Significant accrual of soil organic carbon through long-term rice cultivation in paddy fields in China.

Paddy fields serve as significant reservoirs of soil organic carbon (SOC) and their potential for terrestrial carbon (C) sequestration is closely associated with changes in SOC pools. However, there has been a dearth of comprehensive studies quantifying changes in SOC pools following extended periods of rice cultivation across a broad geographical scale. Using 104 rice paddy sampling sites that have been in continuous cultivation since the 1980s across China, we studied the changes in topsoil (0-20 cm) labile organic C (LOC I), semi-labile organic C (LOC II), recalcitrant organic C (ROC), and total SOC. We found a substantial increase in both the content (48%) and density (39%) of total SOC within China's paddy fields between the 1980s to the 2010s. Intriguingly, the rate of increase in content and density of ROC exceeded that of LOC (I and II). Using a structural equation model, we revealed that changes in the content and density of total SOC were mainly driven by corresponding shifts in ROC, which are influenced both directly and indirectly by climatic and soil physicochemical factors; in particular temperature, precipitation, phosphorous (P) and clay content. We also showed that the δ13 CLOC were greater than δ13 CROC , independent of the rice cropping region, and that there was a significant positive correlation between δ13 CSOC and δ13 Cstraw . The δ13 CLOC and δ13 CSOC showed significantly negative correlation with soil total Si, suggesting that soil Si plays a part in the allocation of C into different SOC pools, and its turnover or stabilization. Our study underscores that the global C sequestration of the paddy fields mainly stems from the substantial increase in ROC pool.

Critical role of water conditions in the responses of autumn phenology of marsh wetlands to climate change on the Tibetan Plateau.

The Tibetan Plateau, housing 20% of China's wetlands, plays a vital role in the regional carbon cycle. Examining the phenological dynamics of wetland vegetation in response to climate change is crucial for understanding its impact on the ecosystem. Despite this importance, the specific effects of climate change on wetland vegetation phenology in this region remain uncertain. In this study, we investigated the influence of climate change on the end of the growing season (EOS) of marsh wetland vegetation across the Tibetan Plateau, utilizing satellite-derived Normalized Difference Vegetation Index (NDVI) data and observational climate data. We observed that the regionally averaged EOS of marsh vegetation across the Tibetan Plateau was significantly (p < .05) delayed by 4.10 days/decade from 2001 to 2020. Warming preseason temperatures were found to be the primary driver behind the delay in the EOS of marsh vegetation, whereas preseason cumulative precipitation showed no significant impact. Interestingly, the responses of EOS to climate change varied spatially across the plateau, indicating a regulatory role for hydrological conditions in marsh phenology. In the humid and cold central regions, preseason daytime warming significantly delayed the EOS. However, areas with lower soil moisture exhibited a weaker or reversed delay effect, suggesting complex interplays between temperature, soil moisture, and EOS. Notably, in the arid southwestern regions of the plateau, increased preseason rainfall directly delayed the EOS, while higher daytime temperatures advanced it. Our results emphasize the critical role of hydrological conditions, specifically soil moisture, in shaping marsh EOS responses in different regions. Our findings underscore the need to incorporate hydrological factors into terrestrial ecosystem models, particularly in cold and dry regions, for accurate predictions of marsh vegetation phenological responses to climate change. This understanding is vital for informed conservation and management strategies in the face of current and future climate challenges.

Storage and Distribution of Organic Carbon and Nutrients in Acidic Soils Developed on Sulfidic Sediments: The Roles of Reactive Iron and Macropores.

In a boreal acidic sulfate-rich subsoil (pH 3-4) developing on sulfidic and organic-rich sediments over the past 70 years, extensive brownish-to-yellowish layers have formed on macropores. Our data reveal that these layers ("macropore surfaces") are strongly enriched in 1 M HCl-extractable reactive iron (2-7% dry weight), largely bound to schwertmannite and 2-line ferrihydrite. These reactive iron phases trap large pools of labile organic matter (OM) and HCl-extractable phosphorus, possibly derived from the cultivated layer. Within soil aggregates, the OM is of a different nature from that on the macropore surfaces but similar to that in the underlying sulfidic sediments (C-horizon). This provides evidence that the sedimentary OM in the bulk subsoil has been largely preserved without significant decomposition and/or fractionation, likely due to physiochemical stabilization by the reactive iron phases that also existed abundantly within the aggregates. These findings not only highlight the important yet underappreciated roles of iron oxyhydroxysulfates in OM/nutrient storage and distribution in acidic sulfate-rich and other similar environments but also suggest that boreal acidic sulfate-rich subsoils and other similar soil systems (existing widely on coastal plains worldwide and being increasingly formed in thawing permafrost) may act as global sinks for OM and nutrients in the short run.

