Changes in Soil Microbial Functioning in Coastal Wetlands Exposed to Environmental Stressors and Subsidies

Environmental perturbations are ubiquitous features of ecosystems and shape ecological structure and function. Climate change will alter the intensity and frequency of disturbances and expose ecosystems to novel combinations of useful inputs (subsidies) and harmful inputs (stressors). Coastal wetlands are particularly vulnerable to changing environmental conditions and are increasingly exposed to effects of interacting subsidies and stressors. In particular, the Florida Coastal Everglades, which has experienced accelerated change due to a history of water management practices, is vulnerable to new disturbances associated with climate change. The low-lying Florida Everglades faces multiple disturbances from storm surge, nutrient enrichment, and sea-level rise which will influence its responses to future environmental perturbations. Microbial communities are often used to characterize environmental change because of their high surface area to volume ratio, permeable membrane, and quick turnover rates. Therefore, assessing how microbial function changes can provide insights into how subsidies and stressors interact to alter biogeochemical cycles. I tested how nutrient enrichment can alter ecosystem responses to stress and found that it did not promote recovery in mangrove plants. I examined how long-term exposure to salinity and phosphorus (the limiting nutrient in the Everglades) affected microbial enzyme activity and found that salinity alone acts as a suppressor of enzyme activity but phosphorus addition can mitigate salinity stress in sawgrass soil. I tested how pulses of salinity can affect the microbially-mediated breakdown of organic material and found that the microbial community was functionally redundant and unaffected by saltwater pulses; however, microbial activity was consistently lower in the brackish marsh compared to the freshwater marsh. I investigated how gradients of salinity and phosphorus affected freshwater and brackish soils and determined previous exposure to saltwater intrusion dominates affects microbial function and soil composition. Across these experiments, I found that environmental perturbations alter the microbial-mediated processing of nutrients and carbon, and legacies of previous disturbances influence the microbial response to new disturbance regimes

Saltwater and phosphorus drive unique soil biogeochemical processes in freshwater and brackish wetland mesocosms

Coastal ecosystems are exposed to saltwater intrusion but differential effects on biogeochemical cycling are uncertain. We tested how elevated salinity and phosphorus (P) individually and interactively affect microbial activities and biogeochemical cycling in freshwater and brackish wetland soils. In experimental mesocosms, we added crossed gradients of elevated concentrations of soluble reactive P (SRP) (0, 20, 40, 60, 80 μg/L) and salinity (0, 4, 7, 12, 16 ppt) to freshwater and brackish peat soils (10, 14, 17, 22, 26 ppt) for 35 d. We quantified changes in water chemistry [dissolved organic carbon (DOC), ammonium ((Formula presented.)), nitrate + nitrite (N + N), SRP concentrations], soil microbial extracellular enzyme activities, respiration rates, microbial biomass C, and soil chemistry (%C, %N, %P, C:N, C:P, N:P). DOC, (Formula presented.), and SRP increased in freshwater but decreased in brackish mesocosms with elevated salinity. DOC similarly decreased in brackish mesocosms with added P, and N + N decreased with elevated salinity in both freshwater and brackish mesocosms. In freshwater soils, water column P uptake occurred only in the absence of elevated salinity and when P was above 40 µg/L. Freshwater microbial EEAs, respiration rates, and microbial biomass C were consistently higher compared to those from brackish soils, and soil phosphatase activities and microbial respiration rates in freshwater soils decreased with elevated salinity. Elevated salinity increased arylsulfatase activities and microbial biomass C in brackish soils, and elevated P increased microbial respiration rates in brackish soils. Freshwater soil %C, %N, %P decreased and C:P and N:P increased with elevated salinity. Elevated P increased %C and C:N in freshwater soils and increased %P but decreased C:P and N:P in brackish soils. Freshwater soils released more C and nutrients than brackish soils when exposed to elevated salinity, and both soils were less responsive to elevated P than expected. Freshwater soils became more nutrient-depleted with elevated salinity, whereas brackish soils were unaffected by salinity but increased P uptake. Microbial activities in freshwater soils were inhibited by elevated salinity and unaffected by added P, but brackish soil microbial activities slightly increased with elevated salinity and P

Effects of Nutrient-Limitation on Disturbance Recovery in Experimental Mangrove Wetlands

