Permafrost carbon cycle and its dynamics on the Tibetan Plateau.

Our knowledge on permafrost carbon (C) cycle is crucial for understanding its feedback to climate warming and developing nature-based solutions for mitigating climate change. To understand the characteristics of permafrost C cycle on the Tibetan Plateau, the largest alpine permafrost region around the world, we summarized recent advances including the stocks and fluxes of permafrost C and their responses to thawing, and depicted permafrost C dynamics within this century. We find that this alpine permafrost region stores approximately 14.1 Pg (1 Pg=1015 g) of soil organic C (SOC) in the top 3 m. Both substantial gaseous emissions and lateral C transport occur across this permafrost region. Moreover, the mobilization of frozen C is expedited by permafrost thaw, especially by the formation of thermokarst landscapes, which could release significant amounts of C into the atmosphere and surrounding water bodies. This alpine permafrost region nevertheless remains an important C sink, and its capacity to sequester C will continue to increase by 2100. For future perspectives, we would suggest developing long-term in situ observation networks of C stocks and fluxes with improved temporal and spatial coverage, and exploring the mechanisms underlying the response of ecosystem C cycle to permafrost thaw. In addition, it is essential to improve the projection of permafrost C dynamics through in-depth model-data fusion on the Tibetan Plateau.

Metagenomic insights into microbial community structure and metabolism in alpine permafrost on the Tibetan Plateau.

Permafrost, characterized by its frozen soil, serves as a unique habitat for diverse microorganisms. Understanding these microbial communities is crucial for predicting the response of permafrost ecosystems to climate change. However, large-scale evidence regarding stratigraphic variations in microbial profiles remains limited. Here, we analyze microbial community structure and functional potential based on 16S rRNA gene amplicon sequencing and metagenomic data obtained from an ∼1000 km permafrost transect on the Tibetan Plateau. We find that microbial alpha diversity declines but beta diversity increases down the soil profile. Microbial assemblages are primarily governed by dispersal limitation and drift, with the importance of drift decreasing but that of dispersal limitation increasing with soil depth. Moreover, genes related to reduction reactions (e.g., ferric iron reduction, dissimilatory nitrate reduction, and denitrification) are enriched in the subsurface and permafrost layers. In addition, microbial groups involved in alternative electron accepting processes are more diverse and contribute highly to community-level metabolic profiles in the subsurface and permafrost layers, likely reflecting the lower redox potential and more complicated trophic strategies for microorganisms in deeper soils. Overall, these findings provide comprehensive insights into large-scale stratigraphic profiles of microbial community structure and functional potentials in permafrost regions.

A novel method for colposcopic shunting in HPV-positive women: Quantitative detection of HPV E7 oncoprotein.

The objective of the study was to evaluate the clinical application potential of quantitatively detecting human papillomavirus (HPV) E7 oncoprotein in HPV-positive women, with the goal of detecting potential high-grade cervical squamous intraepithelial lesions (HSIL) and cervical cancer improving the accuracy of colposcopic shunting in these patients.HPV-positive women (N = 611) were selected for quantitatively detecting HPV E7 protein levels by magnetic particle-based chemiluminescence immunoassay before colposcopy. Receiver operating characteristic (ROC) curve analysis was performed (n = 400) to determine diagnostic detection thresholds for HPV E7 oncoprotein. ThinPrep cytology test (TCT) and Aptima HPV E6/E7 mRNA analysis were also performed (n = 211). The diagnostic performance of these three diagnostic methods in detecting HSIL and cervical cancer was compared with the gold standard of pathological diagnosis. The area under the ROC curve was 0.724. The diagnostic detection threshold of HPV E7 oncoprotein was ≥10.88 ng/mL. The sensitivity (SEN), specificity (SPE), positive predictive value (PPV), negative predictive value (NPV), and Youden index of HPV E7 oncoprotein for the identification of HSIL and cervical cancer were 78.7 %, 77.9 %, 72.2 %, 83.3 %, and 56.6 %, respectively, which were higher than those of TCT and HPV E6/E7 mRNA.The results indicate that quantitative detection of HPV E7 oncoprotein can effectively shunt HPV-positive women and reduce unnecessary colposcopy and biopsy. It can detect potential HSIL and cervical cancer in a timely manner and prevent high-risk patients from missing diagnosis.

