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Copper (Cu) is a bio-essential element and a potentially toxic pollutant in the plant-soil systems. Analysis of stable Cu isotopes can be a powerful tool for tracing the biogeochemical cycling of Cu in plant-soil systems. In this review, we examined the analysis method of stable Cu isotope ratios in plants and soils, and discussed the biogeochemical processes, including redox reactions, mineral dissolution, abiotic and biotic sorption, which fractionate Cu isotopes in plant-soil systems. We also reviewed the variability of the isotopic signature in different plants and plant tissues, as well as different soil types and profiles to discuss the relationship between the biogeochemical transformation of Cu and its isotope fractionation in plant-soil systems. The collected data show that delta 65Cu values range from - 2.59 to + 1.73 parts per thousand in plant-soil systems, and increment 65Cu values range from - 1.00 to - 0.11 parts per thousand between the plant and soil. The variation in the increment 65Cu value between the plant and soil is mainly in response to the different uptake strategies during the acquisition of Cu from soils. Cu isotope analyses are proved to be a suitable technique during the biogeochemical transformation of Cu in plant-soil systems, especially during redox reactions. Ultimately, research challenges and future directions for Cu isotope techniques as a proxy for Cu biogeochemical cycles are also proposed. This review is beneficial for soil safety, food safety, and the sustainable development of agriculture and human health.

Aims Previous studies have shown that silicon (Si) can affect plant growth and yield by regulating the availability of other nutrients. However, the mechanisms by which Si affects plant biomass accumulation in coastal wetlands are not well explored. Methods We conducted a sampling campaign across the whole growing season of Phragmites australis under waterlogging and drought conditions in coastal wetland, and quantified the effects of Si availability on biomass accumulation. Results Compared with drought condition, the waterlogged condition improved the utilization efficiency of nitrogen (N) and phosphorus (P) of P. australis regulated by higher Si contents. Meanwhile, the increased Si contents promoted the utilization of N and P in leaf, suggesting that the increase in Si contents optimizes the photosynthetic process. Lignin contents in P. australis decreased with the increasing Si contents, which confirmed that Si can replace structural carbon components. In addition, principal component analysis (PCA) showed aboveground biomass accumulation of P. australis was synchronized with Si accumulation, indicating that Si was a beneficial element to promote biomass accumulation. Conclusions Our study implies that increasing Si availability is conducive to biomass accumulation of P. australis in waterlogged wetlands, which will provide important scientific references for the management of coastal wetland ecosystem and the increase of global 'blue carbon' sequestration.

Aims The natural abundances of stable carbon (C) and nitrogen (N) isotopes (delta C-13 and delta N-15) are extensively used to indicate the C and N biogeochemical cycles at large spatial scales. However, the spatial patterns of delta C-13 and delta N-15 in plant-soil systems of grasslands in northern China and their main driving factors across regional climatic gradient are still not well understood. Methods We measured plant and soil delta C-13 and delta N-15 compositions as well as their associated environmental factors across 2000 km climatic gradient (-0.2 to 9 degrees C; 152 to 502 mm) in grasslands of northern China. Results The soil delta C-13 and delta N-15 values in surface were lower than those in bottom for temperate typical steppe but had no significant differences for temperate meadow steppe and temperate desert steppe. Soil delta C-13 values declined with increasing soil organic carbon (SOC) but increased as mean annual temperature (MAT). These changes were attributed to the microbial decomposition rate. The delta N-15 values in soil and plant were negatively correlated with MAT and mean annual precipitation (MAP), which were mainly related to the low soil organic matter mineralization rate and the shift of dominant species from C-4 to C-3. Conclusions Our results indicate the spatial patterns and different influencing factors on delta C-13 and delta N-15 values along the climatic gradient in grasslands of northern China. The findings will provide scientific references for future research on the C and N biogeochemical cycles of temperate grasslands.

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 degrees N) compared with those in both low- (20 degrees N) and high-latitude (38-40 degrees 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 x 10(6) 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.

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 x 10(6) t CO2 a(-1), and the soil PhytOC storage was 383 x 10(6) 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.

