Physical Conditions Regulate the Fungal to Bacterial Ratios of a Tropical Suspended Soil

Abstract

Abstract:As a source of 'suspended soils', epiphytes contribute large amounts of organic matter to the canopy of tropical rain forests. Microbes associated with epiphytes are responsible for much of the nutrient cycling taking place in rain forest canopies. However, soils suspended far above the ground in living organisms differ from soil on the forest floor, and traditional predictors of soil microbial community composition and functioning (nutrient availability and the activity of soil organisms) are likely to be less important. We conducted an experiment in the rain forest biome at the Eden Project in the U.K. to explore how biotic and abiotic conditions determine microbial community composition and functioning in a suspended soil. To simulate their natural epiphytic lifestyle, bird's nest ferns (Asplenium nidus) were placed on a custom-built canopy platform suspended 8 m above the ground. Ammonium nitrate and earthworm treatments were applied to ferns in a factorial design. Extracellular enzyme activity and Phospholipid Fatty Acid (PLFA) profiles were determined at zero, three and six months. We observed no significant differences in either enzyme activity or PLFA profiles between any of the treatments. Instead, we observed decreases in β-glucosidase and N-acetyl-glucosaminidase activity, and an increase in phenol oxidase activity across all treatments and controls over time. An increase in the relative abundance of fungi during the experiment meant that the microbial communities in the Eden Project ferns after six months were comparable with ferns sampled from primary tropical rain forest in Borneo.

Trash-basket epiphytes as secondary foundation species: a review of their distribution and effects on biodiversity and ecosystem functions

“…Gram positive and gram negative bacteria had the highest relative importance in Asplenium nidus, A. phyllitidis, and A. serratum, ranging from 19 to 21%, followed by fungi (5-11%) and actinomycetes (4%) inferred from the analysis of phospholipid fatty acid profiles (Donald et al 2017(Donald et al , 2020. A comparison between A. phyllitidis and A. serratum (from Borneo and Amazonia respectively) show they have the same groups of microorganisms but different compositions, with a greater abundance of fungi in the drier conditions of Borneo (Donald et al 2017(Donald et al , 2020. Fungi are more likely to succeed in the canopy of dry forests because they are able to transfer water through their hyphae (Guhr et al 2016;Donald et al 2017).…”

“…A comparison between A. phyllitidis and A. serratum (from Borneo and Amazonia respectively) show they have the same groups of microorganisms but different compositions, with a greater abundance of fungi in the drier conditions of Borneo (Donald et al 2017(Donald et al , 2020. Fungi are more likely to succeed in the canopy of dry forests because they are able to transfer water through their hyphae (Guhr et al 2016;Donald et al 2017). Fungi also form litter-trapping networks on their own (Snaddon et al 2012).…”

“…However, arboreal soil accumulation and decomposition in situ can take a long time due to the low percentage of debris retained by trees (Nadkarni and Matelson 1991). Accordingly, TBE adaptations to enhance litter capture and retentionincluding their own decaying tissuesmay accelerate arboreal soil formation and thus contribute to structuring ecological communities in the canopy by enhancing nutrient availability at the attachment point of each TBE (Hsu et al 2002), ameliorating water and temperature stress (Turner and Foster 2006;Jian et al 2013;Seidl et al 2020), or providing shelter to microbes, invertebrates, and vertebrates that contribute to debris decomposition and nutrient cycles (Figure 1; Paoletti et al 1991;Ellwood and Foster 2004;Karasawa and Hijii 2006b;Ortega-Solís et al 2017;Donald et al 2017;Seidl et al 2020;Donald et al 2020). The influence of TBE on biodiversity and canopy ecosystem functions varies by forest type and scale of analysis.…”

Tree Species and Epiphyte Taxa Determine the “Metabolomic niche” of Canopy Suspended Soils in a Species-Rich Lowland Tropical Rainforest

“…For example, certain members of the Bromeliaceae family form water and litter-storing phytotelmata and take up nutrients through leaf-absorbing trichomes [29,34,35], whilst Asplenium ferns intercept falling leaf litter, which is then stored as organic matter adjacent to the roots [27,36]. For both epiphytes, this accumulation of leaf litter leads to the formation of organic matter or epiphyte-associated soils that harbor characteristic levels of microbial diversity [37,38]. The extent of this harboring is not well understood, in part due to the difficulty of sampling this habitat, but also due to our inability to characterize the entire spectrum of the metabolome.…”

“…Tree diversity has been shown to influence soil heterogeneity in tropical forests [48], whereby the impacts of differences in litter species composition and their released leachate shapes both forest floor traits [49,50] and suspended canopy soils [24]. Thus, changes in soil traits and contrasting microbe community composition, nutrient status, and enzyme content of forest floor and suspended soils have been observed to depend on tree species [26,38,51]. Canopy soils have higher concentrations of organic matter than ground m. A recent study in a tropical mountain rainforest in Costa Rica revealed that canopy soils harbor very different symbiotic and fungi communities compared to ground soils and have much more enzymatic activity [26].…”