Accelerated organic matter decomposition in thermokarst lakes upon carbon and phosphorus inputs.

Mineralization of dissolved organic matter (DOM) in thermokarst lakes plays a non-negligible role in the permafrost carbon (C) cycle, but remains poorly understood due to its complex interactions with external C and nutrient inputs (i.e., aquatic priming and nutrient effects). Based on large-scale lake sampling and laboratory incubations, in combination with 13 C-stable-isotope labeling, optical spectroscopy, and high-throughput sequencing, we examined large-scale patterns and dominant drivers of priming and nutrient effects of DOM biodegradation across 30 thermokarst lakes along a 1100-km transect on the Tibetan Plateau. We observed that labile C and phosphorus (P) rather than nitrogen (N) inputs stimulated DOM biodegradation, with the priming and P effects being 172% and 451% over unamended control, respectively. We also detected significant interactive effects of labile C and nutrient supply on DOM biodegradation, with the combined labile C and nutrient additions inducing stronger microbial mineralization than C or nutrient treatment alone, illustrating that microbial activity in alpine thermokarst lakes is co-limited by both C and nutrients. We further found that the aquatic priming was mainly driven by DOM quality, with the priming intensity increasing with DOM recalcitrance, reflecting the limitation of external C as energy sources for microbial activity. Greater priming intensity was also associated with higher community-level ribosomal RNA gene operon (rrn) copy number and bacterial diversity as well as increased background soluble reactive P concentration. In contrast, the P effect decreased with DOM recalcitrance as well as with background soluble reactive P and ammonium concentrations, revealing the declining importance of P availability in mediating DOM biodegradation with enhanced C limitation but reduced nutrient limitation. Overall, the stimulation of external C and P inputs on DOM biodegradation in thermokarst lakes would amplify C-climate feedback in this alpine permafrost region.

Photo-produced aromatic compounds stimulate microbial degradation of dissolved organic carbon in thermokarst lakes.

Photochemical and biological degradation of dissolved organic carbon (DOC) and their interactions jointly contribute to the carbon dioxide released from surface waters in permafrost regions. However, the mechanisms that govern the coupled photochemical and biological degradation of DOC are still poorly understood in thermokarst lakes. Here, by combining Fourier transform ion cyclotron resonance mass spectrometry and microbial high-throughput sequencing, we conducted a sunlight and microbial degradation experiment using water samples collected from 10 thermokarst lakes along a 1100-km permafrost transect. We demonstrate that the enhancement of sunlight on DOC biodegradation is not associated with the low molecular weight aliphatics produced by sunlight, but driven by the photo-produced aromatics. This aromatic compound-driven acceleration of biodegradation may be attributed to the potential high abilities of the microbes to decompose complex compounds in thermokarst lakes. These findings highlight the importance of aromatics in regulating the sunlight effects on DOC biodegradation in permafrost-affected lakes.

Priming effect stimulates carbon release from thawed permafrost.

Climate warming leads to widespread permafrost thaw with a fraction of the thawed permafrost carbon (C) being released as carbon dioxide (CO2 ), thus triggering a positive permafrost C-climate feedback. However, large uncertainty exists in the size of this model-projected feedback, partly owing to the limited understanding of permafrost CO2 release through the priming effect (i.e., the stimulation of soil organic matter decomposition by external C inputs) upon thaw. By combining permafrost sampling from 24 sites on the Tibetan Plateau and laboratory incubation, we detected an overall positive priming effect (an increase in soil C decomposition by up to 31%) upon permafrost thaw, which increased with permafrost C density (C storage per area). We then assessed the magnitude of thawed permafrost C under future climate scenarios by coupling increases in active layer thickness over half a century with spatial and vertical distributions of soil C density. The thawed C stocks in the top 3 m of soils from the present (2000-2015) to the future period (2061-2080) were estimated at 1.0 (95% confidence interval (CI): 0.8-1.2) and 1.3 (95% CI: 1.0-1.7) Pg (1 Pg = 1015 g) C under moderate and high Representative Concentration Pathway (RCP) scenarios 4.5 and 8.5, respectively. We further predicted permafrost priming effect potential (priming intensity under optimal conditions) based on the thawed C and the empirical relationship between the priming effect and permafrost C density. By the period 2061-2080, the regional priming potentials could be 8.8 (95% CI: 7.4-10.2) and 10.0 (95% CI: 8.3-11.6) Tg (1 Tg = 1012 g) C year-1 under the RCP 4.5 and RCP 8.5 scenarios, respectively. This large CO2 emission potential induced by the priming effect highlights the complex permafrost C dynamics upon thaw, potentially reinforcing permafrost C-climate feedback.

