) between fungal communities and soil chemical properties significantly differing between canopy cover types (pH: r² = 0.50; H⁺ %-base saturation: r² = 0.48; Ca²⁺ %-base saturation: r² = 0.43). Bacterial community structure correlated with throughfall NO₃^-, NH₄⁺, and Ca²⁺ concentrations (r² = 0.37, p = 0.16). These results suggest that modifications of forest canopy structures are capable of affecting mineral-soil horizon SMC structure via the throughfall mechanism when canopies’ biomass distribution is highly heterogeneous.