Carbon fluxes of China's coastal wetlands and impacts of reclamation and restoration.

Coastal wetlands play an important role in regulating atmospheric carbon dioxide (CO2) concentrations and contribute significantly to climate change mitigation. However, climate change, reclamation, and restoration have been causing substantial changes in coastal wetland areas and carbon exchange in China during recent decades. Here we compiled a carbon flux database consisting of 15 coastal wetland sites to assess the magnitude, patterns, and drivers of carbon fluxes and to compare fluxes among contrasting natural, disturbed, and restored wetlands. The natural coastal wetlands have the average net ecosystem exchange of CO2 (NEE) of -577 g C m-2 year-1, with -821 g C m-2 year-1 for mangrove forests and -430 g C m-2 year-1 for salt marshes. There are pronounced latitudinal patterns for carbon dioxide exchange of natural coastal wetlands: NEE increased whereas gross primary production (GPP) and respiration of ecosystem decreased with increasing latitude. Distinct environmental factors drive annual variations of GPP between mangroves and salt marshes; temperature was the dominant controlling factor in salt marshes, while temperature, precipitation, and solar radiation were co-dominant in mangroves. Meanwhile, both anthropogenic reclamation and restoration had substantial effects on coastal wetland carbon fluxes, and the effect of the anthropogenic perturbation in mangroves was more extensive than that in salt marshes. Furthermore, from 1980 to 2020, anthropogenic reclamation of China's coastal wetlands caused a carbon loss of ~3720 Gg C, while the mangrove restoration project during the period of 2021-2025 may switch restored coastal wetlands from a carbon source to carbon sink with a net carbon gain of 73 Gg C. The comparison of carbon fluxes among these coastal wetlands can improve our understanding of how anthropogenic perturbation can affect the potentials of coastal blue carbon in China, which has implications for informing conservation and restoration strategies and efforts of coastal wetlands.

Nitrogen fertilization enhances organic carbon accumulation in topsoil mainly by improving photosynthetic C assimilation in a salt marsh.

Continuous nitrogen (N) loading alters plant growth and subsequently has the potential to impact soil organic carbon (SOC) accumulation in salt marshes. However, the knowledge gap of photosynthesized carbon (C) allocation in plant-soil-microbial systems hampers the quantification of C fluxes and the clarification of the mechanisms controlling the C budget under N loading in salt marsh ecosystems. To address this, we conducted an N fertilization field observation combined with a 5 h 13C-pulse labeling experiment in a salt marsh dominated by Suaeda. salsa (S. salsa) in the Yellow River Delta (YRD), China. N fertilization increased net 13C assimilation of S. Salsa by 277.97%, which was primarily allocated to aboveground biomass and SOC. However, N fertilization had little effect on 13C allocation to belowground biomass. Correlation analysis showed that 13C incorporation in soil was significantly and linearly correlated with 13C incorporation in shoots rather than in roots both in a 0 N (0 g N m-2 yr-1) and +N (20 g N m-2 yr-1) group. The results suggested that SOC increase under N fertilization was mainly due to an increased C assimilation rate and more efficient downward transfer of photosynthesized C. In addition, N fertilization strongly improved the 13C amounts in the chloroform-labile SOC component by 295.26%. However, the absolute increment of newly fix 13C mainly existed in the form of residual SOC, which had more tendency for burial in the soil. Thus, N fertilization enhanced SOC accumulation although C loss increased via belowground respiration. These results have important implications for predicting the carbon budget under further human-induced N loading.

Ambient precipitation determines the sensitivity of soil respiration to precipitation treatments in a marsh.