Divergent changes in particulate and mineral-associated organic carbon upon permafrost thaw.

Permafrost thaw can stimulate microbial decomposition and induce soil carbon (C) loss, potentially triggering a positive C-climate feedback. However, earlier observations have concentrated on bulk soil C dynamics upon permafrost thaw, with limited evidence involving soil C fractions. Here, we explore how the functionally distinct fractions, including particulate and mineral-associated organic C (POC and MAOC) as well as iron-bound organic C (OC-Fe), respond to permafrost thaw using systematic measurements derived from one permafrost thaw sequence and five additional thermokarst-impacted sites on the Tibetan Plateau. We find that topsoil POC content substantially decreases, while MAOC content remains stable and OC-Fe accumulates due to the enriched Fe oxides after permafrost thaw. Moreover, the proportion of MAOC and OC-Fe increases along the thaw sequence and at most of the thermokarst-impacted sites. The relatively enriched stable soil C fractions would alleviate microbial decomposition and weaken its feedback to climate warming over long-term thermokarst development.

Nitrogen input enhances microbial carbon use efficiency by altering plant-microbe-mineral interactions.

Microbial growth and respiration are at the core of the soil carbon (C) cycle, as these microbial physiological performances ultimately determine the fate of soil C. Microbial C use efficiency (CUE), a critical metric to characterize the partitioning of C between microbial growth and respiration, thus controls the sign and magnitude of soil C-climate feedback. Despite its importance, the response of CUE to nitrogen (N) input and the relevant regulatory mechanisms remain poorly understood, leading to large uncertainties in predicting soil C dynamics under continuous N input. By combining a multi-level field N addition experiment with a substrate-independent 18 O-H2 O labelling approach as well as high-throughput sequencing and mineral analysis, here we elucidated how N-induced changes in plant-microbial-mineral interactions drove the responses of microbial CUE to N input. We found that microbial CUE increased significantly as a consequence of enhanced microbial growth after 6-year N addition. In contrast to the prevailing view, the elevated microbial growth and CUE were not mainly driven by the reduced stoichiometric imbalance, but strongly associated with the increased soil C accessibility from weakened mineral protection. Such attenuated organo-mineral association was further linked to the N-induced changes in the plant community and the increased oxalic acid in the soil. These findings provide empirical evidence for the tight linkage between mineral-associated C dynamics and microbial physiology, highlighting the need to disentangle the complex plant-microbe-mineral interactions to improve soil C prediction under anthropogenic N input.

Divergent Trajectory of Soil Autotrophic and Heterotrophic Respiration upon Permafrost Thaw.

Warming-induced permafrost thaw may stimulate soil respiration (Rs) and thus cause a positive feedback to climate warming. However, due to the limited in situ observations, it remains unclear about how Rs and its autotrophic (Ra) and heterotrophic (Rh) components change upon permafrost thaw. Here we monitored variations in Rs and its components along a permafrost thaw sequence on the Tibetan Plateau, and explored the potential linkage of Rs components (i.e., Ra and Rh) with biotic (e.g., plant functional traits and soil microbial diversity) and abiotic factors (e.g., substrate quality). We found that Ra and Rh exhibited divergent responses to permafrost collapse: Ra increased with the time of thawing, while Rh exhibited a hump-shaped pattern along the thaw sequence. We also observed different drivers of thaw-induced changes in the ratios of Ra:Rs and Rh:Rs. Except for soil water status, plant community structure, diversity, and root properties explained the variation in Ra:Rs ratio, soil substrate quality and microbial diversity were key factors associated with the dynamics of Rh:Rs ratio. Overall, these findings demonstrate divergent patterns and drivers of Rs components as permafrost thaw prolongs, which call for considerations in Earth system models for better forecasting permafrost carbon-climate feedback.

