Search

As productive and essential ecosystems, coastal wetlands have experienced increased environmental impacts such as saltwater intrusion and eutrophication, resulting in significant shifts in microbially mediated ecosystem functions, such as carbon sequestration and nutrient transformations. The soil microbial respiration, a primary process in the transfer of carbon from soil to the atmosphere, is susceptible to environmental changes. However, studies on how salinity affects soil microbial respiration in coastal wetlands have not been fully explored. Soil samples were systematically collected from divergent sampling sites covering medium‐ and extremely‐saline wetlands along a river‐estuary‐coast continuum to investigate mechanisms controlling soil microbial respiration in coastal wetlands. According to the results, the microbial biomass and carbon‐related extracellular enzyme activities were significantly lower in extremely saline (ECe >15 ds m−1, ES) than medium and highly saline soils (ECe <15 ds m−1, MHS) (p < 0.05), indicating a suppressive effect of salinity on soil microbiota. Meanwhile, high‐salinity soils had lower vector length and soil microbial respiration rates, suggesting that soils with low carbon limitation might cause less carbon loss under higher salinity environments. Moreover, it was showed that increased available phosphorus could alleviate microbial carbon limitations. Changes in the microbial functional community demonstrated that the microbial community in favor of metabolic mediates and secondary metabolites substrates (regarded as labile substrates) were more sensitive to salinity. The partial least square path modeling further confirmed that microbial nutrient limitation and microbial biomass contribute more directly to promoting soil microbial respiration. These results have substantial implications for elucidating carbon dynamics in coastal wetlands ecosystems under increased nutrient discharge and sea‐level rise. [ABSTRACT FROM AUTHOR]

As a major plant‐derived soil organic carbon (SOC) component, lignin phenols are unique biomarkers that reflect biogeochemical characteristics under different vegetation compositions and climatic zones in coastal wetlands. However, the latitudinal patterns of plant‐derived lignin phenols to SOC and their link with the stability and controlling mechanisms remain poorly understood.A total of 156 soil samples from 39 sites along a 5000 km coastal transect, were taken to explore the effects of biological and environmental controls on the patterns of lignin phenols. Lignin phenols had contents ranging from 1.91 to 83.3 mg g−1 OC, and a positive correlation was detected in grass‐dominated salt marsh, but a weakly negative correlation in mangrove. Positive correlations between SOC or lignin content and C/V or S/V (the cinnamyl‐ or syringyl‐to‐vanillyl) ratios were found, while overall negative correlations between SOC or lignin content and (Ad/Al)V or (Ad/Al)S (the acid‐to‐aldehyde of vanillyl or syringyl units) ratios were detected, respectively, which confirmed the validity of these lignin biomarker degradation parameters.Our findings revealed that plant C inputs and monomer ratios directly influenced the capacity of lignin phenols in soils. Lignin content and stabilization was mainly controlled by soil properties (i.e. pH, EC sand/clay). Mean annual temperature (MAT) influenced the patterns of lignin phenols both directly by increasing decomposition and indirectly by changing the vegetation and soil biogeochemistry (i.e. microbial substrate availability).Coastal wetlands are characterized by high primary productivity and C burial rate, yet plant‐derived lignin phenols are not as much as we thought compared to microbial residues C. Precise identification and quantification of the origin, decomposition, and determinants of lignin phenols help us understand their contribution to C sequestration and its response to climate and environmental changes. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Soil organic carbon (SOC) in coastal wetlands, also known as "blue C," is an essential component of the global C cycles. To gain a detailed insight into blue C storage and controlling factors, we studied 142 sites across ca. 5000 km of coastal wetlands, covering temperate, subtropical, and tropical climates in China. The wetlands represented six vegetation types (Phragmites australis, mixed of P. australis and Suaeda, single Suaeda, Spartina alterniflora, mangrove [Kandelia obovata and Avicennia marina], tidal flat) and three vegetation types invaded by S. alterniflora (P. australis, K. obovata, A. marina). Our results revealed large spatial heterogeneity in SOC density of the top 1‐m ranging 40–200 Mg C ha−1, with higher values in mid‐latitude regions (25–30° N) compared with those in both low‐ (20°N) and high‐latitude (38–40°N) regions. Vegetation type influenced SOC density, with P. australis and S. alterniflora having the largest SOC density, followed by mangrove, mixed P. australis and Suaeda, single Suaeda and tidal flat. SOC density increased by 6.25 Mg ha−1 following S. alterniflora invasion into P. australis community but decreased by 28.56 and 8.17 Mg ha−1 following invasion into K. obovata and A. marina communities. Based on field measurements and published literature, we calculated a total inventory of 57 × 106 Mg C in the top 1‐m soil across China's coastal wetlands. Edaphic variables controlled SOC content, with soil chemical properties explaining the largest variance in SOC content. Climate did not control SOC content but had a strong interactive effect with edaphic variables. Plant biomass and quality traits were a minor contributor in regulating SOC content, highlighting the importance of quantity and quality of OC inputs and the balance between production and degradation within the coastal wetlands. These findings provide new insights into blue C stabilization mechanisms and sequestration capacity in coastal wetlands. [ABSTRACT FROM AUTHOR]