Microbial Necromass, Lignin, and Glycoproteins for Determining and Optimizing Blue Carbon Formation.

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 (FeSRO). 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.

Storage, patterns and influencing factors for soil organic carbon in coastal wetlands of China.

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.

High potential of stable carbon sequestration in phytoliths of China's grasslands.

Phytolith carbon (C) sequestration plays a key role in mitigating global climate change at a centennial to millennial time scale. However, previous estimates of phytolith-occluded carbon (PhytOC) storage and potential in China's grasslands have large uncertainties mainly due to multiple data sources. This contributes to the uncertainty in predicting long-term C sequestration in terrestrial ecosystems using Earth System Models. In this study, we carried out an intensive field investigation (79 sites, 237 soil profiles [0-100 cm], and 61 vegetation assessments) to quantify PhytOC storage in China's grasslands and to better explore the biogeographical patterns and influencing factors. Generally, PhytOC production flux and soil PhytOC density in both the Tibetan Plateau and the Inner Mongolian Plateau had a decreasing trend from the Northeast to the Southwest. The aboveground PhytOC production rate in China's grassland was 0.48 × 106 t CO2 a-1 , and the soil PhytOC storage was 383 × 106 t CO2 . About 45% of soil PhytOC was stored in the deep soil layers (50-100 cm), highlighting the importance of deep soil layers for C stock assessments. Importantly, the Tibetan Plateau had the greatest contribution (more than 70%) to the PhytOC storage in China's grasslands. The results of multiple regression analysis indicated that altitude and soil texture significantly influenced the spatial distribution of soil PhytOC, explaining 78.1% of the total variation. Soil phytolith turnover time in China's grasslands was mainly controlled by climatic conditions, with the turnover time on the Tibetan Plateau being significantly longer than that on the Inner Mongolian Plateau. Our results offer more accurate estimates of the potential for phytolith C sequestration from ecological restoration projects in degraded grassland ecosystems. These estimates are essential to parameterizing and validating global C models.

Distribution, sources, and decomposition of soil organic matter along a salinity gradient in estuarine wetlands characterized by C:N ratio, δ13 C-δ15 N, and lignin biomarker.

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 δ13 C increased with depth in aerobic soil layers (0-40 cm) but slightly decreased in anaerobic soil layers (40-100 cm). The δ15 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 (δ13 C and δ15 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, δ13 C, δ15 N, and lignin biomarker provides a robust approach to decipher the decomposition and source of sedimentary organic matter along the river-estuary-ocean continuum.

Spartina alterniflora invasion controls organic carbon stocks in coastal marsh and mangrove soils across tropics and subtropics.

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 δ13 C, 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.

A Combined X-ray Absorption and Mössbauer Spectroscopy Study on Fe Valence and Secondary Mineralogy in Granitoid Fracture Networks: Implications for Geological Disposal of Spent Nuclear Fuels.

Underground repository in crystalline bedrock is a widely accepted solution for long-term disposal of spent nuclear fuels. During future deglaciations, meltwater will intrude via bedrock fractures to the depths of future repositories where O2 left in the meltwater could corrode metal canisters and enhance the migration of redox-sensitive radionuclides. Since glacial meltwater is poor in reduced phases, the quantity and (bio)accessibility of minerogenic Fe(II) in bedrock fractures determine to what extent O2 in future meltwater can be consumed. Here, we determined Fe valence and mineralogy in secondary mineral assemblages sampled throughout the upper kilometer of fractured crystalline bedrock at two sites on the Baltic Shield, using X-ray absorption and Mössbauer spectroscopic techniques that were found to deliver matching results. The data point to extensive O2-consuming capacity of the bedrock fractures, because Fe(II)-rich phyllosilicates were abundant and secondary pyrite was dispersed deep into the bedrock with no overall increase in Fe(II) concentrations and Fe(II)/Fe(III) proportions with depth. The results imply that repeated Pleistocene deglaciations did not cause a measurable decrease in the Fe(II) pool. In surficial fractures, largely opened during glacial unloading, ferrihydrite and illite have formed abundantly via oxidative transformation of Fe(II)-rich phyllosilicates and recently exposed primary biotite/hornblende.