Terrestrial carbon sinks in China and around the world and their contribution to carbon neutrality.

Enhancing the terrestrial ecosystem carbon sink (referred to as terrestrial C sink) is an important way to slow down the continuous increase in atmospheric carbon dioxide (CO2) concentration and to achieve carbon neutrality target. To better understand the characteristics of terrestrial C sinks and their contribution to carbon neutrality, this review summarizes major progress in terrestrial C budget researches during the past decades, clarifies spatial patterns and drivers of terrestrial C sources and sinks in China and around the world, and examines the role of terrestrial C sinks in achieving carbon neutrality target. According to recent studies, the global terrestrial C sink has been increasing from a source of (-0.2±0.9) Pg C yr-1 (1 Pg=1015 g) in the 1960s to a sink of (1.9±1.1) Pg C yr-1 in the 2010s. By synthesizing the published data, we estimate terrestrial C sink of 0.20-0.25 Pg C yr-1 in China during the past decades, and predict it to be 0.15-0.52 Pg C yr-1 by 2060. The terrestrial C sinks are mainly located in the mid- and high latitudes of the Northern Hemisphere, while tropical regions act as a weak C sink or source. The C balance differs much among ecosystem types: forest is the major C sink; shrubland, wetland and farmland soil act as C sinks; and whether the grassland functions as C sink or source remains unclear. Desert might be a C sink, but the magnitude and the associated mechanisms are still controversial. Elevated atmospheric CO2 concentration, nitrogen deposition, climate change, and land cover change are the main drivers of terrestrial C sinks, while other factors such as fires and aerosols would also affect ecosystem C balance. The driving factors of terrestrial C sink differ among regions. Elevated CO2 concentration and climate change are major drivers of the C sinks in North America and Europe, while afforestation and ecological restoration are additionally important forcing factors of terrestrial C sinks in China. For future studies, we recommend the necessity for intensive and long term ecosystem C monitoring over broad geographic scale to improve terrestrial biosphere models for accurately evaluating terrestrial C budget and its dynamics under various climate change and policy scenarios.

Disruption of the pentraxin 3/CD44 interaction as an efficient therapy for triple-negative breast cancers.

Due to the heterogeneity and high frequency of genome mutations in cancer cells, targeting vital protumour factors found in stromal cells in the tumour microenvironment may represent an ideal strategy in cancer therapy. However, the regulation and mechanisms of potential targetable therapeutic candidates need to be investigated. An in vivo study demonstrated that loss of pentraxin 3 (PTX3) in stromal cells significantly decreased the metastasis and growth of cancer cells. Clinically, our results indicate that stromal PTX3 expression correlates with adverse prognostic features and is associated with worse survival outcomes in triple-negative breast cancer (TNBC). We also found that transforming growth factor beta 1 (TGF-ß1) induces PTX3 expression by activating the transcription factor CCAAT/enhancer binding protein delta (CEBPD) in stromal fibroblasts. Following PTX3 stimulation, CD44, a PTX3 receptor, activates the downstream ERK1/2, AKT and NF-κB pathways to specifically contribute to the metastasis/invasion and stemness of TNBC MDA-MB-231 cells. Two types of PTX3 inhibitors were developed to disrupt the PTX3/CD44 interaction and they showed a significant effect on attenuating growth and restricting the metastasis/invasion of MDA-MB-231 cells, suggesting that targeting the PTX3/CD44 interaction could be a new strategy for future TNBC therapies.

Stochastic processes regulate belowground community assembly in alpine grasslands on the Tibetan Plateau.

