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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 δ13CLOC were greater than δ13CROC, independent of the rice cropping region, and that there was a significant positive correlation between δ13CSOC and δ13Cstraw. The δ13CLOC and δ13CSOC 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. [ABSTRACT FROM AUTHOR]

Soil organic carbon (SOC) in grasslands not only plays an important role in carbon cycle but also largely affects soil function and thus determines grasslands' productivity. However, the influence mechanism of different factors on SOC storage in grasslands of northern China remains unclear. Based on an extensive field investigation and multiple statistical approaches, we mapped SOC of the grassland in northern China, and explored their correlations with climatic factors, biological factors, human activity intensity (HAI), and soil physicochemical factors. The effects of these factors on SOC were also analyzed by structural equation model. The results showed that the SOC varied across the regions and was strongly correlated with soil chemical factors (i.e., soil total nitrogen [TN], amorphous Al, and amorphous Fe) and soil physical factors (i.e., clay, silt, and sand). HAI, which was mainly influenced by grazing, had a significant but negative impact on SOC. Different from previous studies, we found that climatic factors such as temperature and precipitation indirectly affected SOC by influencing biological factors such as vegetation coverage, vegetation height, Shannon–Weiner index, and species richness. The soil physicochemical properties and HAI were the dominant influencing factors for controlling SOC distribution in the grasslands of northern China. Our findings will further increase the understanding of the carbon cycle in grassland ecosystems, while providing important scientific references for temperate grassland ecosystem management. [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]

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. [ABSTRACT FROM AUTHOR]

Ecological stoichiometry could provide scientific references for restoring degraded grassland. However, most of the research studies conducted on grasslands emphasise the main biogenic elements and ignore silicon (Si) changes, although Si has been considered to affect the nitrogen (N) and phosphorus (P) accumulation in plants. In this study, we aimed to explore the N, P and Si relationships in plants and soil in grasslands of different degradation statuses of northern China. This study conducted intensive sampling, including samples of eight plant species, the aboveground parts of plant communities and corresponding soil profiles from 69 sites. In addition to the N, P and Si concentrations, multiple soil (e.g., grain size) and vegetation (e.g., coverage) parameters were measured or calculated. Our results showed that the N, P and dissolved Si (DSi) in soil profiles were mainly determined by soil properties, such as the soil water content and pH, whereas plant elemental concentrations were determined by plant families and species. The N concentrations within the same plant species were also significantly influenced by mean annual temperature. Additionally, we used the soil and vegetation parameters to indicate the degradation level and found that grassland degradation could decrease soil elemental concentrations, soil N/P and the N or P concentrations of plant species. The DSi in soil had significantly positive correlation with the plant N and P concentrations (p < 0.05) and showed high independent effect on them. Given the benefits of Si on plants, future management practices relating to grassland restoration should consider the Si cycle. [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]

Recovery from grassland degradation caused by climate change and human disturbance is critical to the sustainable development of drylands. Grassland in China accounts for 13% of the global grassland area, and more than half of it is still recovering from degradation. How climate change will contribute to the recovery of degraded grassland has not been clarified through field experiments. Here, we report the effects of manipulated precipitation regimes on above ground net primary productivity (ANPP) in five grassland fields along a degradation gradient using a 3‐year controlled rainfall manipulation experiment in Inner Mongolia, China. We found that differences of approximately 20–30% in growing season precipitation have limited impacts on the ANPP of grasslands, and the responses can depend crucially on the degradation status indicated by vegetation and soil features. ANPP sensitivity to spring drought increased with increasing degradation level, which was mainly determined by vegetation composition. ANPP was generally insensitive to summer drought except for mature grassland, in which drought led to severe soil water deficit, mainly mediated by soil water holding capacity. Our results suggest that the recovery of degraded grassland is expected to be delayed by increasing drought early in the growing season, and mature grassland would be more vulnerable to extreme summer drought. These findings highlight the negative impacts of seasonal drought on degraded grassland and emphasize the importance of accounting for climate change and degradation status in future grassland ecosystem restoration and management policies. [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]

