Salinization of coastal freshwater wetlands; effects of constant versus fluctuating salinity on sediment biogeochemistry

Abstract

Globally, coastal lowlands are becoming more saline by the combined effects of sea level rise, land subsidence and altered hydrological and climatic conditions. Although salinization is known to have a great influence on biogeochemical processes, literature shows contrasting effects that challenge the prediction of future effects. In addition, the effects of fluctuating salinity levels, a more realistic scenario than constant levels, on nutrient cycling in coastal wetland sediments have hardly been examined. A better understanding is therefore crucial for the prediction of future effects and the definition of effective management. To test the effects of constantly brackish water (50 mmol Cl l -1 , 3.2 psu) or fluctuating salinity (5-50 mmol Cl l -1 ), versus constantly low salinity (5 mmol Cl l -1 , 0.32 psu) on nutrient biogeochemistry, we conducted a controlled laboratory experiment with either peat or clay sediments from coastal wetlands. Increased salinity showed to have a fast and large effect. Sediment cation exchange appeared to be the key process explaining both a decrease in phosphorus availability (through calcium mobilization) and an increase in nitrogen availability, their extent being strongly dependent on sediment type. Supply of brackish water decreased surface water turbidity and inhibited sediment methane production but did not affect CO 2 production. Constant and fluctuating salinity levels showed similar longer term effects on nutrient and carbon cycling. The contrasting effects of salinization found for nitrogen and phosphorus, and its effects on water turbidity indicate major ecological consequences for coastal wetlands and have important implications for water management and nature restoration.

Short-Term Exposure of Paddy Soil Microbial Communities to Salt Stress Triggers Different Transcriptional Responses of Key Taxonomic Groups

“…The strong inhibition of CH 4 production agrees well with previous results about how salinization affects methanogenic communities in natural freshwater wetlands (Baldwin et al, 2006; Chambers et al, 2011; van Dijk et al, 2015) and paddy soils (van der Gon and Neue, 1995). Overall, these studies also observed a negative correlation between salinity and methane production.…”

“…This strong induction of AMT gene expression may be related to a highly increased bioavailability of ammonium upon the addition of NaCl to the paddy soil slurries. Due to cation exchange, the fast release of ammonium bound to soil particles is one of the geochemical key processes that occur in response to increasing salinity (Baldwin et al, 2006; van Dijk et al, 2015). …”

“…However, little is known about how seawater salinity affects the structure and function of paddy soil microbial communities, despite the fact that soil microorganisms are crucial for nutrient cycling, soil fertility, and crop productivity (Luo et al, 2016). While only a single study has examined the effects of salinity on methane emission from flooded rice fields (van der Gon and Neue, 1995), some research has been performed to investigate the impact of different levels of seawater salinity on the biogeochemistry (Chambers et al, 2011; van Dijk et al, 2015) and microbial community in soils from natural freshwater wetlands (Baldwin et al, 2006; Morrissey and Franklin, 2015; Nelson et al, 2015). These wetland soils contain highly diverse bacterial communities that are dominated by Proteobacteria, Chloroflexi, Acidobacteria, Nitrospirae, Bacteroidetes, and Verrucomicrobia (Morrissey and Franklin, 2015).…”

Fertilizer legacies meet saltwater incursion: challenges and constraints for coastal plain wetland restoration

“…The prevailing paradigm has been that increased salinity leads to increased availability of P due to sulfide binding to Fe, leading to the release of P, and increasing mineralization of organically bound P. Increases in sediment P availability has been seen with increasing salinity in Chesapeake Bay (Hartzell and Jordan, 2012). However, other studies have reported decreased P availability with increasing salinity (Baldwin et al, 2006;van Dijk et al, 2015). Two plausible reasons we did not observe increased P in surface water are that increased plant uptake prevented the extra soil solution P from leaving the site in surface water, or that salt-induced flocculation of dissolved organic carbon immobilized P .…”

Stoichiometric variation of halophytes in response to changes in soil salinity

“…Climate change will cause rises in sea levels and increase the incidence of drought, both of which can contribute to increased salinisation, a major environmental factor limiting plant growth and productivity (Kenneth 1990;Arag€ u es et al 2015). Excessive accumulation of salt in soil imposes physiological constraints on plants, including osmotic stress, nutrient imbalance and disturbance to photosynthesis, thereby affecting plant growth (Parvaiz & Satyawati 2008;Chaves et al 2009;van Dijk et al 2015), and will alter plant stoichiometry, a key trait controlling nutrient cycling (Finn et al 2015;Zechmeister-Boltenstern et al 2015). Halophytes are remarkable in their ability to regulate their physiology to adapt to changes in saline conditions.…”