Intensified inundation shifts a freshwater wetland from a CO2 sink to a source

Impacts of intensive smooth cordgrass removal on net ecosystem exchange in a saltmarsh-mangrove ecotone of Southeast China

Net ecosystem exchange (NEE) of carbon dioxide (CO2) in disturbed tidal wetlands remain less investigated, albeit the importance of these 'blue carbon' ecosystems in mitigating climate change has been increasingly recognized. The invasion of smooth cordgrass into China's unvegetated tidal wetlands promotes the carbon sink, however little is known about the changes in NEE when the cordgrass is intensively removed. Here, two-year continuous eddy covariance measurements from Nov. 2021 to Oct. 2023 were used to examine how intensive cordgrass removal affects NEE in a cordgrass-dominated saltmarsh-mangrove ecotone of Southeast China. The results showed (a) this wetland acted as a monthly CO2 sink throughout the pre-removal year with nearly 90 % of the annual sink (-719.7 g C m-2 yr-1) in the cordgrass growing season from Apr. to Oct.; (b) the cordgrass removal turned this high-sink wetland into a weak CO2 source at an annual scale (39.0 g C m-2 yr-1), while the change of the sink was diurnally and seasonally unequal with daytime and growing season, respectively, accounting for the majority of the reduction; (c) tidal inundation exerted inhibitive effects on the response of daytime and nighttime NEE to photosynthetically active radiation and air temperature, respectively, with the changes in all-day NEE more driven by photosynthesis than ecosystem respiration. As one of the first assessments on the impacts of cordgrass removal on NEE, this study confirms the reduction in annual CO2 sink is predominantly attributed to the cordgrass removal instead of the climatic difference. This study highlights the importance of the interactive effects among phenological, meteorological, and tidal factors in regulating the seasonality of NEE and its changes along with cordgrass removal. Future longer flux measurements with extended years are needed to complement the present assessment of the cordgrass removal-induced impacts on NEE from a long-term perspective.

Water Level Fluctuations Modulate the Microbiomes Involved in Biogeochemical Cycling in Floodplains

Drastic changes in hydrological conditions within floodplain ecosystems create distinct microbial habitats. However, there remains a lack of exploration regarding the variations in microbial function potentials across the flooding and drought seasons. In this study, metagenomics and environmental analyses were employed in floodplains that experience hydrological variations across four seasons. Analysis of functional gene composition, encompassing nitrogen, carbon, and sulfur metabolisms, revealed apparent differences between the flooding and drought seasons. The primary environmental drivers identified were water level, overlying water depth, submergence time, and temperature. Specific modules, e.g., the hydrolysis of β-1,4-glucosidic bond, denitrification, and dissimilatory/assimilatory nitrate reduction to ammonium, exhibited higher relative abundance in summer compared to winter. It is suggested that cellulose degradation was potentially coupled with nitrate reduction during the flooding season. Phylogenomic analysis of metagenome-assembled genomes (MAGs) unveiled that the Desulfobacterota lineage possessed abundant nitrogen metabolism genes supported by pathway reconstruction. Variation of relative abundance implied its environmental adaptability to both the wet and dry seasons. Furthermore, a novel order was found within Methylomirabilota, containing nitrogen reduction genes in the MAG. Overall, this study highlights the crucial role of hydrological factors in modulating microbial functional diversity and generating genomes with abundant nitrogen metabolism potentials.

Tracing the pathways by which flood duration impacts soil bacteria through soil properties and water-extractable dissolved organic matter: A soil column experiment

Soil microbial communities control biogeochemical processes, nutrient cycling, and organic carbon storage and release in wetlands, which are influenced by flooding. To predict soil nutrient function in wetland ecosystems, understanding the effect of flooding on soil biogeochemical cycling and energy flux, including soil properties, dissolved organic matter (DOM), and microbial communities is essential. This study investigated how different flood durations (1, 3, 8, 16, and 30 d) affect the interactions between physicochemical properties and bacterial communities in a river wetland. The DOM composition was measured using ultraviolet/visible spectrophotometry coupled with fluorescence spectroscopy, and the bacterial communities were identified using 16S rRNA sequencing. Simpson's diversity index varied from 0.92 to 0.94, indicating high bacterial diversity throughout the treatments; the highest and lowest bacterial diversities were found at 1 and 8 flooding days, respectively. The abundance of Desulturomonadales, Clostridiales, Bacteroidales, and Gaiellales was positively correlated with pH, electrical conductivity, water-extractable dissolved organic carbon (WEOC), and water-extractable total dissolved nitrogen (TDN) but negatively correlated with dissolved oxygen (DO) and soil organic matter (SOM), suggesting complex interactions among these factors in response to flooding. Structural equation model revealed that flooding directly increased TDN but indirectly increased WEOC through increasing soil pH; and directly decreased DO and SOM, leading to decreases in total protein-like fraction. Three significant pathways were identified, showing the impacts of flooding on bacterial diversity: (1) flood duration decreased DO, resulting in decreased bacterial diversity; (2) flood duration decreased SOM, leading to increased bacterial diversity; and (3) flood duration decreased DO and SOM, leading to increased bacterial diversity via decreased total protein-like fraction. This study indicated that prolonged flooding has both positive and negative impacts on bacterial diversity, depending on environmental factors. It highlights the importance of flooding in shaping soil bacterial communities, with implications for nutrient cycling and carbon storage in wetlands.