Characteristics and applications of biochar for remediating Cr(VI)-contaminated soils and wastewater.

Chromium (Cr) is a common environmental contaminant due to industrial processes and anthropogenic activities such as mining of chrome ore, electroplating, timber treatment, leather tanning, fertilizer and pesticide, etc. Cr exists mainly in both hexavalent [Cr(VI)] and trivalent [Cr(III)] form, being Cr(VI) with non-degradability and potential to be hidden, thereby affecting surrounding environment and being toxic to human health. Therefore, researches on remediation of Cr pollution in the environment have received much attention. Biochar is a low-cost adsorbent, which has been identified as a suitable material for Cr(VI) immobilization and removal from soil and wastewater. This review incorporates existing literature to provide a detailed examination into the (1) Cr chemistry, the source and current status of Cr pollution, and Cr toxicity and health; (2) feedstock and characterization of biochar; (3) processes and mechanisms of immobilization and removal of Cr by biochar, including oxidation-reduction, electrostatic interactions, complexation, ion exchange, and precipitation; (4) applications of biochar for Cr(VI) remediation and the modification of biochar to improve its performance; (5) factors affecting removal efficiency of Cr(VI) with respect to its physico-chemical conditions, including pH, temperature, initial concentration, reaction time, biochar characteristics, and coexisting contaminants. Finally, we identify current issues, challenges, and put forward recommendations as well as proposed directions for future research. This review provides a thorough understanding of using biochar as an emerging biomaterial adsorbent in Cr(VI)-contaminated soils and wastewater.

Differentiated seasonal vegetation cover dynamics of degraded grasslands in Inner Mongolia recorded by continuous photography technique.

Influence of climate change on the grassland phenology has attracted more and more attentions of ecologists. Although dozens of studies have been conducted, there have been few records examining the phenology differences of grasslands with different plant species compositions. Using continuous photography and image processing methods, this study examined seasonal vegetation cover dynamics of grasslands along a degradation gradient to clarify the influence of vegetation composition on the dynamics of vegetation cover during growing season. Our results revealed that phenological patterns of grasslands differentiated with their degradation status. Abandoned farmland (AF) and severely degraded grassland (SD) with most annuals and least climax species had the earliest start of growing season, while AF and extremely degraded grassland (ED) dominated by grasses had the earliest end of growing season. The start and end of growing season were strongly related to the relative cover of climax species and grasses. The results presented in this study support the possibility of using digital photography to capture the role of plant species composition on vegetation phenology in grasslands.

Effect of bamboo and rice straw biochars on the mobility and redistribution of heavy metals (Cd, Cu, Pb and Zn) in contaminated soil.

Biochar has emerged as an efficient tool to affect bioavailability of heavy metals in contaminated soils. Although partially understood, a carefully designed incubation experiment was performed to examine the effect of biochar on mobility and redistribution of Cd, Cu, Pb and Zn in a sandy loam soil collected from the surroundings of a copper smelter. Bamboo and rice straw biochars with different mesh sizes (<0.25 mm and <1 mm), were applied at three rates (0, 1, and 5% w/w). Heavy metal concentrations in pore water were determined after extraction with 0.01 M CaCl2. Phytoavailable metals were extracted using DTPA/TEA (pH 7.3). The European Union Bureau of Reference (EUBCR) sequential extraction procedure was adopted to determine metal partitioning and redistribution of heavy metals. Results showed that CaCl2-and DTPA-extractable Cd, Cu, Pb and Zn concentrations were significantly (p < 0.05) lower in the bamboo and rice straw biochar treated soils, especially at 5% application rate, than those in the unamended soil. Soil pH values were significantly correlated with CaCl2-extractable metal concentrations (p < 0.01). The EUBCR sequential extraction procedure revealed that the acid extractable fractions of Cd, Cu, Pb and Zn decreased significantly (p < 0.05) with biochar addition. Rice straw biochar was more effective than bamboo biochar in decreasing the acid extractable metal fractions, and the effect was more pronounced with increasing biochar application rate. The effect of biochar particle size on extractable metal concentrations was not consistent. The 5% rice straw biochar treatment reduced the DTPA-extractable metal concentrations in the order of Cd < Cu < Pb < Zn, and reduced the acid extractable pool of Cd, Cu, Pb and Zn by 11, 17, 34 and 6%, respectively, compared to the control. In the same 5% rice straw biochar treatments, the organic bound fraction increased by 37, 58, 68 and 18% for Cd, Cu, Pb and Zn, respectively, compared to the control, indicating that the immobilized metals were mainly bound in the soil organic matter fraction. The results demonstrated that the rice straw biochar can effectively immobilize heavy metals, thereby reducing their mobility and bioavailability in contaminated soils.

