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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. Key Points: Current terrestrial ecosystem models (TEMs) substantially underestimate the interannual variability (IAV) of gross primary production (GPP) in comparison to observations at global flux sitesThe IAV of GPP in TEMs is strongly depended on leaf area index (LAI), which is one of the causes for the underestimation of IAV in GPP and the simulated IAV in LAI from TEMs is much less than the observationPrecisely characterizing the contribution of vegetation physiological changes may improve the performance of predicting IAV in GPP from TEMs [ABSTRACT FROM AUTHOR]

Drought largely affects terrestrial gross primary productivity (GPP) by its duration (i.e., drought time‐scales). However, the large‐scale impact of drought time‐scales on GPP is still not well understood, partly due to limited understanding of ecological processes and difficulties in quantifying drought time‐scales. Here, we examined the responses of GPP to drought in China during the last four decades, using state‐of‐the‐art GPP data estimates and a multiscalar drought metric. Most of the GPP data suggested that the time‐scales at which drought has the largest influence on annual GPP (dominant drought time‐scales) were well related to climate region and vegetation type. The GPP responded to drought more rapidly in arid regions than that in semiarid and subhumid regions, with a mean dominant drought time‐scale of 15 and 20 months, respectively; as plants in arid regions can rapidly adapt to water shortage through physiological strategies, while, in semiarid and subhumid regions plants usually can withstand a degree of water deficits. In a humid region, GPP did not rapidly respond to drought, probably because the high soil water availability buffered the effect of drought. In addition, most of the GPP estimates suggested that grasslands and croplands tended to respond to drought at shorter time‐scales than that of forests, shrublands, and sparse vegetation. The study highlights the importance of improving the effects of water availability on vegetation activity and further consideration of vegetation adaptability to drought in both remote sensing GPP algorithms and parameterizations of current terrestrial ecosystem models. Plain Language Summary: Gross primary productivity (GPP) is the entire photosynthetic production of organic compounds in an ecosystem. It plays an important role in carbon cycles, and is the basis for food, fiber, and wood production. Drought widespreadly and negatively affects GPP by its duration (i.e., drought time‐scales). Thus, it is critical to understand the dominant time‐scale at which drought has the largest influence on GPP. We investigated dominant time‐scales of GPP response to drought in China using six GPP estimates and a multiscalar drought metric. We found that the dominant time‐scales were well related to climate region and vegetation type. The GPP responded to drought more quickly in arid regions than that in semiarid and subhumid regions. This might be due to plants in arid regions can rapidly adapt them to water shortage, while, in semiarid and subhumid regions, plants can withstand a degree of water deficits. In a humid region, GPP did not rapidly respond to drought, probably because the high soil water availability buffered the effect of drought. The grasslands and croplands responded to drought at shorter time‐scales than forests, shrublands, and sparse vegetation. The study could benefit the management and mitigation of drought effects on GPP. Key Points: The magnitudes of gross primary productivity (GPP) response to drought highly vary among different GPP estimates and across time‐scalesGPP responds to drought at shorter time‐scales in arid regions than that in other climate regionsGPP of forests responds to drought at longer time‐scales than that of shrublands, grasslands, and croplands [ABSTRACT FROM AUTHOR]

Changes in climate and land use are causing grasslands to suffer increasingly from abiotic stresses, including soil salinization. Silicon (Si) amendment has been frequently proposed to improve plant resistance to multiple biotic and abiotic stresses and increase ecosystem productivity while controlling the biogeochemical carbon (C) cycle. However, the effects of Si on plant C distribution and accumulation in salt-suffering grasslands are still unclear. In this study, we investigated how salt ions affected major elemental composition in plants and whether Si enhanced biomass C accumulation in grassland species in situ. In samples from the margins of salt lakes, our results showed that the differing distance away from the shore resulted in distinctive phytocoenosis, including halophytes and moderately salt-tolerant grasses, which are closely related to changing soil properties. Different salinity (Na+/K+, ranging from 0.02 to 11.8) in plants caused negative effects on plant C content that decreased from 53.9 to 29.2% with the increase in salinity. Plant Si storage [0.02–2.29 g Si m–2 dry weight (dw)] and plant Si content (0.53 to 2.58%) were positively correlated with bioavailable Si in soils (ranging from 94.4 to 192 mg kg–1). Although C contents in plants and phytoliths were negatively correlated with plant Si content, biomass C accumulation (1.90–83.5 g C m–2 dw) increased due to the increase of Si storage in plants. Plant phytolith-occluded carbon (PhytOC) increased from 0.07 to 0.28‰ of dry mass with the increase of Si content in moderately salt-tolerant grasses. This study demonstrates the potential of Si in mediating plant salinity and C assimilation, providing a reference for potential manipulation of long-term C sequestration via PhytOC production and biomass C accumulation in Si-accumulator dominated grasslands. [ABSTRACT FROM AUTHOR]

