Original CommunicationElevated CO2 and nitrogen addition affect the microbial abundance but not the community structure in salt marsh ecosystem
Introduction
Coastal wetlands including salt marshes often exhibit extremely high productivity and have been found to sequester carbon at a much faster rate than forest ecosystems (Barbier et al., 2011). As many coastal wetlands are exposed to the sea level rise but can maintain a constant elevation depending on the carbon and mineral accumulation rates, the possible modification of the primary production and elevation changes in coastal wetlands in future climate conditions has been drawing much attention (Langely et al., 2008, Gedan et al., 2009, Kirwan and Mudd, 2012, Kirwan et al., 2014). The rise of atmospheric CO2 concentration and the anthropogenic nitrogen input are among the most important factors influencing the structure and function of ecosystems. In addition, as soil microorganisms play important roles in mediating ecological processes, understanding the response of soil microbial communities to environmental factors is critical to fully assessing the impact of environmental factors on ecosystem functioning and stability and predicting future climate changes. As such, there are lots of studies dealing with the effects of elevated CO2 (eCO2) and/or N on primary production (Taub, 2010, Barnaby and Ziska, 2012) or microbial communities in various ecosystems (Blagodatskaya et al., 2012, Bragazza et al., 2012, Dunbar et al., 2012, Eisenhauer et al., 2012, He et al., 2012, Lesaulnier et al., 2008, Ramirez et al., 2012, Weber et al., 2011). However, while the impacts of eCO2 and/or N enrichment on the plant and soil properties of coastal marshes have been relatively well-documented, there is little information about the effect of global change, in particular multiple climate change factors, on microbial composition in salt marsh system (Bowen et al., 2009a, Bowen et al., 2009b). Salt marsh ecosystem also harbors a diverse bacteria, fungi, and archaea (Bowen et al., 2012, Seyler et al., 2014, Weber et al., 2011) and it was reported that microbial composition would directly affect functional processes in estuarine sediments (Reed and Martiny, 2013). In addition, to achieve a broader and better prediction of the interactive effects of environmental changes, studies considering the combined effects of more than two climate change components should be conducted.
In this study, we addressed the effects of multiple climate change factors such as eCO2 and N addition on three major microbial communities (bacteria, fungi, and archaea communities) in salt marsh simultaneously. Through the investigating the microbial communities of which niche and ecophysiology are different each other, it would be possible to fully understand the impact of global change on microbial communities in salt marsh ecosystem. The specific questions are follows: 1) whether eCO2 and/or N affect the overall microbial communities, ii) whether the multiple effects of eCO2 and N are synergistic or not, iii) whether bacteria, fungi, and archaea communities respond to eCO2 and/or N differently, iv) what are specific microbial groups strongly responding to eCO2 and/or N, and finally v) whether other environmental variables significantly affecting the response pattern of microbial groups are present. For addressing these questions, we analyzed the bacterial, archaeal, and fungal community structures of salt marsh system subjected to eCO2 and N addition over 3 years with molecular techniques including pyrosequencing, terminal restriction fragment length polymorphism (T-RFLP) analysis, and quantitative polymerase chain reaction (PCR) concurrently to maximize our understanding of soil microbes and to overcome the limitations of each method.
Section snippets
Study site and sampling
This study was performed at brackish marsh system located on the Rhode River, a sub-estuary of Chesapeake Bay (lat. 38° 53′N, long. 76°33′W). The site was dominated by a C3 sedge, Scheonoplectus americanus, and C4 grasses, Spartina patens and Distichlis spicata. In this study, we selected 3.3 m2 spots with homogeneous vegetation composition and established open top chambers at those spots. We located 20 plots randomly in marsh area and all plots were equally distributed with one C3 and two C4
Analysis of bacteria and archaea pyrosequencing data
After subsampling the original sequence data, we obtained standardized sequences and OTUs for each sample (Table S1 and S2). Based on these data, we analyzed the effect of each treatment on bacterial and archaeal communities. For Bacteria, N treatment significantly affected the Shannon diversity index (Table 1). Most dominant phylum was Proteobacteria, followed by Chloroflexi, Chlorobi, Bacteroidetes, Firmicutes, Actinobacteria, Spriochaetes, and Acidobacteria (Table 1). At the phylum level, 3
Discussion
As described previously, initial vegetation before the manipulation experiment was homogeneous (Langley et al., 2009) and a previous study performed with this experimental set reported distinctively different responses of C3 and C4 plants to eCO2 and N (Langley and Megonigal, 2010). They found that N addition strongly promoted the growth of C4 plant species and distribution of plant was shifted, which ultimately suppressed the CO2-stimulation of plant productivity by the third and fourth years (
Acknowledgements
This study was supported by ERC (No. 2011-0030040) and SGER (2016R1D1A1A02937049).
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Current address: Department of Biology, Villanova University, PA19085, USA.