Composition and transport of silicon in rivers of the Bohai rim with implications for the coastal environment.

The transportation of silicon (Si) by rivers to the sea plays a vital role as an external source of Si budget for coastal environments, impacting the carbon cycle in the ocean. Nevertheless, the transport of reactive silica (RSi) from small rivers to the coastal sea has been frequently disregarded in scientific investigations. This research focused on 24 rivers situated along the Bohai Sea (BS) Rim, encompassing small rivers (SR) and the largest river in the region, the Yellow River (YR), to analyze their concentrations and fluxes of dissolved silicate (DSi), biogenic silica (BSi) and other amorphous forms of Si. The findings indicated seasonal variations in DSi concentrations, with higher levels observed during the flood season. Annually, approximately 105 × 103 t DSi and 200 × 103 t BSi were transported to the BS, with SR and YR contributing equally to the total riverine BSi flux. The smaller rivers were found to increase the BSi fraction of RSi due to elevated biological fixation. The ratios of average DSi and BSi fluxes to the river watershed area of SR were 3.5 and 6 times higher, respectively, compared to those of YR. SR play a critical role in the terrestrial Si export in the BS Rim. Human activities have led to significant deviations in the Si ratios to nitrogen and phosphorus in these rivers from the Redfield-Brzezinski ratio. This discrepancy could impact the phytoplankton community, primary production, and the environment of the BS. The study highlights the substantial contribution of SR to coastal environments, particularly in semi-closed marine environments like the BS.

Evidence that co-existing cadmium and microplastics have an antagonistic effect on greenhouse gas emissions from paddy field soils.

Accumulation of microplastics (MPs) and cadmium (Cd) are ubiquitous in paddy soil. However, the combined effects of MPs and Cd on physiochemical and microbial mechanisms in soils and the attendant implications for greenhouse gas (GHG) emissions, remain largely unknown. Here, we evaluated the influence of polylactic acid (PLA) and polyethylene (PE) MPs on GHG emissions from Cd-contaminated paddy soil using a microcosm experiment under waterlogged and drained conditions. The results showed that PLA significantly increased CH4 and N2O emission fluxes and hence the global warming potential (GWP) of waterlogged soil. Soils treated with MPs+Cd showed significantly reduced GWP compared to those treated only with MPs suggesting that, irrespective of attendant consequences, Cd could alleviate N2O emissions in the presence of MPs. Conversely, the presence of MPs in Cd-contaminated soils tended to alleviate the bioavailability of Cd. Based on a structural equation model analysis, both the MPs-derived dissolved organic matter and the soil bioavailable Cd affected indirectly on soil GHG emissions through their direct influencing on microbial abundance (e.g., Firmicutes, Nitrospirota bacteria). These findings provide new insights into the assessment of GHG emissions and soil/cereal security in response to MPs and Cd coexistence that behaved antagonistically with respect to adverse ecological effects in paddy systems.

Phosphorus depletion is exacerbated by increasing nitrogen loading in the Bohai sea.

Phosphorus (P) is an essential nutrient for algal growth in nearshore ecosystems. In recent years, there has been a shift in nutrient dynamics in nearshore areas, leading to an exacerbation of P limitation, although the underlying mechanisms remain unclear. This study analyzed the P species and budget in the Bohai Sea (BS) from 2011 to 2020, aiming to explore the intrinsic mechanisms of P limitation in the BS. The results show that the main external source of P in the BS was river transport (89%), and the primary fate of P was burial (96%) into the sediment. Due to excessive nitrogen (N) input and biological processes in the BS, the P budget in the BS is unbalanced, resulting in an increase in the N/P ratio, particularly in nearshore areas. Nearshore areas typically have lower concentrations of dissolved inorganic P (DIP) in the water and higher concentrations of reactive P (Reac-P) in the sediments. This pattern is particularly evident in Bohai Bay and the northwest nearshore region, where harmful algal blooms occur frequently. To cope with enhanced P limitation, the biologically driven P regeneration and cycling processes within the BS are accelerated. From 2011 to 2020, the concentration of DIP in the BS during autumn increased, while the content of Reac-P in sediments slightly decreased. Historical data indicate that P depletion in the BS is intensifying and expanding, primarily due to N enrichment and algal production. N enrichment alters the structure and composition of primary production, potentially exacerbating P depletion in the BS. Excessive N may have significant impacts on the P pool, potentially influencing the stability of future coastal ecosystems.