The effects in field manipulation experiments are strongly influenced by amplified interannual variation in ambient climate as the experimental duration increases. Soil respiration (SR), as an important part of the carbon cycle in terrestrial ecosystems, is sensitive to climate changes such as temperature and precipitation changes. A growing body of evidence has indicated that ambient climate affects the temperature sensitivity of SR, which benchmarks the strength of terrestrial soil carbon-climate feedbacks. However, whether SR sensitivity to precipitation changes is influenced by ambient climate is still not clear. In addition, the mechanism driving the above phenomenon is still poorly understood. Here, a long-term field manipulation experiment with five precipitation treatments (-60%, -40%, +0%, +40%, and +60% of annual precipitation) was conducted in a marsh in the Yellow River Delta, China, which is sensitive to soil drying-wetting cycle caused by precipitation changes. Results showed that SR increased exponentially along the experimental precipitation gradient each year and the sensitivity of SR (standardized by per 100 mm change in precipitation under precipitation treatments) exhibited significant interannual variation from 2016 to 2021. In addition, temperature, net radiation, and ambient precipitation all exhibited dramatic interannual variability; however, only ambient precipitation had a significant negative correlation with SR sensitivity. Moreover, the sensitivity of SR was significantly positively related to the sensitivity of belowground biomass (BGB) across 6 years. Structural equation modeling and regression analysis also showed that precipitation treatments significantly affected SR and its autotrophic and heterotrophic components by altering BGB. Our study demonstrated that ambient precipitation determines the sensitivity of SR to precipitation treatments in marshes. The findings underscore the importance of ambient climate in regulating ecosystem responses in long-term field manipulation experiments.

Warming reshaped the microbial hierarchical interactions.

Global warming may alter microbially mediated ecosystem functions through reshaping of microbial diversity and modified microbial interactions. Here, we examined the effects of 5-year experimental warming on different microbial hierarchical groups in a coastal nontidal soil ecosystem, including prokaryotes (i.e., bacteria and archaea), fungi, and Cercozoa, which is a widespread phylum of protists. Warming significantly altered the diversity and structure of prokaryotic and fungal communities in soil and additionally decreased the complexity of the prokaryotic network and fragmented the cercozoan network. By using the Inter-Domain Ecological Network approach, the cross-trophic interactions among prokaryotes, fungi, and Cercozoa were further investigated. Under warming, cercozoan-prokaryotic and fungal-prokaryotic bipartite networks were simplified, whereas the cercozoan-fungal network became slightly more complex. Despite simplification of the fungal-prokaryotic network, the strengthened synergistic interactions between saprotrophic fungi and certain prokaryotic groups, such as the Bacteroidetes, retained these phyla within the network under warming. In addition, the interactions within the fungal community were quite stable under warming conditions, which stabilized the interactions between fungi and prokaryotes or protists. Additionally, we found the microbial hierarchical interactions were affected by environmental stress (i.e., salinity and pH) and soil nutrients. Interestingly, the relevant microbial groups could respond to different soil properties under ambient conditions, whereas under warming these two groups tended to respond to similar soil properties, suggesting network hub species responded to certain environmental changes related to warming, and then transferred this response to their partners through trophic interactions. Finally, warming strengthened the network modules' negative association with soil organic matters through some fungal hub species, which might trigger soil carbon loss in this ecosystem. Our study provides new insights into the response and feedback of microbial hierarchical interactions under warming scenario.

Salt adaptability in a halophytic soybean (Glycine soja) involves photosystems coordination.

BACKGROUND: Glycine soja is a halophytic soybean native to saline soil in Yellow River Delta, China. Photosystem I (PSI) performance and the interaction between photosystem II (PSII) and PSI remain unclear in Glycine soja under salt stress. This study aimed to explore salt adaptability in Glycine soja in terms of photosystems coordination. RESULTS: Potted Glycine soja was exposed to 300 mM NaCl for 9 days with a cultivated soybean, Glycine max, as control. Under salt stress, the maximal photochemical efficiency of PSII (Fv/Fm) and PSI (△MR/MR0) were significantly decreased with the loss of PSI and PSII reaction center proteins in Glycine max, and greater PSI vulnerability was suggested by earlier decrease in △MR/MR0 than Fv/Fm and depressed PSI oxidation in modulated 820 nm reflection transients. Inversely, PSI stability was defined in Glycine soja, as △MR/MR0 and PSI reaction center protein abundance were not affected by salt stress. Consistently, chloroplast ultrastructure and leaf lipid peroxidation were not affected in Glycine soja under salt stress. Inhibition on electron flow at PSII acceptor side helped protect PSI by restricting electron flow to PSI and seemed as a positive response in Glycine soja due to its rapid recovery after salt stress. Reciprocally, PSI stability aided in preventing PSII photoinhibition, as the simulated feedback inhibition by PSI inactivation induced great decrease in Fv/Fm under salt stress. In contrast, PSI inactivation elevated PSII excitation pressure through inhibition on PSII acceptor side and accelerated PSII photoinhibition in Glycine max, according to the positive and negative correlation of △MR/MR0 with efficiency that an electron moves beyond primary quinone and PSII excitation pressure respectively. CONCLUSION: Therefore, photosystems coordination depending on PSI stability and rapid response of PSII acceptor side contributed to defending salt-induced oxidative stress on photosynthetic apparatus in Glycine soja. Photosystems interaction should be considered as one of the salt adaptable mechanisms in this halophytic soybean.

