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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]

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. [ABSTRACT FROM AUTHOR]

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

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

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]

Copper (Cu) isotopes are utilized to track Cu geochemical cycling in weathered gleysols of tropical zones. A significant isotope fractionation of Cu in these soils is primarily redox‐controlled; however, it is rarely reported how the frequency of redox fluctuations affects the soil Cu isotope signature. This study investigated the variations of Cu content and isotope fractionation in two low‐humic gleysol profiles (S1 and S2) from a dry tropical savanna zone. Owing to redox oscillation during weathering, δ65Cu values in profile S2 showed a stronger positive correlation with the mafic index of alteration of reducing environment than S1, and isotopically light Cu is more retained in the Zone II of profile S2 than S1. As the frequency of redox fluctuation increased, the retained stable Cu(I) species and light Cu isotopes increased in the residual soils through re‐adsorption or re‐precipitation by iron oxyhydroxide (i.e., ferrihydrite). Importantly, an Mn‐enriched zone was formed after reduction events in profile S2, and found to be enriched in light Cu isotopes. The heavier Cu fraction might be lost by adsorption on Fe oxyhydroxides in the Mn‐rich zone, while the relatively light Cu might be retained through adsorption on Mn oxyhydroxides. Additionally, a significant Soil Organic Carbon (SOC) contribution to Cu was found due to the high δ65Cu‐SOC correlation (R2 = 0.80) in S1 (depth <1 m). Therefore, our study shows that the Cu isotope signature can respond to redox changes in the terrestrial ecosystem, and these Cu isotope signatures may have significant implications for assessing soil ecological vulnerability under future climate change scenarios. Plain Language Summary: The fractionation of copper (Cu) isotope is a process related to the redox fluctuation during soil Cu biogeochemical cycling. For Cu isotope composition in weathered gleysols of tropical zones, the increased rates of redox fluctuations are assumed to occur during gleysol evolution due to the seasonal exchange of groundwater and river water. However, the impact of the frequency of redox fluctuations on soil Cu isotope signatures is rarely documented. Here, we analyzed the variations of Cu content and isotope fractionation in two low‐humic gleysol profiles with different pedogenetic processes during weathering in the same basin (Mun River Basin), and found that the frequency of redox fluctuations could determine the magnitude of Cu isotope fractionation. We record an increased light Cu isotopes and identify the stable Cu(I) species retained in the residual soils with the increased frequency of redox fluctuation. Several processes contribute to Cu isotope fractionation at different soil horizons, but most isotope fractionation is related to the re‐adsorption or re‐precipitation by iron and manganese oxyhydroxide (i.e., ferrihydrite and pyrolusite), especially at the iron or manganese‐rich zone. Cu isotope fractionation is sensitive to increased redox fluctuations in the terrestrial ecosystem, and may have significant implications for assessing soil ecological vulnerability under future climate change scenarios. Key Points: Extremely light δ65Cu values (−1.20‰ to −1.16‰) were reported to date in the residual soilsThe Cu isotope fractionation was driven by redox fluctuations during gleysols weathering in Mun River BasinCu isotope fractionation was sensitive to increased redox fluctuations in the terrestrial ecosystem [ABSTRACT FROM AUTHOR]

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

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

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]

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

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]

The importance of long‐distance migration from low to high latitudes relative to local spread from northern refugia after the Last Glacial Maximum (LGM) remains a focus of debate for many temperate tree species. We assessed the dynamics of Chinese pine Pinus tabulaeformis, a widespread species endemic to northern China, since the LGM by integrating cytoplasmic DNA data, mapped pollen records and ecological niche modeling. Genetic variation among 544 individuals from 50 populations spanning the entire natural species range revealed eight genetic clusters with distinct geographic distribution, indicating glacial lineages likely originating from multiple local microrefugia. Palynological evidence suggested that the northernmost part of the natural distribution originated from local postglacial spread. Niche modeling indicated high probability of the species being present in the area of the Loess Plateau and coastal areas north of the Yangtze River during the LGM. The three lines of evidence jointly suggest that the species persisted through the last glaciation in the mountains surrounding the Loess Plateau of northern China and that the current distribution of the species originated primarily from the spread of local refugial populations, instead of long‐distance migration. These results cast doubt on the notion that Chinese pine migrated from areas south of the Yangtze River and underscore the importance of northern refugia. [ABSTRACT FROM AUTHOR]

Solid biosilica (phytoliths) deposited in plant tissues is thought to function as structural support, as a cost-effective alternative to lignin, and in herbivore defence, by limiting nutrient access/extraction and abrading herbivore mouthparts. It has been assumed that active phytolith accumulation evolved for these purposes, but these hypotheses remain untested. For example, an influential idea holds that grasses became so silica-rich through antagonistic co-evolution with mammalian grazers during the Cenozoic., We examine whether phytoliths fulfil criteria established for adaptations, focusing on three aspects. First, we evaluate the recent debate concerning whether plant silica wears herbivore mouthparts/teeth. Secondly, we test whether the evolutionary pattern of phytolith accumulation is consistent with adaptive hypotheses by mapping silica content onto time-calibrated land plant and grass phylogenies. Thirdly, we compare with palaeontological evidence for the timing of the 'demand' for the hypothesized function (structural support, herbivore defence)., Our survey demonstrates that phytoliths meet several criteria for adaptations, but key aspects require further study. For example, phytoliths wear teeth but are likely less important than dietary grit, suggesting that silica deterrence is ineffective against large mammalian grazers., Mapping analysis indicates that active silica accumulation evolved numerous times, rather than being ancestral in land plants. However, a clear temporal link between these events and hypothesized functional 'demands' is still missing. For example, we find no convincing evidence for Cenozoic grass-grazer co-evolution., Synthesis. Phytoliths help support and defend plants today, but the adaptive origin of this trait requires further testing. Such tests should integrate the phylogenetic distributions of phytoliths with ecology and biomechanics and use fossil evidence to evaluate the correlation between functional 'demand' and plant evolution. [ABSTRACT FROM AUTHOR]

Legume species were more and more commonly introduced to degraded grassland for human-aided restoration. This study aimed to test whether the natural spread of legume contributes to reversal of grassland degradation through making an extensive grassland vegetation and soil survey in Inner Mongolia of China. The results showed that legume biomass increased along a gradient of soil coarsening that was also a gradient of grassland degradation. The total biomass rather than legume biomass was limited by mean annual precipitation at plot level. The presence of legumes increased species richness in degraded steppe vegetation. Constancy of intermediate and climax species of vegetation succession at sites with legumes was higher than that at sites without legumes, implying that legumes might contribute positively to the reversal of grassland degradation. Our study suggests that naturally distributed legumes could benefit the reversal of grassland degradation through promoting plant community succession rather than total plant biomass. Planting legumes would be an effective measure to accelerate the recovery process of degraded grassland with coarsened soil in regions similar to our study region. Copyright © 2015 John Wiley & Sons, Ltd. [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]