Purpose The karst region in southwestern China is undergoing soil erosion and rocky desertification. The different silicon (Si) fractions along the hillslopes in this mountainous region could benefit plant growth and alleviate the ecological deterioration. However, extensive distribution of carbonate rocks may lead to limited plant available Si. The mountainous terrain in karst region also leads to more Si output, which seriously affects the biogeochemical cycle of Si in this area. Yet, the soil Si fractions in the karst region have not been fully evaluated. Methods Soil profiles and their corresponding plants were sampled from two typical karst mountains in Guizhou, China. The different fractions of non-crystalline Si in soil, accounting for the most important pool for Si availability to plants, were analyzed by the improved sequential chemical extraction and Si concentrations in plants grown in this region were also measured. Results The concentration and storage of non-crystalline Si were higher at lower slopes (storage was 2.44, 2.73, and 3.25 kg center dot m(-2) for upper, middle, and lower slopes, respectively) than other slope positions. Grasses dominated at lower slopes and contained significantly higher Si (mean +/- SD: 14.42 +/- 6.63 mg center dot g(-1)) than trees and shrubs (1.94 +/- 1.78 and 1.29 +/- 1.00 mg center dot g(-1), respectively), which were primarily distributed on upper slopes. However, Si concentrations of the same plant species in different slope positions had no significant correlation with soil acid Na acetate-Si, the Si regarded as directly available for plants. Conclusions This study suggests that plant species and soil properties have a significant impact on the soil Si distribution of hillslopes in karst region. Soil erosion may decrease non-crystalline Si concentrations in soils and impair Si uptake in grasses, which need to be considered in ecosystem management in this region.

Black shales are characterized by a high content of organic carbon (C). Few studies have focused on the influence of land use on soil organic C (SOC) fractions from soils derived from black shale (black shale soils). The objective of this study was to elucidate the influence of land use on SOC fractions in black shale soils combining chemical determination and stable C isotope analysis techniques. Herein, we determined labile organic C (LOC), semilabile organic C (Semi-LOC), and recalcitrant organic C (ROC) fractions in various depths of soils in paddy fields (0-70 cm) and forests (0-120 cm) from black shale distribution region in Hunan province, China, and then investigated delta C-13 values of these soils. Results showed that the contents of LOC, Semi-LOC, and ROC in paddy soils (1.63-7.35 g kg(-1), 0.35-1.21 g kg(-1), and 3.75-14.8 g kg(-1), respectively) and forest soils (0.73-4.94 g kg(-1), 0.12-0.89 g kg(-1), and 1.44-8.96 g kg(-1), respectively) are significantly decreased with increasing depth. The contribution made by LOC to SOC in paddy soils was significantly lower than that in forest soils, while the contribution made by ROC to SOC was significantly higher in paddy soils than that in forest soils. In these two land uses, the delta C-13 values were higher in SOC compared to the ROC fraction, while the delta C-13 values were close in the ROC fraction below 20 cm soil depth. Our study indicated that i) new C is mainly limited to the surface soil layer (0-10 cm) in forests, while it can be leached along the soil profiles in paddy fields; ii) the estimated ROC pool is similar to 900 Pg within the 0-100 cm soil layer in terrestrial ecosystems, which should better represent the ability of soil C sequestration.

To understand the past carbon accumulation of forest-steppe ecotone and to identify the main drivers of the long-term carbon dynamics, we selected Huangqihai Lake and analyzed the sediment records. We measured the organic carbon content (TOC; %) of sedimentary samples and quantified the carbon accumulation rate (CAR; g C m(-2) yr(-1)). Furthermore, the climate, soil erosion, and vegetation development of the past 6800 years were reconstructed using physicochemical parameters and pollen records. Human activities were also obtained from a 2200-year history record. Our results showed that the CAR was high during 5800 similar to 4100 cal yr BP (40 similar to 60 g C m(-2) yr(-1)), which is mainly attributed to the high sediment accumulation rate (SAR) during this period. Pearson's correlation, redundancy analysis and hierarchical variation partitioning analyses suggested that the CAR was influenced by the SAR and TOC, while vegetation dynamics (broadleaved tree percentage and vegetation coverage) and local soil erosion were the main drivers of the TOC and SAR. Especially when the vegetation was dominated by broadleaved forests, the CAR was significantly high due to the high gross primary productivity and carbon density of forest compared with steppe. Our study highlights the direct influence of local vegetation and soil erosion on the CAR, whereas climate might influence indirectly by changing local vegetation and soil conditions. Moreover, our results showed that human activities had positive influences on the carbon accumulation dynamics in this region since 2200 cal yr BP by influencing the SAR.