Root Abscisic Acid Contributes to Defending Photoinibition in Jerusalem Artichoke (Helianthus tuberosus L.) under Salt Stress.

The aim of the study was to examine the role of root abscisic acid (ABA) in protecting photosystems and photosynthesis in Jerusalem artichoke against salt stress. Potted plants were pretreated by a specific ABA synthesis inhibitor sodium tungstate and then subjected to salt stress (150 mM NaCl). Tungstate did not directly affect root ABA content and photosynthetic parameters, whereas it inhibited root ABA accumulation and induced a greater decrease in photosynthetic rate under salt stress. The maximal photochemical efficiency of PSII (Fv/Fm) significantly declined in tungstate-pretreated plants under salt stress, suggesting photosystem II (PSII) photoinhibition appeared. PSII photoinhibition did not prevent PSI photoinhibition by restricting electron donation, as the maximal photochemical efficiency of PSI (ΔMR/MR0) was lowered. In line with photoinhibition, elevated H2O2 concentration and lipid peroxidation corroborated salt-induced oxidative stress in tungstate-pretreated plants. Less decrease in ΔMR/MR0 and Fv/Fm indicated that PSII and PSI in non-pretreated plants could maintain better performance than tungstate-pretreated plants under salt stress. Consistently, greater reduction in PSII and PSI reaction center protein abundance confirmed the elevated vulnerability of photosystems to salt stress in tungstate-pretreated plants. Overall, the root ABA signal participated in defending the photosystem's photoinhibition and protecting photosynthesis in Jerusalem artichoke under salt stress.

Effects of age and stand density of mother trees on early Pinus thunbergii seedling establishment in the coastal zone, China.

Effects of age and stand density of mother tree on seed germination, seedling biomass allocation, and seedling growth of Pinus thunbergii were studied. The results showed that age of mother tree did not have significant influences on seed germination, but it was significant on seedling biomass allocation and growth. Seedlings from the minimum and maximum age of mother tree had higher leaf mass ratio and lower root mass ratio than from the middle age of mother tree. Moreover, they also had higher relative height growth rate and slenderness, which were related to their biomass allocation. Stand density of mother tree mainly demonstrated significant effects on seed germination and seedling growth. Seed from higher stand density of mother tree did not decrease germination rate, but had higher mean germination time, indicating that it delayed germination process. Seedlings of higher stand density of mother tree showed higher relative height growth rate and slenderness. These traits of offspring from higher stand density of mother tree were similar to its mother, indicating significant environmental maternal effects. So, mother tree identity of maternal age and environments had important effects on natural regeneration of the coastal P. thunbergii forest.

Ecological effects of roads on the plant diversity of coastal wetland in the Yellow River Delta.

The 26 sample sites in 7 study plots adjacent to asphalt road and earth road in coastal wetland in the Yellow River Delta were selected to quantify plant diversity using quadrat sampling method in plant bloom phase of July and August 2012. The indice of ß T and Jaccard's coefficient were applied to evaluate the species diversity. The results showed that the plant diversities and alien plants were high in the range of 0-20 m to the road verge. There were more exotics and halophytes in plots of asphalt roadside than that of earth roadside. However, proportion of halophytes in habitats of asphalt roadsides was lower than that of earth roadside. By comparing ß-diversity, there were more common species in the asphalt roadsides than that in the earth roadsides. The similarity of plant communities in studied plots of asphalt roadsides and earth roadsides increased with increasing the distance to road verge. The effect range of roads for plant diversity in study region was about 20 m to road verge. Our results indicate that the construction and maintenance of roads in wetland could increase the plant species diversities of communities and risk of alien species invasion.

