Predictive Value of Serum Albumin, Iron, Ferritin, and Hepcidin for Antiviral Efficacy of IFNα Treatment in Patients with Chronic Hepatitis B.

OBJECTIVE: Interferon-α (IFNα) therapy has been an integral part of the current treatment for hepatitis B virus (HBV) infection. However, the exact effect of IFNα antiviral therapy on liver function and iron metabolism in patients with chronic hepatitis B (CHB) remains unclear. Here, we investigated the characteristics of changes in liver function and iron metabolism indexes in patients with chronic hepatitis B before and after IFNα treatment. Additionally, we determined their predictive value for the therapeutic response of IFNα treatment. METHODS: In this study, 34 patients with CHB before and after IFNα treatment were enrolled. Serum levels of virological indicators, liver function, and iron metabolism markers were detected and analyzed in each patient. ROC curve analysis was performed to compare the predictive value of serum liver function and iron metabolism markers for the therapeutic response of IFN α treatment. RESULTS: A significant decrease in serum HBV DNA (P<0.001) and HBsAg (P<0.001) was observed before and after IFNα treatment. Compared to the patients before IFNα treatment, patients after IFNα treatment showed a significant increase in serum albumin (ALB) (P<0.05) and a significant decrease in serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) (P=0.003 and P=0.034). These findings suggested that the synthetic function of the liver was improved, and liver inflammation was alleviated. Serum HEPC and serum ferritin (SF) levels in patients after IFNα treatment were significantly higher (P<0.001, P<0.001); however, serum iron (SI) levels were significantly lower (P=0.005) than those in patients before IFNα treatment. These findings indicate that IFNα treatment regulated iron metabolism homeostasis in CHB patients. Combined liver function and iron metabolism markers, including ALB, SI, SF, and HEPC, had the highest predictive value for the therapeutic response of IFNα treatment for CHB. CONCLUSION: IFNα treatment improved liver function and iron metabolism homeostasis in patients with CHB. Regular monitoring of serum ALB, SI, SF, and HEPC can help predict the therapeutic response of IFNα treatment for CHB.

Activation of the cGAS-STING pathway by viral dsDNA leading to M1 polarization of macrophages mediates antiviral activity against hepatitis B virus.

BACKGROUND AND AIMS: Activation of the cGAS-STING pathway induces the production of type I interferons, initiating the antiviral immune response, which contributes to the clearance of pathogens. Previous studies have shown that STING agonists promote hepatitis B virus (HBV) clearance; however, few studies have investigated the effect of activating the cGAS-STING pathway in macrophages on HBV. METHODS: The polarization status of HBV particle-stimulated RAW264.7 macrophages was analyzed. After stimulation with HBV particles, the analysis focused on determining whether the DNA sensors in RAW264.7 macrophages recognized the viral double-stranded DNA (dsDNA) and evaluating the activation of the cGAS-STING pathway. Coculture of mouse macrophages and hepatocytes harboring HBV was used to study the antiviral activity of HBV-stimulated RAW264.7 macrophages. RESULTS: After stimulation with HBV particles, HBV relaxed circular DNA (rcDNA) was detected in RAW264.7 macrophages, and the protein expression of phospho-STING, phospho-TBK1, and phospho-IRF3 in the STING pathway was increased, as shown by Western blot analysis, which revealed that M1 polarization of macrophages was caused by increased expression of CD86. RT-PCR analyses revealed elevated expression of M1 macrophage polarization-associated cytokines such as TNFα, IL-1ß, iNOS, and IFNα/ß. In the coculture experiment, both HBsAg and HBeAg expression levels were significantly decreased in AML12-HBV1.3 cells cocultured with the supernatants of HBV-stimulated RAW264.7 macrophages. CONCLUSION: The results suggest that macrophages can endocytose HBV particles. Additionally, viral dsDNA can be recognized by DNA pattern recognition receptors, which in turn activate the cGAS-STING pathway, promoting the M1 polarization of macrophages, while no significant M2 polarization is observed. Macrophages stimulated with HBV particles exhibit enhanced antiviral activity against HBV.