Keywords: phytolith; silica; silicon; biomineralisation; biogeochemistry; carbon sequestration EN phytolith silica silicon biomineralisation biogeochemistry carbon sequestration 1 3 3 04/22/20 20200417 NES 200417 Interest in phytoliths has grown significantly in recent years. Since Ma et al. ([5]) first described a Si transporter in the rice root, considerable interest was raised in establishing the molecular basis of plant Si uptake. So it is quite possible that the work of Zancajo et al. which suggests that bilobate silica cells in sorghum leaves have a different type of occluded organic matter compared with prickles and long cells may yet prove important when we consider carbon sequestration and dating. Phytolith, silica, silicon, biomineralisation, biogeochemistry, carbon sequestration. [Extracted from the article]

During silicate weathering, atmospheric carbon dioxide (CO 2 ) is consumed and base cations are released from silicate minerals to form carbonate and bicarbonate ions, which are finally deposited as carbonate complexes. Continental silicate weathering constitutes a stable carbon sink that is an important influence on long-term climate change, as it sequesters atmospheric carbon dioxide at a million-year time scale. Traditionally, CO 2 sequestered through silicate weathering is estimated by measuring the flux of the base cations to watersheds. However, plants also absorb considerable amounts of base cations. Plant biomass is often removed from ecosystems during harvesting. The base cations are subsequently released after decomposition of the harvested plant materials, and thereby enhance CO 2 consumption related to weathering. Here, we analyze plant biomass storage-harvest fluxes (production and removal of biomass from forests) of base cations in forests across China to quantify the relative contribution of forest trees to the terrestrial weathering-related carbon sink. Our data suggest that the potential CO 2 consumption rate for biomass-related silicate weathering (from the combined action of with afforestation/reforestation, controlled harvesting and rock powder amendment) in Chinese forests is 7.9 ± 4.1 Tg CO 2 yr − 1 . This represents ~ 34% of the chemical weathering rate in China. Globally, forests may increase CO 2 sequestration through biologically-mediated silicate weathering by ~ 32%. [ABSTRACT FROM AUTHOR]

The persistent terrestrial carbon sink regulates long-term climate change, but its size, location, and mechanisms remain uncertain. One of the most promising terrestrial biogeochemical carbon sequestration mechanisms is the occlusion of carbon within phytoliths, the silicified features that deposit within plant tissues. Using phytolith content-biogenic silica content transfer function obtained from our investigation, in combination with published silica content and aboveground net primary productivity ( ANPP) data of leaf litter and herb layer in China's forests, we estimated the production of phytolith-occluded carbon (PhytOC) in China's forests. The present annual phytolith carbon sink in China's forests is 1.7 ± 0.4 Tg CO2 yr −1, 30% of which is contributed by bamboo because the production flux of PhytOC through tree leaf litter for bamboo is 3-80 times higher than that of other forest types. As a result of national and international bamboo afforestation and reforestation, the potential of phytolith carbon sink for China's forests and world's bamboo can reach 6.8 ± 1.5 and 27.0 ± 6.1 Tg CO2 yr−1, respectively. Forest management practices such as bamboo afforestation and reforestation may significantly enhance the long-term terrestrial carbon sink and contribute to mitigation of global climate warming. [ABSTRACT FROM AUTHOR]