The biogeochemical cycle of Si plays an important role in maintaining the function and stability of ecosystems. Although there have been many studies on long‐term silicate weathering, there are few reports on Si fractions and cycling in early ecosystem development. Using optimized sequential chemical extraction processes, this study quantified soil Si fractions of the eluvial horizon and deposition layers in Hailuogou Glacier Retreat Area of Gongga Mountain, Southwest China. The results showed that plant‐available Si (NaAc–Si) content decreased significantly from 33.40 to 2.16 mg kg−1 with the increase of soil age. There was a significantly positive correlation (p <.01) between soil pH and NaAc–Si. Within the labile Si fractions, the largest fraction was amorphous Si (Na2CO3–Si) (165.35 ± 81.50 mg kg−1), followed by pedogenic oxides/hydroxides chemisorbed Si (Oxalate–Si) (135.13 ± 66.83 mg kg−1), Si adsorbed on the surface of inorganic soil particles (Acetic–Si) (9.66 ± 4.27 mg kg−1), and soluble Si (CaCl2–Si) (4.89 ± 1.68 mg kg−1). Compared with Stage 1 (i.e., a variety of pioneer species symbiosis stage) after glacier retreat, the storage of these four labile Si fractions in Stage 3 (i.e., climax species stage) decreased by 45.95% (CaCl2–Si), 42.98% (Acetic–Si), 67.34% (Oxalate–Si), and 75.24% (Na2CO3–Si), respectively, due to the absorption and utilization of Si by plants. Our study implies that the multiple transformations of different Si fractions in soils and the absorption and return of Si by plants can keep the CaCl2–Si pool in dynamic equilibrium, which provides a basis for the stability of ecosystem functions during the vegetation evolution after glacier retreat. Core Ideas: Plant‐available Si decreased significantly with the time of glacier retreat.Within the labile Si fractions, the largest fraction was Na2CO3–Si, followed by Oxalate–Si, Acetic–Si, and CaCl2–Si.Amorphous Si was the main source of dissolved Si in soil.Si can alleviate the biotic and abiotic stresses of plants, which may reduce soil erosion after glacier retreat. [ABSTRACT FROM AUTHOR]

Coastal wetlands are among the most productive ecosystems and store large amounts of organic carbon (C)—the so termed "blue carbon." However, wetlands in the tropics and subtropics have been invaded by smooth cordgrass (Spartina alterniflora) affecting storage of blue C. To understand how S. alterniflora affects soil organic carbon (SOC) stocks, sources, stability, and their spatial distribution, we sampled soils along a 2500 km coastal transect encompassing tropical to subtropical climate zones. This included 216 samplings within three coastal wetland types: a marsh (Phragmites australis) and two mangroves (Kandelia candel and Avicennia marina). Using δ13C, C:nitrogen (N) ratios, and lignin biomarker composition, we traced changes in the sources, stability, and storage of SOC in response to S. alterniflora invasion. The contribution of S. alterniflora‐derived C up to 40 cm accounts for 5.6%, 23%, and 12% in the P. australis, K. candel, and A. marina communities, respectively, with a corresponding change in SOC storage of +3.5, −14, and −3.9 t C ha−1. SOC storage did not follow the trend in aboveground biomass from the native to invasive species, or with vegetation types and invasion duration (7–15 years). SOC storage decreased with increasing mean annual precipitation (1000–1900 mm) and temperature (15.3–23.4℃). Edaphic variables in P. australis marshes remained stable after S. alterniflora invasion, and hence, their effects on SOC content were absent. In mangrove wetlands, however, electrical conductivity, total N and phosphorus, pH, and active silicon were the main factors controlling SOC stocks. Mangrove wetlands were most strongly impacted by S. alterniflora invasion and efforts are needed to focus on restoring native vegetation. By understanding the mechanisms and consequences of invasion by S. alterniflora, changes in blue C sequestration can be predicted to optimize storage can be developed. [ABSTRACT FROM AUTHOR]