Interferon-induced polarization of M1 macrophages mediates antiviral activity against the hepatitis B virus via the hepcidin-ferroportin axis.

BACKGROUNDS & AIMS: Given its ability to inhibit HBV replication, Interferon alpha (IFN-α) treatment has been confirmed to be effective in managing Chronic Hepatitis B (CHB). However, its underlying mechanisms are incompletely understood. METHODS: Herein, we investigated the antiviral properties of IFN-α by introducing IFN-α expression plasmids into a well-established HBV Hydrodynamic Injection (HDI) mouse model and examined the impact of IFN-α or hepcidin treatment on macrophages derived from THP-1 cells. The cytokine profiles were analyzed using the cytometry microsphere microarray technology, and flow cytometry was used to analyze the polarization of macrophages. Additionally, the IL-6/JAK2/STAT3 signaling pathway and the hepcidin-ferroportin axis were analyzed to better understand the macrophage polarization mechanism. RESULTS: As evidenced by the suppression of HBV replication, injection of an IFN-α expression plasmid and supernatants of IFN-α-treated macrophages exerted anti-HBV effects. The IFN-α treatment up-regulated IL-6 in mice with HBV replication, as well as in IFN-α-treated HepG2 cells and macrophages. Furthermore, JAK2/STAT3 signaling and hepcidin expression was promoted, inducing iron accumulation via the hepcidin-ferroportin axis, which caused the polarization of M1 macrophages. Furthermore, under the effect of IFN-α, IL-6 silencing or blockade downregulated the JAK2/STAT3 signaling pathway and hepcidin, implying that increased hepcidin expression under IFN-α treatment was dependent on the IL-6/JAK2/STAT3 pathway. CONCLUSION: The IL-6/JAK2/STAT3 signaling pathway is activated by IFN-α which induces hepcidin expression. The resulting iron accumulation then induces the polarization of M1 macrophages via the hepcidin-ferroportin axis, yielding an immune response which exerts antiviral effects against HBV replication.

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.

Characteristics of immunological events in Epstein-Barr virus infection in children with infectious mononucleosis.

Backgrounds & aims: Epstein-Barr virus (EBV) infection occurs commonly in children and may cause acute infectious mononucleosis (AIM) and various malignant diseases. Host immune responses are key players in the resistance to EBV infection. We here assessed the immunological events and laboratory indicators of EBV infection, as well as determined the clinical usefulness of evaluating the severity and efficacy of antiviral therapy in AIM patients. Methods: We enrolled 88 children with EBV infection. The immune environment was defined by immunological events such as frequencies of lymphocyte subsets, phenotypes of T cells, and their ability to secrete cytokines, and so on. This environment was analyzed in EBV-infected children with different viral loads and in children in different phases of infectious mononucleosis (IM) from disease onset to convalescence. Results: Children with AIM had higher frequencies of CD3+ T and CD8+ T cells, but lower frequencies of CD4+ T cells and CD19+ B cells. In these children, the expression of CD62L was lower and that of CTLA-4 and PD-1 was higher on T cells. EBV exposure induced granzyme B expression, but reduced IFN-γ secretion, by CD8+ T cells, whereas NK cells exhibited reduced granzyme B expression and increased IFN-γ secretion. The frequency of CD8+ T cells was positively correlated with the EBV DNA load, whereas the frequencies of CD4+ T cells and B cells were negatively correlated. During the convalescent phase of IM, CD8+ T cell frequency and CD62L expression on T cells were restored. Moreover, patient serum levels of IL-4, IL-6, IL-10, and IFN-γ were considerably lower throughout the convalescent phase than throughout the acute phase. Conclusion: Robust expansion of CD8+ T cells, accompanied by CD62L downregulation, PD-1 and CTLA-4 upregulation on T cells, enhanced granzyme B production, and impaired IFN-γ secretion, is a typical characteristic of immunological events in children with AIM. Noncytolytic and cytolytic effector functions of CD8+ T cells are regulated in an oscillatory manner. Furthermore, the AST level, number of CD8+ T cells, and CD62L expression on T cells may act as markers related to IM severity and the effectiveness of antiviral treatment.