Understanding biogeographical patterns and underlying processes of belowground community assembly is crucial for predicting soil functions and their responses to global environmental change. However, little is known about potential differences of belowground community assembly among bacteria, fungi, protists and soil animals, particularly for alpine ecosystems. Based on the combination of large-scale field sampling, high-throughput marker-gene sequencing and multiple statistical analyses, we explored patterns and drivers of belowground community assembly in alpine grasslands on the Tibetan Plateau. Our results revealed that the distance-decay rates varied among trophic levels, with organisms of higher trophic level having weaker distance-decay pattern. The spatial and environmental variables explained limited variations of belowground communities. By contrast, the stochastic processes, mainly consisting of dispersal limitation and drift, played a primary role in regulating belowground community assembly. Moreover, the relative importance of stochastic processes varied among trophic levels, with the role of dispersal limitation weakening whereas that of drift enhancing in the order of bacteria, fungi, protists and soil animals. These findings advance our understanding of patterns and mechanisms driving belowground community assembly in alpine ecosystems and provide a reference basis for predicting the dynamics of ecosystem functions under changing environment.

Depth-dependent drivers of soil microbial necromass carbon across Tibetan alpine grasslands.

Microbial necromass carbon (C) has been considered an important contributor to persistent soil C pool. However, there still lacks large-scale systematic observations on microbial necromass C in different soil layers, particularly for alpine ecosystems. Besides, it is still unclear whether the relative importance of biotic and abiotic variables such as plant C input and mineral properties in regulating microbial necromass C would change with soil depth. Based on the combination of large-scale sampling along a ~2200 km transect across Tibetan alpine grasslands and biomarker analysis, together with a global data synthesis across grassland ecosystems, we observed a relatively low proportion of microbial-derived C in Tibetan alpine grasslands compared to global grasslands (topsoil: 45.4% vs. 58.1%; subsoil: 41.7% vs. 53.7%). We also found that major determinants of microbial necromass C depended on soil depth. In topsoil, both plant C input and mineral protection exerted dominant effects on microbial necromass C. However, in subsoil, the physico-chemical protection provided by soil clay particles, iron-aluminum oxides, and exchangeable calcium dominantly facilitated the preservation of microbial necromass C. The differential drivers over microbial necromass C between soil depths should be considered in Earth system models for accurately forecasting soil C dynamics and its potential feedback to global warming.

Response to Comment on 'Soil carbon persistence governed by plant input and mineral protection at regional and global scales'.

We demonstrated that ignoring the non-linear relationship between topsoil Δ14 C and plant carbon (C) input in Wu et al.'s analysis was the fundamental reason for the discrepancy between their analysis and ours. By considering such a non-linear relationship, plant C input still predominantly governs the topsoil C turnover.

Temperature sensitivity of permafrost carbon release mediated by mineral and microbial properties.

Temperature sensitivity (Q 10) of permafrost carbon (C) release upon thaw is a vital parameter for projecting permafrost C dynamics under climate warming. However, it remains unclear how mineral protection interacts with microbial properties and intrinsic recalcitrance to affect permafrost C fate. Here, we sampled permafrost soils across a 1000-km transect on the Tibetan Plateau and conducted two laboratory incubations over 400- and 28-day durations to explore patterns and drivers of permafrost C release and its temperature response after thaw. We find that mineral protection and microbial properties are two types of crucial predictors of permafrost C dynamics upon thaw. Both high C release and Q 10 are associated with weak organo-mineral associations but high microbial abundances and activities, whereas high microbial diversity corresponds to low Q 10 The attenuating effects of mineral protection and the dual roles of microbial properties would make the permafrost C-climate feedback more complex than previously thought.

Methanogenic Community, CH4 Production Potential and Its Determinants in the Active Layer and Permafrost Deposits on the Tibetan Plateau.

Permafrost thaw could increase methane (CH4) emissions, which largely depends on CH4 production driven by methanogenic archaea. However, large-scale evidence regarding key methanogenic taxa and their relative importance to abiotic factors in mediating methanogenesis remains limited. Here, we explored the methanogenic community, potential CH4 production and its determinants in the active layer and permafrost deposits based on soil samples acquired from 12 swamp meadow sites along a ∼1000 km permafrost transect on the Tibetan Plateau. Our results revealed lower CH4 production potential, mcrA gene abundance, and richness in the permafrost layer than those in the active layer. CH4 production potential in both soil layers was regulated by microbial and abiotic factors. Of the microbial properties, marker OTUs, rather than the abundance and diversity of methanogens, stimulated CH4 production potential. Marker OTUs differed between the two soil layers with hydrogenotrophic Methanocellales and facultative acetoclastic Methanosarcina predominant in regulating CH4 production potential in the permafrost and active layer, respectively. Besides microbial drivers, CH4 production potential increased with the carbon/nitrogen (C/N) ratio in both soil layers and was also stimulated by soil moisture in the permafrost layer. These results provide empirical evidence for model improvements to better predict permafrost carbon feedback to climate warming.