Soil organic carbon (SOC) stocks have been greatly depleted across the globe by conversion of wetlands to croplands, agroforestry, and urban areas. Here, we investigated SOC distribution and turnover in four land use types: a wetland, cropland, forestland, and construction land in the Baiyangdian Wetland, Northern China. The C : N ratios were up to 1.70‐times larger in cropland, forestland, and construction land than in the original wetland because of faster N losses compared to C following wetland conversion. The δ13C values of SOC increased with depth in wetland, and showed an overall depletion compared with the other three land use types. Acid‐to‐Aldehyde ratios of syringyl in wetland were 0.72–1.14‐, 0.72–1.72‐, and 1.18–1.43‐times, and cinnamyl/vanillyl ratios were 0.56–1.05‐, 0.22–0.48‐, and 0.40–0.76‐times those of cropland, forestland, and construction land, which reflects faster lignin decomposition rate in wetlands. The β value was defined by the slope of the linear regression between the logarithm of SOC and δ13SOC values, and decreased from cropland over construction land and forestland to wetland, reflecting the faster SOC turnover with the lower β values. However, SOC content and storage were up to 2.29‐ and 2.07‐times higher in wetlands than in soils of other land use types. The combination of C : N ratios, δ13C, and lignin monomer composition can explain the decrease of β value (corresponding to faster SOC turnover), and can be used as effective proxies to evaluate the sources and turnover of SOC in response to land use changes. [ABSTRACT FROM AUTHOR]

Chromium (Cr) is a potentially toxic metal originating from natural processes and anthropogenic activities such as the iron-steel, electroplating, and leather industries, which is carcinogen to living organisms and has an ecological risk. Hence, research into the remediation of Cr pollution has attracted widespread attention. Bioremediation techniques have advantages of causing little disturbance to soil and water, low cost, simple and convenient operation, and less secondary pollution. In this review, we briefly describe the chemical properties of Cr, sources of Cr pollution, environmental quality, toxicological/health effects of Cr, and analytical methods. We also discuss the factors that govern methods for the bioremediation of Cr and compare their advantages and disadvantages. In particular, we focus on efforts to establish Cr bioremediation processes and their mechanisms. The main mechanisms include biosorption, bioaccumulation, complexation, electrostatic attraction, Cr(VI) reduction to Cr(III), and ion exchange, which decrease the Cr(VI) concentrations and convert Cr(VI) into Cr(III) lowering its toxicity and making it environmentally benign. However, bioremediation is still a challenging technique and most studies remain at the laboratory stage. Therefore we suggest areas for future research and provide theoretical guidance and a scientific basis for the application of biosorbents for Cr(VI) bioremediation in soils and wastewater. [ABSTRACT FROM AUTHOR]