Patterns and drivers of anaerobic nitrogen transformations in sediments of thermokarst lakes.

Significant attention has been given to the way in which the soil nitrogen (N) cycle responds to permafrost thaw in recent years, yet little is known about anaerobic N transformations in thermokarst lakes, which account for more than one-third of thermokarst landforms across permafrost regions. Based on the N isotope dilution and tracing technique, combined with qPCR and high-throughput sequencing, we presented large-scale measurements of anaerobic N transformations of sediments across 30 thermokarst lakes over the Tibetan alpine permafrost region. Our results showed that gross N mineralization, ammonium immobilization, and dissimilatory nitrate reduction rates in thermokarst lakes were higher in the eastern part of our study area than in the west. Denitrification dominated in the dissimilatory nitrate reduction processes, being two and one orders of magnitude higher than anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA), respectively. The abundances of the dissimilatory nitrate reduction genes (nirK, nirS, hzsB, and nrfA) exhibited patterns consistent with sediment N transformation rates, while α diversity did not. The inter-lake variability in gross N mineralization and ammonium immobilization was dominantly driven by microbial biomass, while the variability in anammox and DNRA was driven by substrate supply and organic carbon content, respectively. Denitrification was jointly affected by nirS abundance and organic carbon content. Overall, the patterns and drivers of anaerobic N transformation rates detected in this study provide a new perspective on potential N release, retention, and removal upon the formation and development of thermokarst lakes.

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.

Dissolved organic carbon concentration and biodegradability across the global rivers: A meta-analysis.

Riverine dissolved organic carbon (DOC) exerts a vital role in the global carbon cycle. Although substantial studies have explored DOC dynamics in aquatic ecosystems, it remains unknown about the patterns and drivers of riverine DOC concentration and biodegradability at the global scale. Here, by synthesizing 396 data points from 42 published literatures worldwide, we explored the spatial variations and controls of riverine DOC concentration and biodegradability. Our results revealed that the DOC concentration varied largely across the global rivers, with an average concentration of 10.4 mg L-1. The variations of DOC concentration were influenced by mean annual precipitation, vegetation type, soil type, permafrost degradation and river order. Furthermore, our results illustrated that the riverine DOC also had high biodegradability, showing an average of 16.4% loss within less than 90 days. The biodegradability of DOC was dominantly regulated by inorganic nutrients and DOC composition, but the controlling factors exhibited significant differences between small streams and large rivers. For small streams, DOC composition was the primary driver influencing DOC biodegradability, while for large rivers, nitrogen availability became the dominant factor affecting DOC biodegradability. This study highlights the high concentration and biodegradability of riverine DOC, which could exert an important role in the feedbacks between the global carbon cycle and climate warming.

Phosphorus rather than nitrogen regulates ecosystem carbon dynamics after permafrost thaw.

Ecosystem carbon (C) dynamics after permafrost thaw depends on more than just climate change since soil nutrient status may also impact ecosystem C balance. It has been advocated that nitrogen (N) release upon permafrost thaw could promote plant growth and thus offset soil C loss. However, compared with the widely accepted C-N interactions, little is known about the potential role of soil phosphorus (P) availability. We combined 3-year field observations along a thaw sequence (constituted by four thaw stages, i.e., non-collapse and 5, 14, and 22 years since collapse) with an in-situ fertilization experiment (included N and P additions at the level of 10 g N m-2  year-1 and 10 g P m-2  year-1 ) to evaluate ecosystem C-nutrient interactions upon permafrost thaw. We found that changes in soil P availability rather than N availability played an important role in regulating gross primary productivity and net ecosystem productivity along the thaw sequence. The fertilization experiment confirmed that P addition had stronger effects on plant growth than N addition in this permafrost ecosystem. These two lines of evidence highlight the crucial role of soil P availability in altering the trajectory of permafrost C cycle under climate warming.

Soil capacity of intercepting different rainfalls across subtropical plantation: Distinct effects of plant and soil properties.

