Abstract
Microbial succession during leaf breakdown was investigated in a small forested stream in west-central Georgia, USA, using multiple culture-independent techniques. Red maple (Acer rubrum) and water oak (Quercus nigra) leaf litter were incubated in situ for 128 days, and litter breakdown was quantified by ash-free dry mass (AFDM) method and microbial assemblage composition using phospholipid fatty acid analysis (PLFA), ribosomal intergenic spacer analysis (RISA), denaturing gradient gel electrophoresis (DGGE), and bar-coded next-generation sequencing of 16S rRNA gene amplicons. Leaf breakdown was faster for red maple than water oak. PLFA revealed a significant time effect on microbial lipid profiles for both leaf species. Microbial assemblages on maple contained a higher relative abundance of bacterial lipids than oak, and oak microbial assemblages contained higher relative abundance of fungal lipids than maple. RISA showed that incubation time was more important in structuring bacterial assemblages than leaf physicochemistry. DGGE profiles revealed high variability in bacterial assemblages over time, and sequencing of DGGE-resolved amplicons indicated several taxa present on degrading litter. Next-generation sequencing revealed temporal shifts in dominant taxa within the phylum Proteobacteria, whereas γ-Proteobacteria dominated pre-immersion and α- and β-Proteobacteria dominated after 1 month of instream incubation; the latter groups contain taxa that are predicted to be capable of using organic material to fuel further breakdown. Our results suggest that incubation time is more important than leaf species physicochemistry in influencing leaf litter microbial assemblage composition, and indicate the need for investigation into seasonal and temporal dynamics of leaf litter microbial assemblage succession.
Highlights
Allochthonous inputs are the major source of energy and nutrients within food webs of many small forested streams [1], with this input primarily entering streams as leaf litter from surrounding riparian vegetation [2,3,4,5]
Tukey’s pairwise comparisons of initial current velocity among runs indicated that the run sampled on day 2 was more similar to the runs sampled on days 32 and 64 than any other runs, but given that leaf litter microbial assemblage data on day 2 was more similar to days 0, 4, and 8, this difference in current velocity likely had little effect on the leaf litter microbial assemblage relative to sampling date
Breakdown rates strongly differed between leaf species in our study, with observed rates being faster for red maple than water oak, a result consistent with previous work [1]
Summary
Allochthonous (external) inputs are the major source of energy and nutrients within food webs of many small forested streams [1], with this input primarily entering streams as leaf litter from surrounding riparian vegetation [2,3,4,5]. The structural and energetic importance of leaf litter to forested streams makes it an integral part of overall ecosystem integrity and function [1, 7, 10]. Leaf breakdown, defined as the decomposition of vascular plant detritus described by Webster and Benfield [1], consists of 3 primary phases—leaching, conditioning, and fragmentation. Colonization and conditioning by fungi and bacteria softens litter and facilitates further decomposition within days after immersion [12, 13]. Litter is subsequently fragmented by physical abrasion and processing by macroinvertebrate consumers (i.e., shredders) [12, 14, 15], which, in combination, accelerate breakdown [16]
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