Over the past century, conversion to agriculture has greatly reduced the global extent of coastal wetlands leading to degradation and loss of these ecosystems. However, it remains unclear how this land conversion affects the confluent soil organic and inorganic carbon (SOC and SIC) storage as well as their localizations in soil matrix. Here, we investigated these issues using wet sieving at two coastal saline–alkali sites in northern China. Conversion of marshes to cropland (>60 years) decreased the portion of large macroaggregates (>2 mm) and correspondingly increased the portion of microaggregates (0.053–0.25 mm) at both sites. Land conversion decreased SOC contents by 31–67% in all fractions (>2, 0.25–2, 0.053–0.25, and <0.053 mm) in the topsoil (0–15 cm) and subsoil (15–30 cm). In contrast, irrigation‐ and NH4HCO3 fertilization‐derived carbonates increased SIC storages in almost all fractions due to the saline–alkali soil conditions, especially for the subsoil. This increases in SIC almost offset and compensate for the SOC losses at both sites. Consequently, the irrigation‐ and NH4HCO3‐induced SIC accumulation should be included in the full C balance of saline–alkali soils. It should be noted, however, that the progressive loss of SOC due to cultivation will lead to soil degradation in fertility and ecological function, thereby hampering long‐term sustainability of coastal ecosystems. Therefore, the compensation of SIC for the loss of SOC is not sustainable in the longer term. [ABSTRACT FROM AUTHOR]

Evapotranspiration (ET) plays a key role in linking water, energy, and carbon cycles. However, at regional scales, variations and controls of ET are still not well understood, due to the substantial uncertainties in ET estimates. Here, the variations in ET over China during the last three decades and their drivers and controls were analysed at monthly, seasonal, and annual scales, using well‐evaluated land surface model simulations. The results suggested that (a) the spatially averaged ET over China increased during 1979–2012 at each temporal scale. Overall, the ET significantly increased (p <.05) in most of western China and decreased in several regions in eastern China during most months and seasons. (a) The increases in spatially averaged ET during the nongrowing season were mainly determined by warming. The increased annual ET, seasonal ET, and monthly ET within the growing season were jointly caused by vegetation greening and climate warming. (c) During November–March, ET was primarily determined by temperature in most of central China and was determined by water availability in the remaining regions. The annual ET and the monthly ET within April–October were mainly controlled by water availability in most of northern and western China, and several regions in central China, by solar radiation in most of central and southern China and by temperature in some regions in the Tibetan Plateau. The study highlights the large effects of warming and greening on ET over China during the last three decades. The results may benefit water resource management under climate change. [ABSTRACT FROM AUTHOR]

Phytolith-occluded carbon (PhytOC) has significant potential for long-term biogeochemical carbon (C) sequestration because of its high resistance against decomposition. It may also play a crucial role in slowing the increase in global CO concentrations and mitigating climate warming. As phytolith C sequestration flux is usually correlated with phytolith content, C content of phytoliths and above-ground net primary productivity in plants, we hypothesize that application of fertilizers may increase phytolith C sequestration in some degraded grasslands. In this study, we conducted a field experiment to investigate the effects of external application of nitrogen (N) at six levels (0, 10, 20, 30, 40, and 50 g N m year) from 2011 to 2013 on the potential for phytolith C sequestration in degraded grasslands. Analysis showed that N application increased the PhytOC production flux in the extremely degraded grassland from 0.003 to 0.021 t CO ha year and the flux increased with the level of N fertilization peaking in the 20 g N m year treatment at 700 % of the control flux, but decreased at higher N doses. Assuming half of China's grasslands are fertilized with N to recover from degradation and the phytolith C sequestration flux of degraded grasslands amended with N is half of the 700 % increase, the potential of phytolith C sequestration in China's grasslands could be increased at least 60 %. This study demonstrates that optimization of nutritional supply is a promising approach to increase long-term phytolith C sequestration in degraded grasslands. [ABSTRACT FROM AUTHOR]

In plants, silicon (Si) accumulates in phytoliths, which can contain 0.1-6.0 % occluded organic carbon (phytolith-occluded organic carbon, PhytOC). Globally, phytolith formation has implications for the long-term terrestrial carbon balance, and thus, global climate change. In this study, we analyzed the Si contents of 108 species of plants from Inner Mongolia and Hebei, China, to demonstrate the relationships between plant Si content and various phylogenetic and ecological characteristics, and to estimate PhytOC production in the forests of north China. Variations in Si content were related to the plant's higher-level phylogenetic position, according to the Angiosperm Phylogeny Group III classification. In angiosperms, the commelinoid monocot orders accumulated substantially more Si than did the other monocot orders, and the asteroid dicot orders accumulated more Si than did the other dicot orders. In an ecological context, herbs had the highest Si content (0.47 ± 0.26 % dry weight), followed by trees (0.33 ± 0.13 % dry weight) and shrubs (0.24 ± 0.09 % dry weight). The total PhytOC production rate in the forests of north China corresponded to 0.48 ± 0.20 × 10 t CO year, approximately 44 % of which was contributed by the understory vegetation comprising herbs and shrubs. This finding indicates that the phytolith carbon sequestration potential of the forests may be further enhanced by afforestation/reforestation, preserving the understory vegetation, and maximizing the aboveground net primary productivity of all forest layers, especially herbs that accumulate high Si contents. [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]

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]