Silicon (Si) plays an important role in improving soil nutrient availability and plant carbon (C) accumulation and may therefore impact the biogeochemical cycles of C, nitrogen (N), and phosphorus (P) in terrestrial ecosystems profoundly. However, research on this process in grassland ecosystems is scarce, despite the fact that these ecosystems are one of the most significant accumulators of biogenic Si (BSi). In this study, we collected the aboveground parts of four widespread grasses and soil profile samples in northern China and assessed the correlations between Si concentrations and stoichiometry and accumulation of C, N, and P in grasses at the landscape scale. Our results showed that Si concentrations in plants were significantly negatively correlated (p< 0.01) with associated C concentrations. There was no significant correlation between Si and N concentrations. It is worth noting that since the Si concentration increased, the P concentration increased from less than 0.10% to more than 0.20% and therefore C:P and N:P ratios decreased concomitantly. Besides, the soil noncrystalline Si played more important role in C, N, and P accumulation than other environmental factors (e.g., MAT, MAP, and altitude). These findings indicate that Si may facilitate grasses in adjusting the utilization of nutrients (C, N, and P) and may particularly alleviate P deficiency in grasslands. We conclude that Si positively alters the concentrations and accumulation of C, N, and P likely resulting in the variation of ecological stoichiometry in both vegetation and litter decomposition in soils. This study further suggests that the physiological function of Si is an important but overlooked factor in influencing biogeochemical cycles of C and P in grassland ecosystems.

Silicon (Si) plays an important role in improving soil nutrient availability and plant carbon (C) accumulation and may therefore impact the biogeochemical cycles of C, nitrogen (N), and phosphorus (P) in terrestrial ecosystems profoundly. However, research on this process in grassland ecosystems is scarce, despite the fact that these ecosystems are one of the most significant accumulators of biogenic Si (BSi). In this study, we collected the aboveground parts of four widespread grasses and soil profile samples in northern China and assessed the correlations between Si concentrations and stoichiometry and accumulation of C, N, and P in grasses at the landscape scale. Our results showed that Si concentrations in plants were significantly negatively correlated (p< 0.01) with associated C concentrations. There was no significant correlation between Si and N concentrations. It is worth noting that since the Si concentration increased, the P concentration increased from less than 0.10% to more than 0.20% and therefore C:P and N:P ratios decreased concomitantly. Besides, the soil noncrystalline Si played more important role in C, N, and P accumulation than other environmental factors (e.g., MAT, MAP, and altitude). These findings indicate that Si may facilitate grasses in adjusting the utilization of nutrients (C, N, and P) and may particularly alleviate P deficiency in grasslands. We conclude that Si positively alters the concentrations and accumulation of C, N, and P likely resulting in the variation of ecological stoichiometry in both vegetation and litter decomposition in soils. This study further suggests that the physiological function of Si is an important but overlooked factor in influencing biogeochemical cycles of C and P in grassland ecosystems.

Background and aims Silicon (Si) deficiency is a major constraint on rice production. The objective of this study was to evaluate the long-term influence of phytolith-rich straw return and groundwater table management on labile Si fractions in paddy soil and subsequent plant Si uptake. Methods A field experiment was conducted over 36 years in subtropical China with different application doses of phytolith-rich straw and a groundwater table of either 20 or 80 cm. An optimized sequential chemical extraction procedure allowed us to determine labile Si fractions, represented by CaCl2-Si, Acetic-Si, H2O2-Si, Oxalate-Si, and Na2CO3-Si. Additional analyses included the determination of amorphous silica particles in soil, phytoliths in supplied straw, Si in planted rice straw, and the dissolution rate of phytoliths extracted from supplied straw. Results Long-term application of phytolith-rich straw significantly increased the H2O2-Si and Na2CO3-Si contents. The CaCl2-Si (5.21-7.91 mg kg(- 1)), H2O2-Si (50.0-72.4 mg kg(- 1)) and Na2CO3-Si (3.33-4.60 g kg(- 1)) contents were positively correlated with soil organic carbon. The Si content (13.6-28.9 g kg(-& x200d;1)) in planted rice straw significantly (p < 0.05) increased with the application dose of phytolith-rich straw under both groundwater tables. This effect was significantly (p < 0.05) greater under 80 cm groundwater table than under 20 cm groundwater table for matching straw amendments. Conclusions This study indicates that long-term application of phytolith-rich straw and groundwater management significantly increase soil Si bioavailability by promoting accumulation of organic matter and phytoliths, and enhancing the soil-plant Si cycle.