Silicon in paddy fields: Benefits for rice production and the potential of rice phytoliths for biogeochemical carbon sequestration.

Silicon (Si) biogeochemical cycling is beneficial for crop productivity and carbon (C) sequestration in agricultural ecosystem, thus offering a nonnegligible role in alleviating global warming and food crisis. Compared with other crops, rice plants have a greater quantity of phytolith production, because they are able to take up a lot of Si. However, it remains unclear on Si supply capacity of paddy soils across the world, general rice yield-increasing effect after Si fertilizer addition, and factors affecting phytolith production and potential of phytolith C sequestration in paddy fields. This study used a meta-analysis of >3500 data from 87 studies to investigate Si supply capacity of global paddy soils and elaborate the benefits of Si regarding rice productivity and phytolith C sequestration in paddy fields. Analytical results showed that the Si supply capacity of paddy soils was insufficient in the major rice producing countries/regions. Dealing with this predicament, Si fertilization was an effective strategy to supply plant-available Si to improve rice productivity. Our meta-analysis results further revealed that Si fertilization led to the average increasing rate of 36 % and 39 % in rice yield and biomass, which could reach up to 52 % and 46 % with the increasing doses of Si fertilizer, respectively. Especially, this strategy also improved the potential of phytolith C sequestration through the increased phytolith content and rice biomass, despite that this potential might have a decline in old paddy soils (≥ 7000 year) compared to in young paddy soils (≤ 1000 year) due to the slow migration and dissolution of phytoliths at millennial scale. Our findings thus indicate that a deep investigation on the benefits of Si in agroecosystem will further improve our understanding on regulating crop production and the potential of biogeochemical C sequestration within phytoliths in global cropland.

Mobility and environmental impact of cadmium (Cd) during weathering of carbonaceous black shales in western Hunan, China.

Carbonaceous black shale generally contains high concentration of Cd, with weathering leading to Cd release to environment. In this study, the mobility of Cd during weathering was quantified using geochemical assessment on black shale from western Hunan, China. Results suggested that Cd was heterogeneously distributed in shale profiles with concentrations ranging from 0.16 to 109.9 (mg/kg). Cd distribution was heterogeneous resulting from the parent shale inheritance and the mobility of Cd during weathering. Black shales weathered to a moderate degree with Cd mobility characterized by both enrichment in and release from weathered shales. Cd enrichment in weathered shales resulted from the re-enrichment of Cd in secondary minerals formed during the initial stage of carbonate (and phosphorite) dissolution, and the secondary stage of sulfide oxidation. The release of Cd was caused by decomposition of the secondary Cd-bearing minerals. Cadmium was extensively released during pedogenesis, and Cd release mass flux was estimated to range from 1.26 to 9.50 (g/m2) with a mean of 6.60 g/m2. Thus, black shale weathering may lead to the releasing of large amount of Cd resulting in Cd contamination to local environments.

Source identification of sedimentary organic carbon in coastal wetlands of the western Bohai Sea.