Coastal wetlands are exposed to high-energy storms that influence plant and soil structure. To understand how nutrient availability interacts with storm-induced plant stress, we tested how defoliation interacts with nutrient enrichment to affect carbon (C) and nutrient (nitrogen, N; phosphorus, P) cycling and storage within soils and plants. In outdoor experimental mesocosms, we defoliated red mangrove saplings (Rhizophora mangle), added 30 g of inorganic P to peat soils, and quantified plant [elemental stoichiometry (C:N, C:P, N:P), leaf count, and above- and below- ground biomass] and soil responses [C:N, C:P, N:P, litter breakdown rate (k), soil CO2 efflux] during a 42-d recovery period. Mangroves rapidly regrew all removed leaves and recovered nearly 30% of leaf biomass. Mangrove biomass %P increased by 50% with added P; however, soil stoichiometry remained unchanged. Defoliation reduced Soil CO2 efflux by 40% and root litter k by 30%. Phosphorus was quickly incorporated into mangrove biomass and stimulated nighttime soil CO2 efflux. This work highlights the importance of testing interactions of nutrient availability and plant stress on plant and soil biogeochemical cycling and suggests that plants quickly incorporate available nutrients into biomass and defoliation can lead to reduced soil C losses

Saltwater intrusion and soil carbon loss: Testing effects of salinity and phosphorus loading on microbial functions in experimental freshwater wetlands

Wetlands can store significant amounts of carbon (C), but climate and land-use change increasingly threaten wetland C storage potential. Carbon stored in soils of freshwater coastal wetlands is vulnerable to rapid saltwater intrusion associated with sea-level rise and reduced freshwater flows. In the Florida Everglades, unprecedented saltwater intrusion is simultaneously exposing wetlands soils to elevated salinity and phosphorus (P), in areas where C-rich peat soils are collapsing. To determine how elevated salinity and P interact to influence microbial contributions to C loss, we continuously added P (~0.5 mg P d−1) and salinity (~6.9 g salt d−1) to freshwater Cladium jamaicense (sawgrass) peat monoliths for two years. We measured changes in porewater chemistry, microbial extracellular enzyme activities, respiration rates, microbial biomass, root litter breakdown rates (k), and soil elemental composition after short (57 d), intermediate- (392 d), and long-term (741 d) exposure. After 741 days, both β-1,4-glucosidase activity (P \u3c 0.01) and β-1,4-cellobiosidase activity (P \u3c 0.01) were reduced with added salinity in soils at 7.5–15 cm depth. Soil microbial biomass C decreased by 3.6× at 7.5–15 cm (P \u3c 0.01) but not 0–7.5 cm depth (P \u3e 0.05) with added salinity and was unaffected by added P. Soil respiration rates decreased after 372 d exposure to salinity (P = 0.05) and did not change with P exposure. Root litter k increased by 1.5× with added P and was unaffected by salinity exposure (P \u3e 0.01). Soil %C decreased by approximately 1.3× after 741 days of salinity exposure compared to freshwater controls (P \u3c 0.01). Elevated salinity and P accelerated wetland soil C loss primarily through leaching of DOC and increased root litter k. Our results indicate that freshwater wetland soils are sensitive to short- and long-term exposure to saltwater intrusion. Despite suppression of some soil microbial processes with added salinity, salt and P exposure appear to drive net C losses from coastal wetland soils

Saltwater intrusion and soil carbon loss: Testing effects of salinity and phosphorus loading on microbial functions in experimental freshwater wetlands

Wetlands can store significant amounts of carbon (C), but climate and land-use change increasingly threaten wetland C storage potential. Carbon stored in soils of freshwater coastal wetlands is vulnerable to rapid saltwater intrusion associated with sea-level rise and reduced freshwater flows. In the Florida Everglades, unprecedented saltwater intrusion is simultaneously exposing wetlands soils to elevated salinity and phosphorus (P), in areas where C-rich peat soils are collapsing. To determine how elevated salinity and P interact to influence microbial contributions to C loss, we continuously added P (~0.5 mg P d−1) and salinity (~6.9 g salt d−1) to freshwater Cladium jamaicense (sawgrass) peat monoliths for two years. We measured changes in porewater chemistry, microbial extracellular enzyme activities, respiration rates, microbial biomass, root litter breakdown rates (k), and soil elemental composition after short (57 d), intermediate- (392 d), and long-term (741 d) exposure. After 741 days, both β-1,4-glucosidase activity (P \u3c 0.01) and β-1,4-cellobiosidase activity (P \u3c 0.01) were reduced with added salinity in soils at 7.5–15 cm depth. Soil microbial biomass C decreased by 3.6× at 7.5–15 cm (P \u3c 0.01) but not 0–7.5 cm depth (P \u3e 0.05) with added salinity and was unaffected by added P. Soil respiration rates decreased after 372 d exposure to salinity (P = 0.05) and did not change with P exposure. Root litter k increased by 1.5× with added P and was unaffected by salinity exposure (P \u3e 0.01). Soil %C decreased by approximately 1.3× after 741 days of salinity exposure compared to freshwater controls (P \u3c 0.01). Elevated salinity and P accelerated wetland soil C loss primarily through leaching of DOC and increased root litter k. Our results indicate that freshwater wetland soils are sensitive to short- and long-term exposure to saltwater intrusion. Despite suppression of some soil microbial processes with added salinity, salt and P exposure appear to drive net C losses from coastal wetland soils

