Exploring anaerobic CO2 production response to elevated nitrate levels in Gulf of Mexico coastal wetlands: Phenomena and relationships

https://doi.org/10.1016/j.scitotenv.2019.136158Get rights and content

Highlights

  • Swamp and marshes show different responses of CO2 production to elevated nitrate.

  • Nitrate promotes CO2 production in swamp but inhibits CO2 emission in marsh soils.

  • Soil DOM in swamp has abundant polysaccharides while marshes have rich phenolics.

  • Swamp dominated with nitrate reducers while marsh soils had more sulfate reducers.

  • Different soil DOM and dominant microbes likely cause varying anaerobic respiration.

Abstract

Recent studies have shown the effect of nitrate (NO3) on carbon gas emissions from wetland soils that contradict thermodynamic predictions. In this study, CO2 production in three Mississippi River deltaic plain wetland soils (forest swamp, freshwater and saline marshes) with the presence of different NO3 levels (0.2, 2.0, and 3.2 mM) was evaluated in an anaerobic microcosm. Molecular composition of dissolved organic matter (DOM) of these soils was investigated using pyrolysis-GC/MS, and soil microbial community was characterized based on phosphorus lipid fatty acid (PLFA) method to elucidate the underlying mechanisms. Addition of NO3 promoted CO2 production in swamp forest soil, but inhibited CO2 emission from marsh soils. Pyrolysis-GC/MS analysis showed that swamp soil contained more polysaccharides, whereas both marsh soils had high abundance of phenolic compounds. Total PLFAs of forest swamp soil were 34% and 66% higher than freshwater and saline marsh soils, respectively. The PLFA profiles indicated different microbial distribution along a salinity gradient with the forest swamp having a higher proportion of fungi and NO3 reducers but lower sulfate (SO42) reducers than marsh soils. Overall, the study indicated that the inherent differences in soil DOM and microbial community led to the contrasting response in soil CO2 respiration between forest swamp and marsh ecosystems to NO3 loading. These differences should be considered in determining the fate of nitrate entering Louisiana coastal wetlands from river diversions and other sources and their management.

Introduction

Wetlands play an important role on carbon (C) cycling in nature (Zhang et al., 2002). Despite being only about 6.4% of the terrestrial surface, wetlands account for 20–25% of the global organic C (OC) reservoir (Stocker et al., 2013). From a global warming perspective, wetland participates in C interactions between terrestrial ecosystem and atmosphere by serving as both a sink of atmospheric carbon dioxide (CO2) and an emitting source of methane (CH4) (Mitsch and Gosselink, 2007; Lenart, 2009; Junk et al., 2013; Meng et al., 2016). Carbon dioxide as the primary greenhouse gas (GHG), releases to atmosphere when wetland is under drainage (aerobic) or disturbed conditions (Page and Dalal, 2011) and CO2 efflux from saltwater wetland soils is a significant component of the global C budget (Alongi et al., 2004; Donato et al., 2011). Many factors affect C mineralization, including vegetation type (Neff and Hooper, 2002; Kandel et al., 2013), nutrient availability (Bridgham et al., 1998), microbial activity (Das et al., 2017), salinity (Weston et al., 2011; Wang et al., 2017), pH (Goodwin and Zeikus, 1987), and climate change (Keller et al., 2004). In particular, the CO2 production is closely related to the activity of decomposers in wetlands under anaerobic condition (Bridgham and Richardson, 1992), which depends on the availability of C sources (Qiu et al., 2015) as well as specific electron acceptors as a replacement of O2 to decompose organic matter in soil and oxidize the latter to CO2 (Ponnamperuma, 1972; Patrick and Reddy, 1978; Zehnder and Stumm, 1988). Nitrate (NO3), which is a dominant and most preferred terminal electron acceptor based on its thermodynamic potency, can oxidize organic C in anoxic natural wetland soil (Kaplan et al., 1979; Seitzinger, 1988; Hamersley and Howes, 2005).