The occlusion of carbon (C) by phytoliths, the recalcitrant silicified structures deposited within plant tissues, is an important persistent C sink mechanism for croplands and other grass-dominated ecosystems. By constructing a silica content-phytolith content transfer function and calculating the magnitude of phytolith C sink in global croplands with relevant crop production data, this study investigated the present and potential of phytolith C sinks in global croplands and its contribution to the cropland C balance to understand the cropland C cycle and enhance long-term C sequestration in croplands. Our results indicate that the phytolith sink annually sequesters 26.35±10.22 Tg of carbon dioxide (CO2) and may contribute 40±18% of the global net cropland soil C sink for 1961–2100. Rice (25%), wheat (19%) and maize (23%) are the dominant contributing crop species to this phytolith C sink. Continentally, the main contributors are Asia (49%), North America (17%) and Europe (16%). The sink has tripled since 1961, mainly due to fertilizer application and irrigation. Cropland phytolith C sinks may be further enhanced by adopting cropland management practices such as optimization of cropping system and fertilization. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

As an important long-term terrestrial carbon sequestration mechanism, biogeochemical sequestration of carbon within phytoliths may play a significant role in the global carbon cycle and climate change. The aim of this study is to explore the potential of carbon bio-sequestration within phytoliths produced by wetland plants. The results show that the occluded carbon content of phytoliths in wetland plants ranges from 0.49% to 3.97%, with a CV (coefficient of variation) value of 810%. The data also indicate that the phytolith-occluded carbon (PhytOC) content of biomass for wetland plants depends not only on the phytolith content of biomass, but also the efficiency of carbon occlusion within phytoliths during plant growth in herb-dominated fens. The fluxes of carbon bio-sequestration within phytoliths of herb-dominated fen plants range from 0.003 to 0.077 t CO equivalents t-e-CO ha a. In China, 0.04×10 to 1.05×10 t CO equivalents per year may be sequestrated in phytoliths of herbaceous-dominated fen plants. Globally, taking a fen area of 1.48×10 ha and the largest phytolith carbon biosequestration flux (0.077 t-e-CO ha a) for herb-dominated fen plants, about 1.14×10 t CO equivalents per year would have been sequestrated in phytoliths of fen plants. If other wetland plants have similar PhytOC production flux with herb-dominated fen plants (0.077 t-e-CO ha a), about 4.39×10 t-e-CO a may be sequestrated in the phytoliths of world wetland plants. The data indicate that the management of wetland ecosystems (e.g. selection of plant species) to maximize the production of PhytOC have the potential to bio-sequestrate considerable quantities of atmospheric CO. [ABSTRACT FROM AUTHOR]

Among the most promising approaches of long-term atmospheric CO2 sequestration is terrestrial biogeochemical carbon sequestration. One of the most promising terrestrial biogeochemical carbon sequestration mechanisms is the occlusion of carbon within phytoliths, the silicified features that deposit within plant tissues. Using phytolith content-biogenic silica content transfer function obtained from our investigation, in combination with published silica content and above-ground net primary productivity ( ANPP) data of China's grasslands, we estimated the production of phytoliths and phytolith-occluded carbon ( PhytOC) in grasslands. The results show that the average above-ground phytolith production rates of China's grasslands (10.9 106 t yr−1 or 1.45% of world grasslands) are much lower than those of other grasslands (e.g. North American nonwoody grasslands) mainly because of much lower ANPP. Assuming a median content of PhytOC of 1.5%, the average above-ground PhytOC production rates of China's grasslands and world grasslands are estimated to be 0.6 106 t CO2 yr−1 and 41.4 106 t CO2 yr−1, respectively. The management of grasslands to maximize ANPP has the potential to result in considerable quantities of phytoliths and securely bio-sequestered carbon. [ABSTRACT FROM AUTHOR]

