Plant community structure mediates potential methane production and potential iron reduction in wetland mesocosms

S E Andrews,V Bouchard,R Varner,R Schultz,M J Ducey,S D Frey

doi:10.1890/es12-00314.1

Abstract

Wetlands are the largest natural source of methane to the atmosphere, but factors controlling methane emissions from wetlands are a major source of uncertainty in greenhouse gas budgets and projections of future climate change. We conducted a controlled outdoor mesocosm experiment to assess the effects of plant community structure (functional group richness and composition) on potential methane production and potential iron reduction in freshwater emergent marshes. Four plant functional groups (facultative annuals, obligate annuals, reeds, and tussocks) were arranged in a full‐factorial design and additional mesocosms were assigned as no‐plant controls. Soil samples from the top 10 cm were collected three times during the growing season to determine potential methane production and potential iron reduction (in unamended soils and in soils amended with 200 mM formate). These data were compared to soil organic matter, soil pH, and previously published data on above and belowground plant biomass. We found that functional group richness was less important than the presence of specific functional groups (reeds or tussocks) in mediating potential iron reduction. In our mesocosms, where oxidized iron was abundant and electron donors were limiting, iron reducing bacteria outcompeted methanogens, keeping methane production barely detectable in unamended lab incubations. When the possibility of re‐oxidizing iron was eliminated via anaerobic incubations and the electron donor limitation was removed by adding formate, potential methane production increased and followed the same patterns as potential iron reduction. Our findings suggest that in the absence of abundant oxidized iron and/or the presence of abundant electron donors, wetlands dominated by either reeds or tussocks may have increased methane production compared to wetlands dominated by annuals. Depending on functional traits such as plant transport and rhizospheric oxygenation capacities, this could potentially lead to increased methane emissions in some wetlands. Additional research examining the role these plant functional groups play in other aspects of methane dynamics will be useful given the importance of methane as a greenhouse gas.

Highlights

The structure and function of ecosystems are being greatly altered by human activities (Vitousek et al 1997), and the resulting loss of species worldwide has led to increasing concern for the consequences of reduced biodiversity across a wide range of ecosystems (Naeem et al 1994, Chapin et al 2000, Loreau et al 2001, Zedler et al 2001, Hooper et al 2005, Lovett et al 2009, Geyer et al 2011)
We hypothesized that potential methane production and potential iron reduction would be positively correlated with (1) increases in belowground plant biomass, (2) increases in plant functional group richness, and (3) the presence of reeds and/or tussocks
We hypothesized that potential methane production and potential iron reduction would increase with belowground biomass and that factors we expected would lead to increased belowground biomass would be correlated with increases in potential methane production and potential iron reduction

Summary

Introduction

The structure and function of ecosystems are being greatly altered by human activities (Vitousek et al 1997), and the resulting loss of species worldwide has led to increasing concern for the consequences of reduced biodiversity across a wide range of ecosystems (Naeem et al 1994, Chapin et al 2000, Loreau et al 2001, Zedler et al 2001, Hooper et al 2005, Lovett et al 2009, Geyer et al 2011). Freshwater wetland ecosystems provide a number of valuable services, including biodiversity support, water quality maintenance and improvement, flood control, and carbon storage (Zedler and Kercher 2005). While there are a number of studies documenting changes in wetland biodiversity as a consequence of human activities (Findlay and Houlahan 1997, Findlay and Bourdages 2000, Houlahan and Findlay 2004, Rosas et al 2006, Schooler et al 2006), there have been fewer studies on links between changes in biodiversity and functions that support wetland services (Engelhardt and Ritchie 2001, Mahaney et al 2006, Bouchard et al 2007, Schultz et al 2011). It is especially important that we understand the role that plant community structure plays in mediating wetland properties such as methane production and oxidation, organic matter dynamics, and plant biomass (Joabsson et al 1999, Engelhardt and Richie 2001, Vann and Megonigal 2003, Bouchard et al 2007)

Methods

Results

Conclusion