Forest management practices play an important role in soil water conservation. However, the soil water-holding capacity and associated drivers under different management practices remain uncertain, especially when the precipitation varies substantially at the regional scale. Here, we used hydrogen stable isotope to explore the contribution of rainfall to soil water (CRSW) under light, moderate and heavy precipitation in Pinus massoniana plantations with multiple management practices (pure stand, mixed stand, understory removal, light-intensity thinning and high-intensity thinning) in subtropical China. We further used variation partitioning analysis and structural equation modeling to identify the dominant driver affecting CRSW. Our results showed that after light rainfall, the highest CRSW (28.7%) was found in the high-intensity thinning plantation. However, after heavy rainfall, the high-intensity thinning plantation received the lowest CRSW (43.3%), while the mixed stand showed the highest CRSW (67.1%). These results demonstrated that the mixed stand of P. massoniana had a stronger capacity for soil water conservation, whereas high-intensity thinning showed poorer capacity. Furthermore, our results revealed that plant properties (i.e., tree, root and litter biomass) were the dominant controls of the CRSW under light rainfall, while soil properties (i.e., bulk density, total porosity, field capacity) were the primary drivers under moderate and heavy rainfall, indicating that the determinants influencing plantation capacity for intercepting rainfall vary with different levels of precipitation. These results highlight the importance of the level of precipitation in determining the dominant driver of CRSW. More importantly, these results suggest that the mixed stand, rather than high-intensity thinning, was better forest management since the former has a higher capacity for intercepting heavy rainfall.

Stream Dissolved Organic Matter in Permafrost Regions Shows Surprising Compositional Similarities but Negative Priming and Nutrient Effects.

Permafrost degradation is delivering bioavailable dissolved organic matter (DOM) and inorganic nutrients to surface water networks. While these permafrost subsidies represent a small portion of total fluvial DOM and nutrient fluxes, they could influence food webs and net ecosystem carbon balance via priming or nutrient effects that destabilize background DOM. We investigated how addition of biolabile carbon (acetate) and inorganic nutrients (nitrogen and phosphorus) affected DOM decomposition with 28-day incubations. We incubated late-summer stream water from 23 locations nested in seven northern or high-altitude regions in Asia, Europe, and North America. DOM loss ranged from 3% to 52%, showing a variety of longitudinal patterns within stream networks. DOM optical properties varied widely, but DOM showed compositional similarity based on Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis. Addition of acetate and nutrients decreased bulk DOM mineralization (i.e., negative priming), with more negative effects on biodegradable DOM but neutral or positive effects on stable DOM. Unexpectedly, acetate and nutrients triggered breakdown of colored DOM (CDOM), with median decreases of 1.6% in the control and 22% in the amended treatment. Additionally, the uptake of added acetate was strongly limited by nutrient availability across sites. These findings suggest that biolabile DOM and nutrients released from degrading permafrost may decrease background DOM mineralization but alter stoichiometry and light conditions in receiving waterbodies. We conclude that priming and nutrient effects are coupled in northern aquatic ecosystems and that quantifying two-way interactions between DOM properties and environmental conditions could resolve conflicting observations about the drivers of DOM in permafrost zone waterways.

Concentration and biodegradability of dissolved organic carbon derived from soils: A global perspective.

Dissolved organic carbon (DOC), as active and mobile carbon, plays a critical role in terrestrial and aquatic ecosystems. However, it remains unclear how the concentration and biodegradability of soil-derived DOC (extracted from pore water or soil leachates) vary over a global scale and what determines the variations in DOC concentration and biodegradability. Here we addressed this issue by synthesizing the dataset involved in 121 sites from 39 literatures worldwide, and analyzed the patterns and drivers of DOC concentration and biodegradability. Our results showed that the DOC concentration in either pore water or soil leachates varied considerably, with mean values of 33.2 mg L-1 in pore water and 213.5 mg kg-1 in soil leachates, respectively. Mean annual precipitation (MAP) was the dominant control on the variability in soil-derived DOC concentration. Our results also revealed that the biodegradability of DOC in pore water was significantly lower than that in soil leachates, with the means of 16.5% versus 28.7%, respectively. Specific UV absorbance (SUVA254, a parameter used for evaluating dissolved aromatic carbon content) was the primary indicator predicting the spatial variation in DOC biodegradability, whereas MAP exerted limited effects on DOC biodegradability. These results demonstrate the high biodegradability of soil-derived DOC, highlighting its crucial role in the global carbon cycle under climate change.