Climate warming negatively affects plant water-use efficiency in a seasonal hydroperiod wetland

Climate warming has substantial influences on plant water-use efficiency (PWUE), which is defined as the ratio of plant CO2 uptake to water loss and is central to the cycles of carbon and water in ecosystems. However, it remains uncertain how does climate warming affect PWUE in wetland ecosystems, especially those with seasonally alternating water availability during the growing season. In this study, we used a continuous 10-year (2011-2020) eddy covariance (EC) dataset from a seasonal hydroperiod wetland coupled with a 15-year (2003-2017) satellite-based dataset (called PML-V2) and an in situ warming experiment to examine the climate warming impacts on wetland PWUE. The 10-year EC observational results revealed that rising temperatures had significant negative impacts on the interannual variations in wetland PWUE, and increased transpiration (Et) rather than changes in gross primary productivity (GPP) dominated these negative impacts. Furthermore, the 15-year satellite-based evidence confirmed that, in the study region, climate warming had significant negative consequences for the interannual variations in wetland PWUE by enhancing wetland Et. Lastly, at the leaf-scale, the light response curves of leaf photosynthesis, leaf Et, and leaf-scale PWUE indicated that wetland plants need to consume more water during the photosynthesis process under warmer conditions. These findings provide a fresh perspective on how climate warming influences carbon and water cycles in wetland ecosystems.

Freshwater wetland plants respond nonlinearly to inundation over a sustained period

PREMISE

Wetland plants regularly experience physiological stresses resulting from inundation; however, plant responses to the interacting effects of water level and inundation duration are not fully understood.

METHODS

We conducted a mesocosm experiment on two wetland species, sawgrass (Cladium jamaicense) and muhly grass (Muhlenbergia filipes), that co-dominate many freshwater wetlands in the Florida Everglades. We tracked photosynthesis, respiration, and growth at water levels of -10 (control), 10 (shallow), and 35 cm (deep) with reference to soil surface over 6 months.

RESULTS

The response of photosynthesis to inundation was nonlinear. Specifically, photosynthetic capacity (A_max ) declined by 25% in sawgrass and by 70% in muhly grass after 1-2 months of inundation. After 4 months, A_max of muhly grass in the deep-water treatment declined to near zero. Inundated sawgrass maintained similar leaf respiration and growth rates as the control, whereas inundated muhly grass suppressed both respiration and growth. At the end of the experiment, sawgrass had similar nonstructural carbohydrate pools in all treatments. By contrast, muhly grass in the deep-water treatment had largely depleted sugar reserves but maintained a similar starch pool as the control, which is critical for post-stress recovery.

CONCLUSIONS

Overall, the two species exhibited nonlinear and contrasting patterns of carbon uptake and use under inundation stress, which ultimately defines their strategies of surviving regularly flooded habitats. The results suggest that a future scenario with more intensive inundation, due to the water management and climate change, may weaken the dominance of muhly grass in many freshwater wetlands of the Everglades.

Integrating Aquatic Metabolism and Net Ecosystem CO2 Balance in Short- and Long-Hydroperiod Subtropical Freshwater Wetlands