• Phragmites australis is efficient in taking up silica (Si), storing the biogenic Si (BSi), and releasing Si, thus playing a crucial role in the biogeochemical cycling of Si in wetlands. • BSi can substitute lignin as a structural component, due to their similar eco-physiological functions, reduces costs associated with lignin biosynthesis. • BSi accumulation increased total biomass C storage, and BSi changed the composition and quality of litter substrates. • Competing processes of carbon cycles (i.e., biomass quantity vs quality) can be influenced by Si cycling in wetlands. Silicon (Si) is one of the most abundant elements in the Earth's crust but its role in governing the biogeochemical cycling of other elements remains poor understood. There is a paucity of information on the role of Si in wetland plants, and how this may alter wetland C production and storage. Therefore, this study investigated Si distribution, nutrient stoichiometry and lignin abundance in Phragmites australis from a wetland system in China to better understand the biogeochemical cycling and C storage. Our data show that Si content (ranging between 0.202% to 6.614%) of Phragmites australis is negatively correlated with C concentration (38.150%–47.220%). Furthermore, Si content was negatively antagonistically related to the concentration of lignin-derived phenols in the stem (66.763–120.670 mg g-1 C) and sheath (65.400–114.118 mg g-1 C), but only a weak relationship was observed in the leaf tissue (36.439–55.905 mg g-1 C), which is relevant to the photosynthesis or stabilization function of the plant tissues. These results support the notion that biogenic Si (BSi) can substitute lignin as a structural component, due to their similar eco-physiological functions, reduces costs associated with lignin biosynthesis. The accumulation of BSi increased total biomass C storage and nutrient accumulation due to greater productivity of Phragmites australis. On the other hand, BSi regulated litter composition and quality (e.g., nutrient stoichiometry and lignin) that provide a possibility for the factors affecting litter decomposition. Thus competing processes (i.e., biomass quantity vs quality) can be influenced by Si cycling in wetlands. [ABSTRACT FROM AUTHOR]

Abstract Grassland ecosystems play an important role in the global terrestrial silicon (Si) cycle, and Si is a beneficial element and structural constituent for the growth of grasses. In previous decades, grasslands have been degraded to different degrees because of the drying climate and intense human disturbance. However, the impact of grassland degradation on the distribution and bioavailability of soil Si is largely unknown. Here, we investigated vegetation and soil conditions of 30 sites to characterize different degrees of degradation for grasslands in the agro-pastoral ecotone of northern China. We then explored the impact of grassland degradation on the distribution and bioavailability of soil Si, including total Si and four forms of noncrystalline Si in three horizons (0–10, 10–20 and 20–40 cm) of different soil profiles. The concentrations of noncrystalline Si in soil profiles significantly decreased with increasing degrees of degradation, being 7.35 ± 0.88 mg g−1, 5.36 ± 0.39 mg g−1, 3.81 ± 0.37 mg g−1 and 3.60 ± 0.26 mg g−1 in non-degraded, lightly degraded, moderately degraded and seriously degraded grasslands, respectively. Moreover, the storage of noncrystalline Si decreased from higher than 40 t ha−1 to lower than 23 t ha−1. The corresponding bioavailability of soil Si also generally decreased with grassland degradation. These processes may not only affect the Si pools and fluxes in soils but also influence the Si uptake in plants. We suggest that grassland degradation can significantly affect the global grassland Si cycle. Grassland management methods such as fertilizing and avoiding overgrazing can potentially double the content and storage of noncrystalline Si in soils, thereby enhancing the soil Si bioavailability by >17%. Graphical abstract Unlabelled Image Highlights • Grasslands play a crucial role in the Si cycle. • The content and storage of noncrystalline Si decrease with grassland degradation. • Grassland degradation can cause the decreased soil Si bioavailability. • Degradation can significantly affect the Si cycle in global grasslands. [ABSTRACT FROM AUTHOR]