Soil carbon persistence governed by plant input and mineral protection at regional and global scales.

Elucidating the processes underlying the persistence of soil organic matter (SOM) is a prerequisite for projecting soil carbon feedback to climate change. However, the potential role of plant carbon input in regulating the multi-layer SOM preservation over broad geographic scales remains unclear. Based on large-scale soil radiocarbon (∆14 C) measurements on the Tibetan Plateau, we found that plant carbon input was the major contributor to topsoil carbon destabilisation despite the significant associations of topsoil ∆14 C with climatic and mineral variables as well as SOM chemical composition. By contrast, mineral protection by iron-aluminium oxides and cations became more important in preserving SOM in deep soils. These regional observations were confirmed by a global synthesis derived from the International Soil Radiocarbon Database (ISRaD). Our findings illustrate different effects of plant carbon input on SOM persistence across soil layers, providing new insights for models to better predict multi-layer soil carbon dynamics under changing environments.

Altered microbial structure and function after thermokarst formation.

Permafrost thaw could induce substantial carbon (C) emissions to the atmosphere, and thus trigger a positive feedback to climate warming. As the engine of biogeochemical cycling, soil microorganisms exert a critical role in mediating the direction and strength of permafrost C-climate feedback. However, our understanding about the impacts of thermokarst (abrupt permafrost thaw) on microbial structure and function remains limited. Here we employed metagenomic sequencing to analyze changes in topsoil (0-15 cm) microbial communities and functional genes along a permafrost thaw sequence (1, 10, and 16 years since permafrost collapse) on the Tibetan Plateau. By combining laboratory incubation and a two-pool model, we then explored changes in soil labile and stable C decomposition along the thaw sequence. Our results showed that topsoil microbial α-diversity decreased, while the community structure and functional gene abundance did not exhibit any significant change at the early stage of collapse (1 year since collapse) relative to non-collapsed control. However, as the time since the collapse increased, both the topsoil microbial community structure and functional genes differed from the control. Abundances of functional genes involved in labile C degradation decreased while those for stable C degradation increased at the late stage of collapse (16 years since collapse), largely driven by changes in substrate properties along the thaw sequence. Accordingly, faster stable C decomposition occurred at the late stage of collapse compared to the control, which was associated with the increase in relative abundance of functional genes for stable C degradation. These results suggest that upland thermokarst alters microbial structure and function, particularly enhances soil stable C decomposition by modulating microbial functional genes, which could reinforce a warmer climate over the decadal timescale.

Permafrost nitrogen status and its determinants on the Tibetan Plateau.

It had been suggested that permafrost thaw could promote frozen nitrogen (N) release and modify microbial N transformation rates, which might alter soil N availability and then regulate ecosystem functions. However, the current understanding of this issue is confined to limited observations in the Arctic permafrost region, without any systematic measurements in other permafrost regions. Based on a large-scale field investigation along a 1,000 km transect and a laboratory incubation experiment with a 15 N pool dilution approach, this study provides the comprehensive evaluation of the permafrost N status, including the available N content and related N transformation rates, across the Tibetan alpine permafrost region. In contrast to the prevailing view, our results showed that the Tibetan alpine permafrost had lower available N content and net N mineralization rate than the active layer. Moreover, the permafrost had lower gross rates of N mineralization, microbial immobilization and nitrification than the active layer. Our results also revealed that the dominant drivers of the gross N mineralization and microbial immobilization rates differed between the permafrost and the active layer, with these rates being determined by microbial properties in the permafrost while regulated by soil moisture in the active layer. In contrast, soil gross nitrification rate was consistently modulated by the soil NH 4 + content in both the permafrost and the active layer. Overall, patterns and drivers of permafrost N pools and transformation rates observed in this study offer new insights into the potential N release upon permafrost thaw and provide important clues for Earth system models to better predict permafrost biogeochemical cycles under a warming climate.