How aquatic primary productivity influences the carbon (C) sequestering capacity of wetlands is uncertain. We evaluated the magnitude and variability in aquatic C dynamics and compared them to net ecosystem CO2 exchange (NEE) and ecosystem respiration (Reco) rates within calcareous freshwater wetlands in Everglades National Park. We continuously recorded 30-min measurements of dissolved oxygen (DO), water level, water temperature (Twater), and photosynthetically active radiation (PAR). These measurements were coupled with ecosystem CO2 fluxes over 5 years (2012–2016) in a long-hydroperiod peat-rich, freshwater marsh and a short-hydroperiod, freshwater marl prairie. Daily net aquatic primary productivity (NAPP) rates indicated both wetlands were generally net heterotrophic. Gross aquatic primary productivity (GAPP) ranged from 0 to − 6.3 g C m−2 day−1 and aquatic respiration (RAq) from 0 to 6.13 g C m−2 day−1. Nonlinear interactions between water level, Twater, and GAPP and RAq resulted in high variability in NAPP that contributed to NEE. Net aquatic primary productivity accounted for 4–5% of the deviance explained in NEE rates. With respect to the flux magnitude, daily NAPP was a greater proportion of daily NEE at the long-hydroperiod site (mean = 95%) compared to the short-hydroperiod site (mean = 64%). Although we have confirmed the significant contribution of NAPP to NEE in both long- and short-hydroperiod freshwater wetlands, the decoupling of the aquatic and ecosystem fluxes could largely depend on emergent vegetation, the carbonate cycle, and the lateral C flux.

Flooding variations affect soil bacterial communities at the spatial and inter-annual scales

Hydrological variations have substantial effects on the diversity and composition of soil bacterial communities in wetlands. At the spatial scale, the responses of soil bacterial diversity and composition to hydrological variations in wetlands have been extensively investigated. However, at the temporal scale, especially at the inter-annual scale, the corresponding bacterial responses are rarely reported. Therefore, we explored the effects of flooding variations on the diversity and composition of soil bacterial communities at a lakeshore wetland in two hydrological contrasting years. Three flooding variables, i.e. flooding duration (FD), total duration of the growing season (TGD), and exposure duration of the growing season (EGD), were used to characterize flooding regime. Soil bacterial communities were determined using 16S rRNA gene sequencing method. We found a very high soil bacterial diversity at the lakeshore wetland. The Shannon's indexes of soil bacterial communities varied from 5.61 to 7.11 in two years. Soil bacterial α-diversity followed a unimodal curve along the elevation gradient, and was significantly lower in the flooding year than in the drought year. Principal coordinate analysis demonstrated that the compositions of soil bacterial communities were separated in order of elevation and year along the first and second axes, respectively. The apparent habitat preferences of soil bacterial families were closely connected with their respiratory traits, and this trend was stronger at the inter-annual scale than at the spatial scale. Soil bacterial compositions were predominantly determined by the direct (by changing respiratory traits) and indirect (by changing soil pH) effects of TGD at the spatial scale, while they were simultaneously regulated by the direct effects of three flooding variables at the inter-annual scale. Our results enhance the understanding of soil microbial communities in wetlands and have large implications for developing general theories to predicting soil microbial functions.

Sea-level rise will reduce net CO2 uptake in subtropical coastal marshes

Coastal marshes have a significant capacity to sequester carbon; however, sea-level rise (SLR) is expected to result in prolonged flooding and saltwater intrusion in coastal regions. To explore the effects of SLR projections on net CO₂ uptake in coastal marshes, we conducted a "double-check" investigation, including the eddy covariance (EC) measurements of the CO₂ fluxes in subtropical coastal marshes along inundation and salinity gradients, in combination with a mesocosm experiment for analyzing CO₂ flux components under waterlogging and increased salinity conditions. During the same measurement periods, the net ecosystem CO₂ exchange (NEE_EC based on the EC dataset) in an oligohaline marsh was higher than that in a low-elevation mesohaline marsh, whereas the NEE_EC was lower than that in a high-elevation freshwater marsh. The declines in NEE_EC between the marshes could be attributed to a greater decrease in gross primary production relative to ecosystem respiration. Waterlogging slightly increased the NEE_ms (NEE based on the mesocosms) because of inhibited soil respiration and slight changes in plant photosynthesis and shoot respiration. However, the NEE_ms measured during the drainage period decreased significantly due to the stimulated soil respiration. The NEE_ms decreased with increasing salinity (except under mild salinity), and waterlogging exacerbated the adverse impacts of salinity. The amplificatory effect of decreases in both leaf photosynthesis and growth under hydrological stresses contributed more to reduce the NEE_ms than to respiratory effluxes. Both waterlogging and increased salinity reduced the root biomass, soil microbial biomass, and activities of assayed soil enzymes (except for cellulase under waterlogging conditions), leading to limited soil respiration. The declines in plant growth, photosynthesis, and soil respiration could also be attributed to the decrease in soil nutrients under waterlogging and increased salinity conditions. We propose that the coupling of SLR-driven hydrological effects lowers the capacity of CO₂ uptake in subtropical coastal marshes.