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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

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.

Vegetation gross primary production (GPP) is the largest terrestrial carbon flux and plays an important role in regulating the carbon sink. Current terrestrial ecosystem models (TEMs) are indispensable tools for evaluating and predicting GPP. However, to which degree the TEMs can capture the interannual variability (IAV) of GPP remains unclear. With large data sets of remote sensing, in situ observations, and predictions of TEMs at a global scale, this study found that the current TEMs substantially underestimate the GPP IAV in comparison to observations at global flux towers. Our results also showed the larger underestimations of IAV in GPP at nonforest ecosystem types than forest types, especially in arid and semiarid grassland and shrubland. One cause of the underestimation is that the IAV in GPP predicted by models is strongly dependent on canopy structure, that is, leaf area index (LAI), and the models underestimate the changes of canopy physiology responding to climate change. On the other hand, the simulated interannual variations of LAI are much less than the observed. Our results highlight the importance of improving TEMs by precisely characterizing the contribution of canopy physiological changes on the IAV in GPP and of clarifying the reason for the underestimated IAV in LAI. With these efforts, it may be possible to accurately predict the IAV in GPP and the stability of the global carbon sink in the context of global climate change. Vegetation gross primary production (GPP) is the largest terrestrial carbon flux and plays an important role in regulating the carbon sink. Current terrestrial ecosystem models (TEMs) are indispensable tools for evaluating and predicting GPP. However, to which degree the TEMs can capture the interannual variability (IAV) of GPP remains unclear. With large data sets of remote sensing, in situ observations, and predictions of TEMs at a global scale, this study found that the current TEMs substantially underestimate the GPP IAV in comparison to observations at global flux towers. Our results also showed the larger underestimations of IAV in GPP at nonforest ecosystem types than forest types, especially in arid and semiarid grassland and shrubland. One cause of the underestimation is that the IAV in GPP predicted by models is strongly dependent on canopy structure, that is, leaf area index (LAI), and the models underestimate the changes of canopy physiology responding to climate change. On the other hand, the simulated interannual variations of LAI are much less than the observed. Our results highlight the importance of improving TEMs by precisely characterizing the contribution of canopy physiological changes on the IAV in GPP and of clarifying the reason for the underestimated IAV in LAI. With these efforts, it may be possible to accurately predict the IAV in GPP and the stability of the global carbon sink in the context of global climate change.

Refereed/Peer-reviewed Silicon nanoparticles (SiNPs) have been widely used to immobilize toxic trace metal(loid)s (TTMs) in contaminated croplands. However, the effect and mechanisms of SiNP application on TTM transportation in response to phytolith formation and phytolith-encapsulated-TTM (PhytTTM) production in plants are unclear. This study demonstrates the promotion effect of SiNP amendment on phytolith development and explores the associated mechanisms of TTM encapsulation in wheat phytoliths grown on multi-TTM contaminated soil. The bioconcentration factors between organic tissues and phytoliths of As and Cr (> 1) were significantly higher than those of Cd, Pb, Zn and Cu, and about 10 % and 40 % of the total As and Cr that bioaccumulated in wheat organic tissues were encapsulated into the corresponding phytoliths under high-level SiNP treatment. These observations demonstrate that the potential interaction of plant silica with TTMs is highly variable among elements, with As and Cr being the two most strongly concentrated TTMs in the phytoliths of wheat treated with SiNPs. The qualitative and semi-quantitative analyses of the phytoliths extracted from wheat tissues suggest that the high pore space and surface area (≈ 200 m² g⁻¹) of phytolith particles could have contributed to the embedding of TTMs during silica gel polymerization and concentration to form PhytTTMs. The abundant Sisingle bondO functional groups and high silicate-minerals in phytoliths are dominant chemical mechanisms for the preferential encapsulation of TTMs (i.e., As and Cr) by wheat phytoliths. Notably, the organic carbon and bioavailable Si of soils and the translocation of minerals from soil to plant aerial parts can impact TTM sequestration by phytoliths. Thus, this study has implications for the distribution or detoxification of TTMs in plants via preferential PhytTTM production and biogeochemical cycling of PhytTTMs in contaminated cropland following exogenous Si supplementation.

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.

Asian dust has been identified in subtropical soils of China. Neodymium (Nd) and lead (Pb) elemental and isotopic geochemistry of soils in Southeast China were used to assess the significance of local versus extraneous sources. The eNd(0) values were close to the parent rocks (+ 2.9) in the young soils (NSJ); while their values were always negative (~ − 3.7) in the old soils (OMJ), implying dust deposition. The young soils contained less Asian dust contribution (

Background and aims Silicon (Si) deficiency is a major constraint on rice production. The objective of this study was to evaluate the long-term influence of phytolith-rich straw return and groundwa ...

