Estimation of Throughfall and Stemflow Bacterial Flux in a Subtropical Oak‐Cedar Forest

Abstract

Transport pathways of microbes between ecosystem spheres (atmosphere, phyllosphere, and pedosphere) represent major fluxes in nutrient cycles and have the potential to affect microbially mediated biogeochemical processes. Novel data on bacterial fluxes from the phyllosphere to the pedosphere during rainfall via throughfall (rain dripping from/through the canopy) and stemflow (rain funneled down tree stems) are reported. Bacterial concentrations were quantified using flow cytometry and validated with quantitative polymerase chain reaction assays in rainfall samples from an oak‐cedar forest in coastal Georgia (southeastern U.S.). Bacteria concentrations (cells mL−1) and storm‐normalized fluxes (cells m−2 h−1, cells m−2 mm−1) were greater for cedar versus oak. Total bacterial flux was 1.5 × 1016 cells ha−1 yr−1. These previously unexamined bacterial fluxes are interpreted in the context of major elemental pools and fluxes in forests and could represent inoculum‐level sources of bacteria (if alive), and organic matter and inorganic solute inputs (if lysed) to soils.

Bacterial Community Composition of Throughfall and Stemflow

“…The path rainwater travels through the canopy does not appear to determine the community structure of bacteria delivered to the forest surface during storms. These findings are counterintuitive to previous findings; specifically, that (i) TF and SF bacterial concentrations are two orders of magnitude greater than in open rainfall at the same site (Bittar et al, 2018), (ii) SF and TF primarily flow along bark and leaf surfaces, respectively, and (iii) bark and leaf surfaces have been reported to host distinct bacterial communities (Lambais et al, 2014). It is seemingly paradoxical that rainwater entrained as TF and SF in the canopy becomes enriched in the same bacterial taxa carried by the rainwater itself.…”

“…Findings by Lymperopoulou et al (2016) suggest that bacterial taxa located on plant leaf surfaces impact bacterial communities found in surrounding air, supporting the hypothesis that looselyassociated phyllosphere bacteria are shared with environments surrounding the plant surface. This report and other work at this site (Bittar et al, 2018) suggests that weakly-attached bacteria are numerically abundant. However, the level of activity of these bacteria on the leaf surface and how they shape the structure FIGURE 1 | Relative abundances of taxa at the family level for each flux in each sampled storm.…”

“…Previous work at this site demonstrated that SF may supply a rapid and substantial bacterial flux (30 × 10 9 cells m −2 h −1 ) to soils during storms (Bittar et al, 2018). Members of the most abundant phylum in canopy-derived hydrologic fluxes, Proteobacteria, play key roles in carbon, sulfur, and nitrogen cycling (Kersters et al, 2006).…”

“…TF is the portion of rain that drips from the canopy or passes through gaps, while SF is that which contacts and subsequently flows down stems, branches, and trunks. Recent work has shown that TF and SF contain up to 2 orders of magnitude higher concentrations of bacterial cells than open rainfall, resulting in >10 16 cells ha −1 being delivered to the forest floor annually (Bittar et al, 2018). No work has assessed the taxonomic identities of bacteria transported by TF and SF, leaving open questions with strong ecohydrological implications: (1) Which bacterial taxa are TF and SF transporting to the surface?…”

Interstorm Variability in the Biolability of Tree-Derived Dissolved Organic Matter (Tree-DOM) in Throughfall and Stemflow

“…A bacterial inoculum was added (2 mL) to each vial along with 2 mL of Nitrogen-Phosphorous-Potassium (NPK) nutrient solution (10:10:10) to prevent nutrient limitation from constraining biodegradation. As throughfall and stemflow at this site contain 10 4 -10 6 bacteria mL −1 [24], the inoculum was prepared for each storm from a volume-weighted composite of freshly collected throughfall and stemflow samples filtered through a 50 µm mesh to remove microbial grazers and coarse particulates. Caps were placed loosely on the bottles to allow air movement, then samples were incubated for 1, 2, 4, and 14 days at 25 • C in the dark on a shaker table (60 rpm).…”