Wet and dry atmospheric depositions of inorganic nitrogen during plant growing season in the coastal zone of Yellow River Delta.

The ecological problems caused by dry and wet deposition of atmospheric nitrogen have been widespread concern in the world. In this study, wet and dry atmospheric depositions were monitored in plant growing season in the coastal zone of the Yellow River Delta (YRD) using automatic sampling equipment. The results showed that SO4 (2-) and Na(+) were the predominant anion and cation, respectively, in both wet and dry atmospheric depositions. The total atmospheric nitrogen deposition was ~2264.24 mg m(-2), in which dry atmospheric nitrogen deposition was about 32.02%. The highest values of dry and wet atmospheric nitrogen deposition appeared in May and August, respectively. In the studied area, NO3 (-)-N was the main nitrogen form in dry deposition, while the predominant nitrogen in wet atmospheric deposition was NH4 (+)-N with ~56.51% of total wet atmospheric nitrogen deposition. The average monthly attribution rate of atmospheric deposition of NO3 (-)-N and NH4 (+)-N was ~31.38% and ~20.50% for the contents of NO3 (-)-N and NH4 (+)-N in 0-10 cm soil layer, respectively, suggested that the atmospheric nitrogen was one of main sources for soil nitrogen in coastal zone of the YRD.

Moderate increase of precipitation stimulates CO2 production by regulating soil organic carbon in a saltmarsh.

Saltmarsh is widely recognized as a blue carbon ecosystem with great carbon storage potential. Yet soil respiration with a major contributor of atmospheric CO2 can offset its carbon sink function. Up to date, mechanisms ruling CO2 emissions from saltmarsh soil remain unclear. In particular, the effect of precipitation on soil CO2 emissions is unclear in coastal wetlands, due the lack of outdoor data in real situations. We conducted a 7-year field manipulation experiment in a saltmarsh in the Yellow River Delta, China. Soil respiration in five treatments (-60%, -40%, +0%, +40%, and + 60% of precipitation) was measured in the field. Topsoils from the last 3 years (2019-2021) were analyzed for CO2 production potential by microcosm experiments. Furthermore, quality and quantity of soil organic carbon and microbial function were tested. Results show that only the moderate precipitation rise of +40% induced a 66.2% increase of CO2 production potential for the microcosm experiments, whereas other data showed a weak impact. Consistently, soil respiration was also found to be strongest at +40%. The CO2 production potential is positively correlated with soil organic carbon, including carbon quantity and quality. But microbial diversity did not show any positive response to precipitation sizes. r-/K-strategy seemed to be a plausible explanation for biological factors. Overall, our finding reveal that a moderate precipitation increase, not decrease or a robust increase, in a saltmarsh is likely to improve soil organic carbon quality and quantity, and bacterial oligotroph:copiotroph ratio, ultimately leading to an enhanced CO2 production.

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.

Winter soil respiration from different vegetation patches in the Yellow River Delta, China.

Vegetation type and density exhibited a considerable patchy distribution at very local scales in the Yellow River Delta, due to the spatial variation of soil salinity and water scarcity. We proposed that soil respiration is affected by the spatial variations in vegetation type and soil chemical properties and tested this hypothesis in three different vegetation patches (Phragmites australis, Suaeda heteroptera and bare soil) in winter (from November 2010 to April 2011). At diurnal scale, soil respiration all displayed single-peak curves and asymmetric patterns in the three vegetation patches; At seasonal scale, soil respiration all declined steadily until February, and then increased to a peak in next April. But, the magnitude of soil respiration showed significant differences among the three sites. Mean soil respiration rates in winter were 0.60, 0.45 and 0.17 µmol CO(2) m(-2) s(-1) for the Phragmites australis, Suaeda heteroptera and bare soil, respectively. The combined effect of soil temperature and soil moisture accounted for 58-68 % of the seasonal variation of winter soil respiration. The mean soil respiration revealed positive and linear correlations with total N, total N and SOC storages at 0-20 cm depth, and plant biomass among the three sites. We conclude that the patchy distribution of plant biomass and soil chemical properties (total C, total N and SOC) may affect decomposition rate of soil organic matter in winter, thereby leading to spatial variations in soil respiration.