Ethylenediaminetetraacetic acid (EDTA)-enhanced soil washing is an important technique of removing potentially toxic elements (PTEs) from contaminated soils. Conventional mechanical shaking (CMS) method is widely adopted in soil washing. However, it consumes a considerable amount of time and energy. Extraction methods which need less contact time or power, such as static equilibrium extraction (SEE) and ultrasound-assisted extraction (UAE), are potential to replace CMS in industrial application. The washing process of the soil from a contaminated site was optimized by investigating various extraction conditions, including EDTA concentration, contact time, and different extraction methods, such as CMS, SEE, and UAE, in this study. The results indicated that EDTA greatly enhanced the removal of cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) from the soil. The EDTA was more effective in removing Pb and Cu than Cd and Zn, regardless of the extraction method. This might be mainly attributed to the strong complexation capacity of EDTA with Pb and Cu and geochemical distribution of the target PTEs. The three extraction methods were almost equally efficient, while SEE required less energy and UAE needed shortened contact time compared with CMS. Based on the above results, SEE and UAE are promising alternatives to CMS for cleaning up PTEs-contaminated soils.

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 O

In this study, we quantified the vegetation changes in the Haihe river basin (HRB) and analyzed their drivers during 1982–2012 by using satellite normalized difference vegetation index (NDVI) data,...

Phytolith-occluded carbon (PhytOC) is mainly derived from the products of photosynthesis, which can be preserved in soils and sediments for hundreds-to-thousands of years due to the resilient nature of the amorphous phytolith silica. Therefore, stable and radioactive carbon (C) isotopes of phytoliths can be effectively utilized in paleoecological and archeological research. However, there still exists debate about the applicability of C isotopes of phytoliths, as a “two-pool” hypothesis to characterize PhytOC sources has been proposed, whereby a component of the PhytOC is derived from soil organic matter (SOM) absorbed through plant roots. Therefore, it is necessary to review this topic to better understand the source of PhytOC. We introduce the stable and radioactive C isotopic compositions of PhytOC, present the impacts of different extraction methods on the study of PhytOC, and discuss the implications of these factors for determining the sources of PhytOC. Based on this review, we suggest that organic matter synthesized by photosynthesis is the main source of PhytOC. However, it is important to make clear whether and how SOM-derived C present in phytoliths influence the controversial “too-old” skew and isotopic fractionation. Though the two-pool hypothesis has been proved by many researches, the carbon isotopes of phytoliths still have potential in paleoecology and archeology, because the main source is photosynthesis and many previous studies put forward the availability of these parameters. This review also shows that phytolith C isotopes may vary with different organic C compounds within phytoliths, which needs further study at the molecular scale. Different phytolith extraction methods can influence 14C dating results.

Aims The natural abundances of stable carbon (C) and nitrogen (N) isotopes (δ13C and δ15N) are extensively used to indicate the C and N biogeochemical cycles at large spatial scales. However, the spatial patterns of δ13C and δ15N in plant-soil system of grasslands in northern China and their main driving factors are still not well understood.Methods We conducted sampling campaigns during 2016-2018 in grasslands of northern China and measured plant and soil δ13C and δ15N compositions to determine effects soil physicochemical properties and climatic factors on spatial distribution of δ13C and δ15N.Results Generally, plant and soil δ13C values increased with the decrease of mean annual precipitation (MAP). The interactions between mean annual temperature (MAT) and soil organic carbon have significant impact on soil δ13C. However, plant and soil δ15N decreased with the increase of MAT. Within all factors, the interactions between MAT and MAP on soil δ15N were significant.Conclusions Our results suggest that C cycling in grasslands of northern China is strongly mediated by plant community and MAT, because C4 species were more prevalent in arid regions. Meanwhile, N cycling is mainly directly regulated by MAT and plant community composition via its effect on the plant δ15N. All of these will provide scientific references for future research on the C and N biogeochemical cycles of temperate grassland ecosystems in northern China.

Cropland contamination by toxic trace metal (loid)s (TTMs) has attracted increasing attention due to the serious consequential threat to crop quality and human health. Mitigation of plant TTM stress by silica amendment has been proposed recently. However, the relationship between the siliceous structure of phytoliths and TTMs in plants, and the environmental implications of phytolith-occluded trace metal (loid)s (PhytTMs) remain unclear. This study assessed the accumulation of five metal (loid)s, including lead (Pb), zinc (Zn), cadmium (Cd), copper (Cu) and arsenic (As), in the organic tissues and phytoliths of wheat grown in a mixed-TTM contaminated soil under both lightly and heavily contaminated conditions. The results show that the concentrations of plant TTMs and PhytTMs were significantly (p < 0.05) positively correlated, and higher in heavily contaminated wheats than those in lightly contaminated ones. The bio-enrichment factors between phytoliths and organic tissues were higher for As (1.83), Pb (0.27) and Zn (0.30) than for Cd (0.03) and Cu (0.14), implying that As, Pb and Zn were more readily co-precipitated with silicon (Si) in phytolith structures than Cd and Cu. Network analysis of the relationship between soil and plant elements with PhytTMs showed that severe contamination could impact the homeostasis of elements in plants by altering the translocation of TTMs between soils, plants, and phytoliths. The accumulation of TTMs in phytoliths was affected by the capacity of Si deposition in tissues and chelation of TTMs with silica, which could impact the role of PhytTMs in global biogeochemical TTM cycles. Refereed/Peer-reviewed