Among the most promising approaches of long-term atmospheric CO2 sequestration is terrestrial biogeochemical carbon sequestration. One of the most promising terrestrial biogeochemical carbon sequestration mechanisms is the occlusion of carbon within phytoliths, the silicified features that deposit within plant tissues. Using phytolith content-biogenic silica content transfer function obtained from our investigation, in combination with published silica content and above-ground net primary productivity ( ANPP) data of China's grasslands, we estimated the production of phytoliths and phytolith-occluded carbon ( PhytOC) in grasslands. The results show that the average above-ground phytolith production rates of China's grasslands (10.9 106 t yr−1 or 1.45% of world grasslands) are much lower than those of other grasslands (e.g. North American nonwoody grasslands) mainly because of much lower ANPP. Assuming a median content of PhytOC of 1.5%, the average above-ground PhytOC production rates of China's grasslands and world grasslands are estimated to be 0.6 106 t CO2 yr−1 and 41.4 106 t CO2 yr−1, respectively. The management of grasslands to maximize ANPP has the potential to result in considerable quantities of phytoliths and securely bio-sequestered carbon. [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]

• Reservoirs dramatically increased POC trapping in the Changjiang River basin. • The proportion of autochthonous, labile POC increased in the Changjiang POC export. • Due to reduced export and the growing labile fraction, carbon sedimentation in the Changjiang estuary is declining. • Global POC trapping in nine large rivers strongly impacted by dams is 12% of the global riverine POC export. The long-term changes in composition and export of particulate organic carbon (POC) by rivers due to dam construction are poorly known. Based on observations, incubation experiments and modelling, this study analyzed the sources, spatial and temporal distribution and annual export of POC in the Changjiang River during recent decades to explore the POC changes due to dam construction. Changes in POC composition resulted from both increasing POC sequestration and carbon fixation in the river basin. The allochthonous POC (from terrestrial input) export by the Changjiang River to its estuary decreased by ∼90% between 1956 and 2002 and 2013–2019. We estimated that 2.3 ± 0.5 Mt C/yr allochthonous POC has been sequestered in the Changjiang watershed since 2013. The autochthonous (entirely from in-stream riverine primary production) contribution increased from 1% to 55% of total POC export during the same period due to improved light transparency caused by decreased sediment discharge. Global POC trapping in nine large rivers strongly impacted by dams is 12% of the global riverine POC export. The reduced POC export and increase in labile autochthonous POC strongly impact the estuarine carbon cycle. Therefore, with continued dam construction in the future, important changes in the riverine and estuarine carbon cycle can be expected. [ABSTRACT FROM AUTHOR]

• Wet alpine meadow ecosystems (WAME) act as a significant carbon sink. • Ecosystem respiration was significantly decreased when soil was nearly-saturated. • Shallow groundwater inhibits soil respiration and favors carbon uptake of WAME. • Warming affects carbon dynamics of WAME through changing soil hydrology. Wet alpine meadows generally act as a significant carbon sink, since their low rate of soil decomposition determines a much smaller ecosystem respiration (Re) than photosynthesis. However, it remains unclear whether the low soil decomposition rate is determined by low temperatures or by nearly-saturated soil moisture. We explored this issue by using five years of measurements from two eddy-covariance sites with low temperature and significantly different soil water conditions. The results showed that both sites were carbon sinks. However, despite a smaller annual gross primary productivity, the wet site with a shallow groundwater showed a much higher carbon use efficiency and larger carbon sink than the dry site (which had a deeper water table) due to its much lower Re. Our analyses showed that Re of the wet site was significantly decreased under the nearly-saturated soil condition during the unfrozen seasons. This effect of nearly-saturated soil water on Re increased with soil depths. In contrast, at the dry site the high soil water content favored Re. The corresponding soil temperature at both sites expectedly showed large and positive effects on Re. These results demonstrated that the high carbon sink of the wet alpine meadow was mainly caused by the inhibiting effects of the nearly-saturated soil condition on soil respiration rather than by the low temperatures. Therefore, we argue that a warming-induced shrinking cryosphere may affect the carbon dynamics of wet and cold ecosystems through changes in soil hydrology and its impact on soil respiration. In addition, our study highlights the different responses of soil respiration to warming across soil depths. The thawing of frozen soil may cause larger CO 2 emission in the top soil, while it may also partially contribute to slowing down soil carbon decomposition in the deep soil through decreasing metabolic activity of aerobic organisms. Image, graphical abstract [ABSTRACT FROM AUTHOR]