Declines in Plant Productivity Drive Carbon Loss from Brackish Coastal Wetland Mesocosms Exposed to Saltwater Intrusion

Coastal wetlands, among the most productive ecosystems, are important global reservoirs of carbon (C). Accelerated sea level rise (SLR) and saltwater intrusion in coastal wetlands increase salinity and inundation depth, causing uncertain effects on plant and soil processes that drive C storage. We exposed peat-soil monoliths with sawgrass (Cladium jamaicense) plants from a brackish marsh to continuous treatments of salinity (elevated (~ 20 ppt) vs. ambient (~ 10 ppt)) and inundation levels (submerged (water above soil surface) vs. exposed (water level 4 cm below soil surface)) for 18 months. We quantified changes in soil biogeochemistry, plant productivity, and whole-ecosystem C flux (gross ecosystem productivity, GEP; ecosystem respiration, ER). Elevated salinity had no effect on soil CO2 and CH4 efflux, but it reduced ER and GEP by 42 and 72%, respectively. Control monoliths exposed to ambient salinity had greater net ecosystem productivity (NEP), storing up to nine times more C than plants and soils exposed to elevated salinity. Submersion suppressed soil CO2 efflux but had no effect on NEP. Decreased plant productivity and soil organic C inputs with saltwater intrusion are likely mechanisms of net declines in soil C storage, which may affect the ability of coastal peat marshes to adapt to rising seas

Functional and Compositional Responses of Periphyton Mats to Simulated Saltwater Intrusion in the Southern Everglades

Periphyton plays key ecological roles in karstic, freshwater wetlands and is extremely sensitive to environmental change making it a powerful tool to detect saltwater intrusion into these vulnerable and valuable ecosystems. We conducted field mesocosm experiments in the Florida Everglades, USA to test the effects of saltwater intrusion on periphyton metabolism, nutrient content, and diatom species composition, and how these responses differ between mats from a freshwater versus a brackish marsh. Pulsed saltwater intrusion was simulated by dosing treatment chambers monthly with a brine solution for 15 months; control chambers were simultaneously dosed with site water. Periphyton from the freshwater marsh responded to a 1-ppt increase in surface water salinity with reduced productivity and decreased concentrations of total carbon, nitrogen, and phosphorus. These functional responses were accompanied by significant shifts in periphytic diatom assemblages. Periphyton mats at the brackish marsh were more functionally resilient to the saltwater treatment (~ 2 ppt above ambient), but nonetheless experienced significant shifts in diatom composition. These findings suggest that freshwater periphyton is negatively affected by small, short-term increases in salinity and that periphytic diatom assemblages, particularly at the brackish marsh, are a better metric of salinity increases compared with periphyton functional metrics due to functional redundancy. This research provides new and valuable information regarding periphyton dynamics in response to changing water sources in the southern Everglades that will allow us to extend the use of periphyton, and their diatom assemblages, as tools for environmental assessments related to saltwater intrusion

Salinity pulses interact with seasonal dry‐down to increase ecosystem carbon loss in marshes of the Florida Everglades

Coastal wetlands are globally important sinks of organic carbon (C). However, to what extent wetland C cycling will be affected by accelerated sea-level rise (SLR) and saltwater intrusion is unknown, especially in coastal peat marshes where water flow is highly managed. Our objective was to determine how the ecosystem C balance in coastal peat marshes is influenced by elevated salinity. For two years, we made monthly in situ manipulations of elevated salinity in freshwater (FW) and brackish water (BW) sites within Everglades National Park, Florida, USA. Salinity pulses interacted with marsh-specific variability in seasonal hydroperiods whereby effects of elevated pulsed salinity on gross ecosystem productivity (GEP), ecosystem respiration (ER), and net ecosystem productivity (NEP) were dependent on marsh inundation level. We found little effect of elevated salinity on C cycling when both marsh sites were inundated, but when water levels receded below the soil surface, the BW marsh shifted from a C sink to a C source. During these exposed periods, we observed an approximately threefold increase in CO2 efflux from the marsh as a result of elevated salinity. Initially, elevated salinity pulses did not affect Cladium jamaicense biomass, but aboveground biomass began to be significantly decreased in the saltwater amended plots after two years of exposure at the BW site. We found a 65% (FW) and 72% (BW) reduction in live root biomass in the soil after two years of exposure to elevated salinity pulses. Regardless of salinity treatment, the FW site was C neutral while the BW site was a strong C source (-334 to -454 g C·m-2 ·yr-1 ), particularly during dry-down events. A loss of live roots coupled with annual net CO2 losses as marshes transition from FW to BW likely contributes to the collapse of peat soils observed in the coastal Everglades. As SLR increases the rate of saltwater intrusion into coastal wetlands globally, understanding how water management influences C gains and losses from these systems is crucial. Under current Everglades\u27 water management, drought lengthens marsh dry-down periods, which, coupled with saltwater intrusion, accelerates CO2 loss from the marsh