Coastal wetlands play a vital role in mitigating climate change, yet the characteristics of buried organic carbon (OC) and carbon cycling are limited due to difficulties in assessing the composition of OC from different sources (allochthonous vs. autochthonous). In this study, we analyzed the total organic carbon (TOC) to total nitrogen (TN) ratio (C/N), stable carbon isotope (δ13C) composition, and n-alkane content to distinguish different sources of OC in the surface sediments of the coastal wetlands on the western coast of the Bohai Sea. The coupling of the C/N ratio with δ13C and n-alkane biomarkers has been proved to be an effective tool for revealing OC sources. The three end-member Bayesian mixing model based on coupling C/N ratios with δ13C showed that the sedimentary OC was dominated by the contribution of terrestrial particulate organic matter (POM), followed by freshwater algae and marine phytoplankton, with relative contributions of 47 ± 21 %, 41 ± 18 % and 12 ± 17 %, respectively. The relative contributions of terrestrial plants, aquatic macrophytes and marine phytoplankton assessed by n-alkanes were 56 ± 8 %, 35 ± 9 % and 9 ± 5 % in the study area, respectively. The relatively high salinity levels and strong hydrodynamic conditions of the Beidagang Reservoir led to higher terrestrial plants source and lower aquatic macrophytes source than these of Qilihai Reservoir based on the assessment of n-alkanes. Both methods showed that sedimentary OC was mainly derived from terrestrial sources (plant-dominated), suggesting that vegetation plays a crucial role in storing carbon in coastal wetlands, thus, the coastal vegetation management needs to be strengthened in the future. Our findings provide insights into the origins and dynamics of OC in coastal wetlands on the western coast of the Bohai Sea and a significant scientific basis for future monitoring of the blue carbon budget balance in coastal wetlands.

Silicon fractionations in coastal wetland sediments: Implications for biogeochemical silicon cycling.

Coastal wetland sediment is important reservoir for silicon (Si), and plays an essential role in controlling its biogeochemical cycling. However, little is known about Si fractionations and the associated factors driving their transformations in coastal wetland sediments. In this study, we applied an optimized sequential Si extraction method to separate six sub-fractions of non-crystalline Si (Sinoncry) in sediments from two coastal wetlands, including Si in dissolved silicate (Sidis), Si in the adsorbed silicate (Siad), Si bound to organic matter (Siorg), Si occluded in pedogenic oxides and hydroxides (Siocc), Si in biogenic amorphous silica (Siba), and Si in pedogenic amorphous silica (Sipa). The results showed that the highest proportion of Si in the Sinoncry fraction was Siba (up to 6.6 % of total Si (Sitot)), followed by the Sipa (up to 1.8 % of Sitot). The smallest proportion of Si was found in the Sidis and Siad fractions with the sum of both being <0.1 % of the Sitot. We found a lower Siocc content (188 ± 96.1 mg kg-1) when compared to terrestrial soils. The Sidis was at the center of the inter-transformation among Si fractions, regulating the biogeochemical Si cycling of coastal wetland sediments. Redundancy analysis (RDA) combined with Pearson's correlations further showed that the basic biogenic elements (total organic carbon and total nitrogen), pH, and sediment salinity collectively controlled the Si fractionations in coastal wetland sediments. Our research optimizes sediment Si fractionation procedure and provides insights into the role of sedimentary Si fractions in controlling Si dynamics and knowledge for unraveling the biogeochemical Si cycling in coastal ecosystems.

Organic matter composition and stability in estuarine wetlands depending on soil salinity.

Coastal wetlands are key players in mitigating global climate change by sequestering soil organic matter. Soil organic matter consists of less stable particulate organic matter (POM) and more stable mineral-associated organic matter (MAOM). The distribution and drivers of MAOM and POM in coastal wetlands have received little attention, despite the processes and mechanisms differ from that in the upland soils. We explored the distribution of POM and MAOM, their contributions to SOM, and the controlling factors along a salinity gradient in an estuarine wetland. In the estuarine wetland, POM C and N were influenced by soil depth and vegetation type, whereas MAOM C and N were influenced only by vegetation type. In the estuarine wetland, SOM was predominantly in the form of MAOM (> 70 %) and increased with salinity (70 %-76 %), leading to long-term C sequestration. Both POM and MAOM increased with SOM, and the increase rate of POM was higher than that of MAOM. Aboveground plant biomass decreased with increasing salinity, resulted in a decrease in POM C (46 %-81 %) and N (52 %-82 %) pools. As the mineral amount and activity, and microbial biomass decreased, the MAOM C (2.5 %-64 %) and N pool (8.6 %-59 %) decreased with salinity. When evaluating POM, the most influential factors were microbial biomass carbon (MBC) and dissolved organic carbon (DOC). Key parameters, including MBC, DOC, soil salinity, soil water content, aboveground plant biomass, mineral content and activity, and bulk density, were identified as influencing factors for both MAOM abundance. Soil water content not only directly controlled MAOM, but together with salinity also indirectly regulated POM and MAOM by controlling microbial biomass and aboveground plant biomass. Our findings have important implications for improving the accumulation and increased stability of soil organic matter in coastal wetlands, considering the global sea level rise and increased frequency of inundation.