Ramifications of increased salinity in tidal freshwater sediments: Geochemistry and microbial pathways of organic matter mineralization

Abstract

[1] The effects of salinity intrusion on the anaerobic microbial and geochemical dynamics of tidal freshwater sediments were investigated using flow-through sediment reactors. In freshwater control sediments, organic matter mineralization was dominated by methanogenesis (62%), followed by sulfate reduction (18%), denitrification (10%), and iron reduction (10%). Upon salinity intrusion, nutrient (ammonium, silicate, phosphate) concentrations increased and rates of methanogenesis declined. Iron-oxide bioavailability increased and microbial iron reduction appeared to account for >60% of organic matter oxidation for several days after salinity intrusion. However, sulfate reduction was the dominant pathway (>50%) of organic matter oxidation within 2 weeks of salinity intrusion, and accounted for >95% of total organic matter mineralization after 4 weeks. Total in situ sediment organic matter mineralization doubled following salinity intrusion. Increased nutrient release, decreased methanogenesis and a rapid shift to sulfate reduction, with a coincident increase overall organic matter mineralization, accompanied salinity intrusion into previously freshwater riverine sediments.

Electron donors and acceptors influence anaerobic soil organic matter mineralization in tidal marshes

“…Among studies that have manipulated the TEA pathway through substrate amendments, some report greater rates of carbon mineralization in the presence of NO 3 À compared to methanogenesis (Pallud et al, 2007;Abell et al, 2009) or SO 4 2À compared to methanogenesis (Reddy and Graetz, 1988;Weston et al, 2006;Pallud et al, 2007). However, in a comparison of 10 different wetland soils, D'Angelo and Reddy (1999) did not observe a difference in organic carbon mineralization rates under denitrifying, sulfate-reducing or methanogenic conditions.…”

“…Yet, other studies suggest that the free energy yield of TEAs does influence mineralization rates in anaerobic environments. Weston et al (2006) reported that a shift from methanogenesis to sulfate reduction in flow-through reactors loaded with tidal freshwater river sediment doubled mineralization rates. Using flow-through reactors and intertidal marsh sediments, Pallud et al (2007) found that nitrate reduction led to faster rates of organic matter decomposition versus sulfate reduction.…”

“…It is already well established that the availability of TEAs affects the last (terminal) step of anaerobic decomposition (Capone and Kiene, 1988;Reddy and D'Angelo, 1994;D'Angelo and Reddy, 1999;Megonigal et al, 2004) by acting through interspecific competition for electron donors (acetate and H 2 ). Indeed, previous work has demonstrated that sea level rise can lead to a shift of electron flow from methanogens to the competitively superior sulfate reducers (Neubauer et al, 2005;Weston et al, 2006). A relatively unexplored question, however, is if this shift in the dominant TEA pathway will influence rates of total anaerobic organic carbon mineralization (i.e., the sum of CO 2 þ CH 4 produced in anaerobic soil organic matter decomposition; herein "overall mineralization"), and how this pattern interacts with electron donor supply.…”

“…The adoption of such an "electron perspective" may be particularly useful in exploring a number of global change scenarios, including increased sulfate (SO 4 2À ) loading from sea level rise (e.g., Weston et al, 2006), and increased NO 3 À loading from cultural eutrophication (e.g., Vitousek et al, 1997) or acid deposition (e.g., Wieder et al,1990), all of which can alter both electron donor and acceptor availability in wetland ecosystems. For example, increased salinity caused by sea level rise may cause a change in soil carbon quality (i.e., potential for microbial mineralization) through changes in plant species composition (Gough and Grace, 1998;Baldwin et al, 2001;Donnelly and Bertness, 2001).…”

Elevated CO2 affects porewater chemistry in a brackish marsh

“…For example, between 1998 and 1999, elevated CO 2 stimulated CH 4 emissions in the C 3 -dominated community by 0.2-0.4 g C m -2 year -1 compared to a stimulation of CO 2 emissions by 34-393 g C m -2 year -1 over the same time period (Marsh et al 2005). Low rates of CH 4 production in this brackish system are likely the result of sulfate inputs from seawater driving the competitively dominant process of sulfate reduction (Kelley et al 1990;Megonigal et al 2004;Weston et al 2006b). Our data show that CH 4 concentrations were generally low in both treatments in the C 3 -and C 4 -dominated plant communities until SO 4 2-concentrations were depleted (Fig.…”