Impacts of Experimental Flooding on Microbial Communities and Methane Fluxes in an Urban Meadow, Baton Rouge, Louisiana

Gary M. King,Katherine Henry

doi:10.3389/fevo.2019.00288

Abstract

The impacts of flooding on microbial communities and their activities in rice paddy soils have been well documented. However, little is known about the responses of urban soil microbial communities. To assess resistance and resilience to short-term flooding, surface soil samples and intact soil cores were collected from an urban meadow bisected by Dawson Creek in Baton Rouge, LA. All samples were collected subsequent to the unprecedented flood of south central Louisiana during August, 2016, which ensued after deposition of some 27 trillion liters of water. During the flood, an area (lower meadow) adjacent to Dawson Creek was inundated for an unknown period lasting at least several days, while an elevated area (upper meadow) was not flooded. Microbial community composition and diversity at each site were assessed for soils collected from core sections at intervals over a 10-cm depth before and after 3 days of experimental flooding ex situ. Cores from both lower and upper meadow sites were also used to assess methane fluxes before and after the experimental flooding. The results indicated that methane fluxes differed between lower and upper meadow sites, and that they were not resistant to flooding. Lower meadow cores emitted methane prior to flooding and rates increased substantially post-flooding; upper meadow cores consumed methane to levels below ambient atmospheric concentrations prior to flooding, but emitted methane post-flooding. In contrast both lower and upper meadow microbial communities were resistant to short-term flooding, with no significant changes observed at either site, or at any depth interval from the surface to 10 cm. However, lower and upper meadow soil communities differed significantly, with distinct distributions of Acidobacteria, Nitrospirae, and Thaumarchaeota among others. Based on responses of soil cores to experimental flooding, the differences between sites in microbial communities do not appear to be residual effects of the August, 2016 flood, but rather appear to arise from physical, chemical, and biological variables that change along a 4-m elevation gradient. Collectively, the results suggest that the composition and diversity for some urban soils might be insensitive to short-term flooding, but that important biogeochemical processes, e.g., methane fluxes, might respond rapidly.

Highlights

Due to long-term shifts from rural areas, more than 80% of America’s population lives in cities, of which many are vulnerable to climate change (e.g., McMichael et al, 2006; Myers and Patz, 2009; Lau et al, 2010; Depietri et al, 2011; Douglas et al, 2012)
The impacts of flooding on natural and agricultural wetlands, especially rice paddy soils, have been studied extensively using cultivation-dependent, cultivationindependent, and biogeochemical and functional analyses (e.g., Bronson et al, 1997; Hengstmann et al, 1999; Noll et al, 2005; Hansel et al, 2008; Unger et al, 2009), the results offer limited predictive insights for urban soils
Experimental flooding in LM cores was associated with higher pH values than in the unflooded cores, but the differences were only marginally significant (p = 0.09)

Summary

Introduction

Due to long-term shifts from rural areas, more than 80% of America’s population lives in cities, of which many are vulnerable to climate change (e.g., McMichael et al, 2006; Myers and Patz, 2009; Lau et al, 2010; Depietri et al, 2011; Douglas et al, 2012). In contrast to paddy soils, which are flooded regularly on a seasonal basis, and for extended periods allowing for microbial community adaptation, urban soils are flooded irregularly for relatively short periods and likely are not flood adapted. Floodplain soils might represent a more suitable model, since they too are flooded intermittently for short periods, but relatively little is known about the microbial communities in them (e.g., Katayama and Kuwatsuka, 1991; Baldwin and MItchell, 2000; Brettar et al, 2002; Rinklebe and Langer, 2006; Langer and Rinklebe, 2009; Parsons et al, 2013), which limits extrapolations to urban systems

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Conclusion