Nitrogen addition alters plant growth in China's Yellow River Delta coastal wetland through direct and indirect effects.

In the coastal wetland, nitrogen is a limiting element for plant growth and reproduction. However, nitrogen inputs increase annually due to the rise in nitrogen emissions from human activity in coastal wetlands. Nitrogen additions may alter the coastal wetlands' soil properties, bacterial compositions, and plant growth. The majority of nitrogen addition studies, however, are conducted in grasslands and forests, and the relationship between soil properties, bacterial compositions, and plant growth driven by nitrogen addition is poorly understood in coastal marshes. We conducted an experiment involving nitrogen addition in the Phragmites australis population of the tidal marsh of the Yellow River Delta. Since 2017, four nitrogen addition levels (N0:0 g • m-2 • year-1, N1:5 g • m-2 • year-1, N2:20 g • m-2 • year-1, N3:50 g • m-2 • year-1) have been established in the experiment. From 2017 to 2020, we examined soil properties and plant traits. In 2018, we also measured soil bacterial composition. We analyzed the effect of nitrogen addition on soil properties, plant growth, reproduction, and plant nutrients using linear mixed-effect models. Moreover, structural equation modeling (SEM) was utilized to determine the direct and indirect effects of nitrogen addition, soil properties, and bacterial diversity on plant growth. The results demonstrated that nitrogen addition significantly affected plant traits of P. australis. N1 and N2 levels generally resulted in higher plant height, diameter, leaf length, leaf breadth, and leaf TC than N0 and N3 levels. Nitrogen addition had significantly impacted soil properties, including pH, salinity, soil TC, and soil TS. The SEM revealed that nitrogen addition had a direct and positive influence on plant height. By modifying soil bacterial diversity, nitrogen addition also had an small indirect and positive impact on plant height. However, nitrogen addition had a great negative indirect impact on plant height through altering soil properties. Thus, nitrogen inputs may directly enhance the growth of P. australis at N1 and N2 levels. Nonetheless, the maximum nitrogen addition (N3) may impede P. australis growth by reducing soil pH. Therefore, to conserve the coastal tidal marsh, it is recommended that an excess of nitrogen input be regulated.

[Effects of simulated precipitation changes on plant community characteristics of wetland in the Yellow River Delta, China]. / 模拟降雨量变化对黄河三角洲湿地植物群落特征的影响.

Under the changing climate scenario, changes in precipitation regimes are expected to alter soil water and salinity conditions, with consequences on the characteristics of plant community in estuarine wetland. Here, we used a six-year (2015-2020) precipitation manipulation experiment to examine how plant community characteristics responded to precipitation changes in the Yellow River Delta. The results showed that soil electrical conductivity significantly decreased, while soil moisture significantly increased with increasing precipitation. Precipitation changes altered plant community composition. Increased precipitation reduced the absolute dominance of Suaeda glauca and Suaeda salsa, but increased that of Triarrhena sacchariflora and Imperata cylindrica. Shannon index and Margalef richness index of plant community significantly increased with increasing precipitation. Compared with the control, both decreased and increased precipitation decreased the plant community abundance, frequency and coverage. The treatment of 60% increased precipitation significantly decreased plant community frequency by 54.9%, while the 60% decreased precipitation, 40% decreased precipitation, 40% increased precipitation and 60% increased precipitation treatment significantly decreased plant abundance by 38.9%, 33.8%, 35.8% and 45.7%, respectively. The aboveground biomass significantly increased with increasing precipitation, but aboveground plant biomass under 60% increased precipitation treatment being lower than that reducing under 40% increased precipitation treatment, probably due to the negative effects of flooding stress. In addition, Margalef richness index had a significantly positive relationship with aboveground biomass. Aboveground biomass, Shannon diversity index, Margalef richness index, and Simpson diversity index were negatively related to soil electrical conductivity, and aboveground plant biomass was positively related to soil moisture. Our results revealed that precipitation changes regulate growth characteristics, species composition, and diversity of plant community by altering soil water and salinity conditions in a coastal wetland.

[C:N:P stoichiometry in plants and soils of Phragmites australis wetland under different water-salt habitats]. / 不同水盐生境下芦苇湿地植被及土壤碳氮磷生态化学计量特征.

