Saltwater intrusion into tidal freshwater marshes alters the biogeochemical processing of organic carbon

R B Franklin,D J Berrier,S C Neubauer

doi:10.5194/bg-10-8171-2013

Abstract

Abstract. Environmental perturbations in wetlands affect the integrated plant-microbial-soil system, causing biogeochemical responses that can manifest at local to global scales. The objective of this study was to determine how saltwater intrusion affects carbon mineralization and greenhouse gas production in coastal wetlands. Working with tidal freshwater marsh soils that had experienced ~ 3.5 yr of in situ saltwater additions, we quantified changes in soil properties, measured extracellular enzyme activity associated with organic matter breakdown, and determined potential rates of anaerobic carbon dioxide (CO2) and methane (CH4) production. Soils from the field plots treated with brackish water had lower carbon content and higher C : N ratios than soils from freshwater plots, indicating that saltwater intrusion reduced carbon availability and increased organic matter recalcitrance. This was reflected in reduced activities of enzymes associated with the hydrolysis of cellulose and the oxidation of lignin, leading to reduced rates of soil CO2 and CH4 production. The effects of long-term saltwater additions contrasted with the effects of short-term exposure to brackish water during three-day laboratory incubations, which increased rates of CO2 production but lowered rates of CH4 production. Collectively, our data suggest that the long-term effect of saltwater intrusion on soil CO2 production is indirect, mediated through the effects of elevated salinity on the quantity and quality of autochthonous organic matter inputs to the soil. In contrast, salinity, organic matter content, and enzyme activities directly influence CH4 production. Our analyses demonstrate that saltwater intrusion into tidal freshwater marshes affects the entire process of carbon mineralization, from the availability of organic carbon through its terminal metabolism to CO2 and/or CH4, and illustrate that long-term shifts in biogeochemical functioning are not necessarily consistent with short-term disturbance-type responses.

Highlights

Biogeochemical processes occurring in wetland soils can be important to the local, regional, and global cycles of elements including carbon, nitrogen, phosphorus, and sulfur
Depth effects were significant for all three treatments and all gas metrics (p ≤ 0.002), except that CH4 production did not change with depth for the +salt treatment (p = 0.46)
Saltwater intrusion can reduce plant productivity and species composition (Latham et al, 1994), potentially changing the amount and quality of autochthonous organic matter added to wetland soils

Summary

Introduction

Biogeochemical processes occurring in wetland soils can be important to the local, regional, and global cycles of elements including carbon, nitrogen, phosphorus, and sulfur These processes can be influenced by changes in environmental conditions such as temperature, soil moisture, oxygen (O2) availability, nutrient supply, and salinity (e.g., Updegraff et al, 1998; Sundareshwar et al, 2003; Baldwin et al, 2006; Bridgham et al, 2008). The mineralization of soil organic matter first requires the breakdown of complex polymers into subunits small enough for microbial uptake. This depolymerization is mediated by extracellular enzymes, which generate soluble sugars that can serve as electron donors to heterotrophic microbes. The terminal steps of C mineralization under anaerobic conditions result in the production of carbon dioxide (CO2) and/or CH4, with natural wetlands accounting for ∼ 20–30 % of global CH4 emissions (Schlesinger, 1997; Conrad, 2009; Bridgham et al, 2013)

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