Phytoliths are silica biomineralization products within plants and have been considered as a promising material to sequester carbon (C). However, there is considerable uncertainty and controversy regarding the C content in phytoliths due to the lack of detailed information on variation of C under different extraction procedures. Herein, we established a series of batch digestion experimental procedures coupled with analyses of phytoliths using Scanning Electron Microscopy and Energy-Dispersive X-ray Spectroscopy to divide phytoliths into three fractions. We then reported an approach for standardizing across hundreds of values found in the literature. Combining this standardized approach with C contents in phytoliths extracted from different digestion degrees, we revaluated the potential production rates of phytolith-occluded carbon (PhytOC) input globally in rice paddy fields. The results showed that the C content in recovered phytoliths exhibited a significantly fitting exponential relationship (p0.01) with digestion degrees and decreased from 30 to 75 g kg

The composition and accumulation of BSi and its role in C sequestration in the YRDW were quantified based on field investigation combined with remote sensing observation in the Yellow River Delta Wetland (YRDW). Results show that the soil BSi pool in the YRDW mainly consists of phytoliths. The pools of BSi, TOC, and BSi-occluded C (OCBSi) for aboveground plants in the YRDW are 7.6 g m−2, 129 g m−2 and 0.13 g m−2, respectively. Surface soil (0–10 cm) stocks of BSi, TOC, and OCBSi are 1780 g m−2, 840 g m−2 and 40 g m−2. Our results showed that BSi preservation efficiencies of BSi (52%) and (OCBSi) (51%) in deep soil layers exceed those of TOC (35%) based on the vertical variation of BSi in the sediment. Approximately 333 t of C fixed by plants is occluded in the phytoliths as OCBSi in the YRDW. Almost half of this OCBSi is preserved in the sediment in stable form. The YRDW is therefore an important sink for BSi and BSi-occluded C.

Remotely sensed vegetation indices (VIs) have been widely used to estimate the aboveground biomass (AGB) carbon stock of coastal wetlands by establishing Vis-related linear models. However, these models always have high uncertainties due to the large spatial variation and fragmentation of coastal wetlands. In this paper, an efficient coastal wetland AGB model for the Bohami Rim coastal wetlands was presented based on multiple data sets. The model was developed statistically with 7 independent variables from 23 metrics derived from remote sensing, topography, and climate data. Compared to previous models, it had better performance, with a root mean square error and r value of 188.32 g m−2 and 0.74, respectively. Using the model, we firstly generated a regional coastal wetland AGB map with a 10 m spatial resolution. Based on the AGB map, the AGB carbon stock of the Bohai Rim coastal wetland was 2.11 Tg C in 2019. The study demonstrated that integrating emerging high spatial resolution multi-remote sensing data and several auxiliary metrics can effectively improve VIs-based coastal wetland AGB models. Such models with emerging freely available data sets will allow for the rapid monitoring and better understanding of the special role that “blue carbon” plays in global carbon cycle.

Silicon (Si) plays an essential role in the biogeochemistry of rivers. This study explored how damming, eutrophication and climate change alters the abundance and flux of DSi in the Changjiang (Yangtze) River based on long-term observations. The results showed that Three Gorges Reservoir (TGR) could enhance DSi transfer only during low-flow time period, and a downstream DSi retention effect by the TGR was found between the Yichang and Jianli stations in the Changjiang River. This resulted in a DSi loss during March and April in the mainstream from Three Gorges Dam (TGD) to Jianli but a DSi addition during July and October along the main channel of the Changjiang River. Long-term data showed a sharp decrease in DSi abundance at the Cuntan, Hankou and Datong stations between the 1960s and 1980s, but a slight increase in DSi between the 1990s and 2010s at these stations. The decrease in DSi during the 1960s -1980s was primarily the result of a decrease trend of silicate weathering, while a slight DSi increase compared to the temperature/DSi relation after the 1990s was largely due to increased DSi retention in the basin by damming and eutrophication. Eutrophication and damming increase DSi trapping in both the river channel and reservoir systems in the low-flow period and thus enhance the nutrient distortion in the coastal ocean.