We examined the effects of channel diversion of Yellow River on the content and stoichiometry of carbon (C), nitrogen (N) and phosphorus (P) in the organs of reeds (stem, leaf, rhizome and fibrous root) and soils in three typical Phragmites australis communities in the Yellow River Delta, including P. australis community in the former Yellow River course abandoned in 1996, P. australis community on the new Yellow River course and the P. australis communities on the intertidal area (far from the abandoned and current channel but affected by the tides). The results showed that foliar C, N and P contents of P. australis were highest in the communities of abandoned Yellow River course. Leaf N, stem C and rhizome P contents were highest in the communities of new Yellow River course. Leaf N and stem C and P contents were highest in the communities of intertidal area. The average leaf C (409.48 g·kg-1) and P (1.09 g·kg-1) contents in the three habitats were lower than national and global average levels, while leaf N content (21.71 g·kg-1) was higher than that of national and global average levels. The mean leaf N:P (20.22) was higher than 16 and the mean soil N:P (0.87) was lower than 14, indicating that the P. australis growth in the three habitats was limited by P. Correlation analysis showed that EC was one of the main factors affecting C:N:P stoichiometry in P. australis. In general, the C and P reserves in P. australis in the study area were low, and N reserve was high. The soil organic carbon content was low, the soil C reserves were large, while the N and P were relatively scarce.

Experimental warming reduces ecosystem resistance and resilience to severe flooding in a wetland.

Climate warming and extreme hydrological events are threatening the sustainability of wetlands across the globe. However, whether climate warming will amplify or diminish the impact of extreme flooding on wetland ecosystems is unknown. Here, we show that climate warming significantly reduced wetland resistance and resilience to a severe flooding event via a 6-year warming experiment. We first found that warming rapidly altered plant community structure by increasing the dominance of low-canopy species. Then, we showed that warming reduced the resistance and resilience of vegetation productivity to a 72-cm flooding event. Last, we detected slower postflooding carbon processes, such as gross ecosystem productivity, soil respiration, and soil methane emission, under the warming treatment. Our results demonstrate how severe flooding can destabilize wetland vegetation structure and ecosystem function under climate warming. These findings indicate an enhanced footprint of extreme hydrological events in wetland ecosystems in a warmer climate.

Fungi and cercozoa regulate methane-associated prokaryotes in wetland methane emissions.

Wetlands are natural sources of methane (CH4) emissions, providing the largest contribution to the atmospheric CH4 pool. Changes in the ecohydrological environment of coastal salt marshes, especially the surface inundation level, cause instability in the CH4 emission levels of coastal ecosystems. Although soil methane-associated microorganisms play key roles in both CH4 generation and metabolism, how other microorganisms regulate methane emission and their responses to inundation has not been investigated. Here, we studied the responses of prokaryotic, fungal and cercozoan communities following 5 years of inundation treatments in a wetland experimental site, and molecular ecological networks analysis (MENs) was constructed to characterize the interdomain relationship. The result showed that the degree of inundation significantly altered the CH4 emissions, and the abundance of the pmoA gene for methanotrophs shifted more significantly than the mcrA gene for methanogens, and they both showed significant positive correlations to methane flux. Additionally, we found inundation significantly altered the diversity of the prokaryotic and fungal communities, as well as the composition of key species in interactions within prokaryotic, fungal, and cercozoan communities. Mantel tests indicated that the structure of the three communities showed significant correlations to methane emissions (p < 0.05), suggesting that all three microbial communities directly or indirectly contributed to the methane emissions of this ecosystem. Correspondingly, the interdomain networks among microbial communities revealed that methane-associated prokaryotic and cercozoan OTUs were all keystone taxa. Methane-associated OTUs were more likely to interact in pairs and correlated negatively with the fungal and cercozoan communities. In addition, the modules significantly positively correlated with methane flux were affected by environmental stress (i.e., pH) and soil nutrients (i.e., total nitrogen, total phosphorus and organic matter), suggesting that these factors tend to positively regulate methane flux by regulating microbial relationships under inundation. Our findings demonstrated that the inundation altered microbial communities in coastal wetlands, and the fungal and cercozoan communities played vital roles in regulating methane emission through microbial interactions with the methane-associated community.