Static chamber method has been used to monitor greenhouse gas (GHG) emission from agricultural soils. When monitoring GHG emission in rice or wheat systems, all the plant stems can be covered in the top chamber. Under this situation, both nitrous oxide (N 2 O) emission from the root free soils and the root area soils can be estimated. However, in maize field, most of the studies placed the chamber within the maize row due to the difficulty to cover the whole maize stalk. N 2 O emission from the root area was not estimated since the majority of maize roots were excluded in the chamber. This may lead to a under estimation of global N 2 O emission from maize cultivation. To evaluate the spatial variability of N 2 O emission from maize field, a two-year field experiment was conducted. N 2 O emissions from three sampling sites including inter-rows, within-rows and root area were monitored under five different fertilization regimes. Results showed that chamber placement site had a significant effect on the N 2 O emission during the maize growing season. Placing the chamber in inter-rows either overestimated or underestimated N 2 O emission. In 2018, N 2 O emission rate in the inter-rows was 69 % higher than the root area on average of the five treatments; whereas, in 2020, N 2 O emission rate in the inter-rows was 2.5 times lower than the root area. N fertilization regimes had a significant effect on N 2 O emission. On average of chamber sites, N 2 O emission under N fertilizer broadcasting was around 2 times lower compared to nest-applied. The highest N 2 O emission rate was observed when both basal and dressing N fertilizer were applied in a nest, which was 1.1–3.2 times higher than the other N fertilizer placement regimes. The N 2 O emission rate from the within-rows was close to the average value of the three sampling sites. Our results suggested that placing chamber in the center of two neighboring maize plants could represent the average area-scaled N 2 O emission in maize field. • N 2 O emissions were monitored from inter-rows, within-rows and root area soil. • Chamber placement site had a significant effect on the N 2 O emission. • N 2 O emission under broadcasting was around 2 times lower than nest-applied N fertilizer. • Placing chamber in the within-rows could represent the area-scaled N 2 O emission. [ABSTRACT FROM AUTHOR]

Coastal wetlands contribute to the mitigation of climate change through the sequestration of "blue carbon". Microbial necromass, lignin, and glycoproteins (i.e., glomalin-related soil proteins (GRSP)), as important components of soil organic carbon (SOC), are sensitive to environmental change. However, their contributions to blue carbon formation and the underlying factors remain largely unresolved. To address this paucity of knowledge, we investigated their contributions to blue carbon formation along a salinity gradient in coastal marshes. Our results revealed decreasing contributions of microbial necromass and lignin to blue carbon as the salinity increased, while GRSP showed an opposite trend. Using random forest models, we showed that their contributions to SOC were dependent on microbial biomass and resource stoichiometry. In N-limited saline soils, contributions of microbial necromass to SOC decreased due to increased N-acquisition enzyme activity. Decreases in lignin contributions were linked to reduced mineral protection offered by short-range-ordered Fe (Fe SRO ). Partial least-squares path modeling (PLS-PM) further indicated that GRSP could increase microbial necromass and lignin formation by enhancing mineral protection. Our findings have implications for improving the accumulation of refractory and mineral-bound organic matter in coastal wetlands, considering the current scenario of heightened nutrient discharge and sea-level rise.

Despite increasing recognition of the critical role of coastal wetlands in mitigating climate change, sea-level rise, and salinity increase, soil organic carbon (SOC) sequestration mechanisms in estuarine wetlands remain poorly understood. Here, we present new results on the source, decomposition, and storage of SOC in estuarine wetlands with four vegetation types, including single Phragmites australis (P, habitat I), a mixture of P. australis and Suaeda salsa (P + S, habitat II), single S. salsa (S, habitat III), and tidal flat (TF, habitat IV) across a salinity gradient. Values of δ 13 C increased with depth in aerobic soil layers (0-40 cm) but slightly decreased in anaerobic soil layers (40-100 cm). The δ 15 N was significantly enriched in soil organic matter at all depths than in the living plant tissues, indicating a preferential decomposition of 14 N-enriched organic components. Thus, the kinetic isotope fractionation during microbial degradation and the preferential substrate utilization are the dominant mechanisms in regulating isotopic compositions in aerobic and anaerobic conditions, respectively. Stable isotopic (δ 13 C and δ 15 N), elemental (C and N), and lignin composition (inherited (Ad/Al)s and C/V) were not completely consistent in reflecting the differences in SOC decomposition or accumulation among four vegetation types, possibly due to differences in litter inputs, root distributions, substrate quality, water-table level, salinity, and microbial community composition/activity. Organic C contents and storage decreased from upstream to downstream, likely due to primarily changes in autochthonous sources (e.g., decreased onsite plant biomass input) and allochthonous materials (e.g., decreased fluvially transported upland river inputs, and increased tidally induced marine algae and phytoplankton). Our results revealed that multiple indicators are essential to unravel the degree of SOC decomposition and accumulation, and a combination of C:N ratios, δ 13 C, δ 15 N, and lignin biomarker provides a robust approach to decipher the decomposition and source of sedimentary organic matter along the river-estuary-ocean continuum., (© 2020 John Wiley & Sons Ltd.)