Abstract
Few studies have investigated bacterial community succession and the role of bacterial decomposition over a continuum of wood decay. Here, we identified how (i) the diversity and abundance of bacteria changed along a chronosequence of decay in Populus grandidentata (bigtooth aspen); (ii) bacterial community succession was dependent on the physical and chemical characteristics of the wood; (iii) interspecific bacterial interactions may mediate community structure. Four hundred and fifty-nine taxa were identified through Illumina sequencing of 16S rRNA amplicons from samples taken along a continuum of decay, representing standing dead trees, downed wood, and soil. Community diversity increased as decomposition progressed, peaking in the most decomposed trees. While a small proportion of taxa displayed a significant pattern in regards to decay status of the host log, many bacterial taxa followed a stochastic distribution. Changes in the water availability and chemical composition of standing dead and downed trees and soil were strongly coupled with shifts in bacterial communities. Nitrogen was a major driver of succession and nitrogen-fixing taxa of the order Rhizobiales were abundant early in decomposition. Recently downed logs shared 65% of their bacterial abundance with the microbiomes of standing dead trees while only sharing 16% with soil. As decay proceeds, bacterial communities appear to respond less to shifting resource availability and more to interspecific bacterial interactions – we report an increase in both the proportion (+9.3%) and the intensity (+62.3%) of interspecific interactions in later stages of decomposition, suggesting the emergence of a more complex community structure as wood decay progresses.
Highlights
Decomposition of coarse woody debris (CWD) – large diameter wood that has senesced – is an essential process linked to the cycling of carbon and nutrients in forest ecosystems (Prewitt et al, 2014; Kielak et al, 2016b)
The most relevant work on bacterial community succession in CWD has been conducted in Europe on Picea abies (Hoppe et al, 2014; Rinta-Kanto et al, 2016), Pinus sylvestris (Kielak et al, 2016b), and Fagus sylvatica (Hoppe et al, 2014)
While bacterial succession has been documented in recently deglaciated soils (Sigler et al, 2002), on leaf surfaces (Redford et al, 2010), and even on human corpses (Hyde et al, 2014), few studies have documented bacterial succession over the course of wood decay using a chronosequence approach (Hoppe et al, 2014; Kielak et al, 2016b; Rinta-Kanto et al, 2016)
Summary
Decomposition of coarse woody debris (CWD) – large diameter wood that has senesced – is an essential process linked to the cycling of carbon and nutrients in forest ecosystems (Prewitt et al, 2014; Kielak et al, 2016b). Prior studies have shown that decreases in wood density and remaining mass, increases in relative moisture, decreases in the carbon:nitrogen (C/N) ratio, and changes in the concentration of N and phosphorus (P) are likely to be especially important drivers (Johnson et al, 2014). These sorts of directional changes in wood characteristics as decomposition progresses provide a suitable study system to evaluate the variable selection scenario proposed by Dini-Andreote et al (2015) where the emergence of new ecological niches follows from the changes in environmental conditions. Changes in the bacterial community may reflect on community interactions (complementary niches, competition, and facilitation), but we still lack studies able to disentangle these interspecific interactions from abiotic interactions – where two taxa do not interact with each other, but rather both are dependent on the same environmental factor
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