Interactive effects of crab herbivory and spring drought on a Phragmites australis-dominated salt marsh in the Yellow River Delta.

Consumers are often overlooked as key drivers of vegetation structure and ecosystem functioning in coastal wetlands. This oversight is particularly apparent in Asia, where much of the variation in coastal wetland plant growth and composition is attributed to physical stress gradients. To address this knowledge gap and quantify the relative importance of consumers in Asian coastal wetlands across temporal variation in environmental stress, we conducted a two-year experiment spanning relatively spring wet (2018) and spring dry (2019) years in which we manipulated the presence of the numerically dominant herbivorous crab, Helice tientsinensis, and evaluated its effects on Phragmites australis growth and structure in a Yellow River Delta salt marsh. In spring wetter 2018, Phragmites biomass and stem density were 75% and 34% higher in Crab Exclusion relative to Ambient Crab plots. In 2019 which experienced spring drought and elevated soil salinity, Phragmites biomass and stem density remained similarly high relative to 2018 in Crab Exclusion plots, but fell further, to only 16% and 39% of levels of 2018 observed in Ambient Crab plots. Phragmites' inflorescences density was also significantly reduced in Ambient Crab than Crab Exclusion plots in 2019. Together, these results highlight the significant role that crab herbivores can play in regulating Phragmites in Yellow River Delta salt marshes and suggest that the magnitude of their top-down control may be amplified, although in a non-additive manner, with spring drought stress in the region.

Reduced magnitude and shifted seasonality of CO2 sink by experimental warming in a coastal wetland.

Coastal wetlands have the highest carbon sequestration rate per unit area among all unmanaged natural ecosystems. However, how the magnitude and seasonality of the CO2 sink in coastal wetlands will respond to future climate warming remains unclear. Here, based on measurements of ecosystem CO2 fluxes in a field experiment in the Yellow River Delta, we found that experimental warming (i.e., a 2.4°C increase in soil temperature) reduced net ecosystem productivity (NEP) by 23.7% across two growing seasons of 2017-2018. Such a reduction in NEP resulted from the greater decrease in gross primary productivity (GPP) than ecosystem respiration (ER) under warming. The negative warming effect on NEP mainly occurred in summer (-43.9%) but not in autumn (+61.3%), leading to a shifted NEP seasonality under warming. Further analyses showed that the warming effects on ecosystem CO2 exchange were mainly controlled by soil salinity and its corresponding impacts on species composition. For example, warming increased soil salinity (+35.0%), reduced total aboveground biomass (-9.9%), and benefited the growth of plant species with high salt tolerance and late peak growth. To the best of our knowledge, this study provides the first experimental evidence on the reduced magnitude and shifted seasonality of CO2 exchange under climate warming in coastal wetlands. These findings underscore the high vulnerability of wetland CO2 sink in coastal regions under future climate change.

Co-regulation of rainfall amount and timing on soil carbon mineralization in a typical salt marsh of the Yellow River Delta, China.

Studying the effects of rainfall regimes such as rainfall amount and timing on soil carbon mineralization is of great importance for our understanding the mechanisms underlying the stability and accumulation of soil carbon in coastal salt marshes. In this study, we examined the responses of soil carbon mineralization (CO2 and CH4 fluxes) from undisturbed soil columns to rainfall events in different seasons (dry and wet seasons) with filed experiments in a primary Suaeda salsa region in the Yellow River Delta salt-marsh wetland, which is far away from the coast and not affected by tides. The results showed that rainfall amount and timing had a significant interaction in affecting soil CO2 flux rates. During the dry season, large rainfall events significantly reduced soil CO2 flux rates but had no significant effect in the wet season, which might be closely related to the significant increase in soil water content and salinity. Rainfall amount, rainfall timing and their interactions had no significant effect on soil CH4 efflux rates. Rainfall timing and rainfall amount did not affect CH4/CO2. CH4/CO2 increased with increasing soil water content and salinity. Soil water content and soil salinity showed similar increases to increasing rainfall amount. Our results suggested that the changing rainfall regime under climate change in the future would have a great impact on soil carbon mineralization and carbon sink function by regulating soil water and salt migration in this region.