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.

The alpine meadow in the Tibetan Plateau (TP) is a fragile ecosystem and very sensitive to climate change and human activities. However, the flux (energy, water, and carbon dioxide) exchanges (FEs) and their controls in this ecosystem are not well understood. Using two years (2015–2016) of observations from three eddy covariance towers and different grazing management types, we quantified the FEs of an alpine meadow ecosystem in the northeastern TP. A boosted regression trees (BRT)-based method was used to determine the environmental controls of FEs and clarify the factors causing the differences in FEs among the sites. The mean annual latent heat flux (LE), sensible heat flux (H), and evapotranspiration (ET) of the sites were 36.3 W m−2, 23.2 W m−2, and 458.2 mm, respectively. The mean annual gross primary productivity (GPP) and ecosystem respiration (Re) of the sites were 504.8 and 317.2 g C m−2, respectively. The mean annual net ecosystem CO2 exchange (NEE) of the sites was –187.6 g C m–2, indicating that the ecosystem was a CO2 sink, not accounting for grazing losses. The BRT-based analysis showed that, (i) The LE (ET) and H were mostly determined by soil temperature (ST) and downward shortwave radiation (DR), while the NEE, GPP, and Re were primarily correlated with ST. As a result, the FEs of the ecosystem were energy-limited; (ii) The high spatial heterogeneity in multiple environmental variables jointly determined the differences in FEs of the ecosystem. The differences in LE (ET) and H among the sites resulted from multiple environmental variables, including DR, ST, precipitation, and SM; and differences in NEE, GPP, and Re were mainly caused by precipitation, SM, and RH. The study highlights that future warming will enhance GPP and Re of the ecosystem, while reducing the CO2 sink because of the larger increases in Re than in GPP.

Abundance and allocation of organic matter, microbial community and their extracellular enzyme activities in soil aggregates had been increasingly concerned for mediating biogeochemical cycle and biodiversity provided by soil. In this study, the variations of soil organic carbon (SOC) accumulation and extracellular enzyme activities in aggregate size fractions of a rice paddy with long-term fertilization treatments were analyzed. These treatments included no fertilizer application (NF) as a control, chemical fertilizer only (CF), chemical fertilizer combined with swine manure (CFM), and with straw residue return (CFS). Hereby, size fractions of soil aggregates respectively of >2000 μm (large macroaggregates), 250–2000 μm (macroaggregates), 53–250 μm (microaggregates), 2–53 μm (fine aggregates), and 53 μm. Whereas, activity of phenol oxidase was generally concentrated in soil aggregate fractions in size between 2 and 2000 μm while that of peroxidase in size between 53 and 2000 μm. Activity of β-glucosidase and cellobiohydrolase was both significantly elevated under CFS and that of peroxidase under CF, CFM and CFS treatments, but phenol oxidase activity was suppressed under CFS treatment. The enzyme activities of β-xylosidase, β-glucosidase, cellobiohydrolase and N-acetyl-glucosaminidase varied greatly (CVs up to 50%), rather with size fractions than with treatments, while those of α-Glucosidase and phenol oxidase varied moderately (CVs up to 35%), rather with treatments than with size fractions. However, those of acid phosphatase and peroxidase changed slightly (CVs up to 20%), regardless of both size fractions and fertilization treatments. Thus, abundance and enrichment of all the soil enzyme activities analyzed depended not on SOC although the overall extracellular enzyme activity was generally well correlated to SOC abundance, among aggregate size fractions. Only activities of carbon-degrading enzymes were greatly enriched in macroaggregates, which were seen reduced under combined organo/mineral fertilization. This study highlighted macroaggregates as biochemically active micro- hotspots and lower activities of carbon-degrading enzymes with SOC accumulation under long-term rational fertilization in the rice paddy.

Acid rain events in Yuebei, a region of northern Guangdong Province, China, have been increasing due to rapid urbanization in recent years. A three-year study was conducted to assess the effect of rainfall on losses of dissolved calcium (Ca2+) from limestone soils in a karst area in Yuebei. Soil leachate was collected from 61 events that produced more than 100 ml leachate per event during a 3-year period. Each rainfall event caused on average a loss of 8.5 × 10−4 g Ca2+ kg−1 soil with a Ca2+ loss of 2.16 × 10−5 g kg−1 per mm of rain. Cumulative net Ca2+ loss from the soil for the 61 events monitored was 0.05 g kg−1. In terms of Ca-loss per mm rain, the largest net Ca2+ loss from the soil occurred with mild acid rain (pH 4.5–5.6), followed by non-acid rain (pH 5.6–6.29) and severe acid rain (pH

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 liv...