However, while the effects of exogenous NO3 on wetland CH4 emission is well known to be inhibitory, its effect on anaerobic CO2 production has been in debate (Dodla et al., 2009). Many studies have found that NO3 promotes CO2 production by increasing nutrient availability, which in turn enhances the microbial C demand with increased microbial activity in ecosystems (Treseder, 2008; Bridgham and Richardson, 2003). Positive correlations between NO3 availability and CO2 production were also found in several wetland soils (Swerts et al., 1996; Basiliko et al., 2005; Dettling et al., 2006). In contrast, there are also studies observing that added electron acceptors, including NO3, SO42, and Fe(III) in wetland soils did not cause an increase in CO2 production (D'Angelo and Reddy, 1999; Chidthaisong and Conrad, 2000; Vile et al., 2003). In a previous study, Dodla et al. (2009) reported a suppressive effect of NO3 on C mineralization in Louisiana coastal freshwater marsh soil. However, the extent of such phenomenon occurrence in the wide range spectrum of wetland ecosystems is unknown. There has been no systematical study to investigate such effects among different wetland ecosystems separated by a natural salinity gradient.

Wetland systems in Louisiana Gulf coastal area are characterized by various geographic distribution and are among the largest in the United States, stretching over 11,000 km2 from Vermillion Bay east to the Chandeluleur Islands (Wang et al., 2015). Part of these wetlands are subjected to disparate influence of NO3 brought in from Mississippi River diversion (Day et al., 2012). The NO3 is thought to be primarily originated from upstreams of both point and non-point sources such as wastewater treatment facilities and runoff of agriculture lands with irregular amount and frequency (David et al., 2010; Mitsch et al., 2001). On the other hand, some part of these wetlands has been affected by NO3 form direct discharge of secondarily treated disinfected, non-toxic municipal effluent from local processing facility for assimilation of nutrients (Hunter et al., 2018). The denitrification of these NO3 potentially cause destabilization of the marsh soils based on the fact that denitrification is coupled to the oxidation of organic matter (Bodker et al., 2015) and is influenced by degree of NO3 exposure (Day et al., 2018). In addition, Louisiana Gulf coastal wetlands are dominated with vegetation and salinity differences, such as swamp, freshwater, and saline marshes. Previous studies have observed different levels of major C gas fluxes between saline and freshwater marshes in Louisiana coastal wetlands (DeLaune and Smith, 1984). Thus, understanding the NO3 impacts on anaerobic CO2 production in these coastal wetlands of a natural salinity gradient is especially important for managing and utilizing these valuable wetland resources.

In this study, the soil CO2 respiration responses of Louisiana Gulf coastal wetland ecosystems (swamp and marshes) along a salinity gradient to elevated NO3 levels was investigated in a microcosm experiment. We also compared molecular composition of soil organic matter (SOM) and soil microbial community between the wetlands. It was hypothesized that the influence of NO3 on CO2 production would be determined by the intrinsic nutrient dynamics, organic C characteristics and associated soil microbiological properties within each of these wetland ecosystems. The overall objective was to explore and evaluate the links among soil CO2 respiration, organic matter chemistry, and microbial community in responding to NO3 levels in different anaerobic wetland ecosystems.

Section snippets

Soil sampling

Three wetland ecosystems, forest swamp (FS), freshwater marsh (FM), and saline marsh (SM) along a natural salinity gradient from the Barataria Basin of the Louisiana Gulf coast were selected for this study. Composite samples of 10 soil cores from top 0 to 25 cm depth at each site were collected. Samples were placed into zip-lock bags and transported to the lab on ice. Visible plant parts were removed and samples were thoroughly mixed before being used for the analysis. The major vegetation

Basic soil properties of study sites

Soil bulk density (BD) of the FS was much higher than those of marshes with similar BD between FM and SM soils (Table 1). The texture of FS soil was dominated by clay as compared to that of FM and SM soils. Saline marsh soil generally had a higher pH than the FM and FS soil. Soil electric conductivity (EC) differed among the three sites with a maximum of 38.9 dS m1 in SM soil followed by FM soil (2.7 dS m1) and FS (1.1 dS m1). Among the three ecosystems, FM soil contained the highest TOC

Conclusion

This study confirmed different response of soils of forest swamp and marsh ecosystems to elevated NO3. Addition of NO3 promoted CO2 production in forest swamp soil while it reduced CO2 respiration in marsh soils regardless of salinity variation. Further analysis indicated that the different response was due to underline inherent variation in soil DOM composition characteristics and microbial community structure between these wetlands. Specifically, swamp soil contained more polysaccharides,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The work was, in part, supported by the USDA National Institute of Food and Agriculture (NIFA)-Agricultural and Food Research Initiative (AFRI) Grant #2009-65102-05975 and the USDA National Institute of Food and Agriculture Hatch Project #10138880.

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