Large-scale evidence for microbial response and associated carbon release after permafrost thaw.

Permafrost thaw could trigger the release of greenhouse gases through microbial decomposition of the large quantities of carbon (C) stored within frozen soils. However, accurate evaluation of soil C emissions from thawing permafrost is still a big challenge, partly due to our inadequate understanding about the response of microbial communities and their linkage with soil C release upon permafrost thaw. Based on a large-scale permafrost sampling across 24 sites on the Tibetan Plateau, we employed meta-genomic technologies (GeoChip and Illumina MiSeq sequencing) to explore the impacts of permafrost thaw (permafrost samples were incubated for 11 days at 5°C) on microbial taxonomic and functional communities, and then conducted a laboratory incubation to investigate the linkage of microbial taxonomic and functional diversity with soil C release after permafrost thaw. We found that bacterial and fungal α diversity decreased, but functional gene diversity and the normalized relative abundance of C degradation genes increased after permafrost thaw, reflecting the rapid microbial response to permafrost thaw. Moreover, both the microbial taxonomic and functional community structures differed between the thawed permafrost and formerly frozen soils. Furthermore, soil C release rate over five month incubation was associated with microbial functional diversity and C degradation gene abundances. By contrast, neither microbial taxonomic diversity nor community structure exhibited any significant effects on soil C release over the incubation period. These findings demonstrate that permafrost thaw could accelerate C emissions by altering the function potentials of microbial communities rather than taxonomic diversity, highlighting the crucial role of microbial functional genes in mediating the responses of permafrost C cycle to climate warming.

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.

Altered water uptake patterns of Populus deltoides in mixed riparian forest stands.

Plant water uptake plays an important role in regulating ecosystem water balance and its productivity. However, previous studies regarding plant water uptake were primarily conducted in wet areas under seasonal drought conditions, with a limited understanding of the proportion and drivers of plant water uptake under humid conditions. Actually, climate change and variations in global precipitation patterns could simultaneously trigger seasonal drought and flooding. Therefore, it is critical to explore patterns and mechanisms for plant water uptake under humid conditions in wet regions. Here, we employed dual stable isotopes of hydrogen and oxygen coupled with a Bayesian mixing model (MixSIAR) to explore the water uptake patterns of Populus deltoides in two types of riparian forests (pure vs. mixed stand of P. deltoides), under different magnitudes of rainfall (7.9, 15.4 and 34.1 mm), in the Middle-Lower Reaches of the Yangtze River in China. We further used both partial correlation and variation partitioning analyses to determine the relative importance of soil variables and plant properties affecting the proportion of P. deltoides water uptake from different soil layers. Our results revealed that compared to pure stands, P. deltoides in mixed stands had a lower water uptake proportion from deep soil layers (60-80, 80-100 cm) and had higher water uptake from shallow soil layers (0-20, 20-40 cm) under 15.4 mm and 34.1 mm rainfall events. Our results also revealed that plant properties such as leaf biomass, fine root biomass, and diameter at breast height were the primary factors influencing water uptake by P. deltoides. This suggests that P. deltoides in mixed stands could increase the proportion of water uptake from shallow soil layers through altering plant attributes. These findings indicate that mixed stands could restrain frequent extreme rainfall events and subsequent flooding, suggesting more resilience towards future climatic variability.

Temperature sensitivity of SOM decomposition governed by aggregate protection and microbial communities.

Temperature sensitivity (Q 10) of soil organic matter (SOM) decomposition is a crucial parameter for predicting the fate of soil carbon (C) under global warming. However, our understanding of its regulatory mechanisms remains inadequate, which constrains its accurate parameterization in Earth system models and induces large uncertainties in predicting terrestrial C-climate feedback. Here, we conducted a long-term laboratory incubation combined with a two-pool model and manipulative experiments to examine potential mechanisms underlying the depth-associated Q 10 variations in active and slow soil C pools. We found that lower microbial abundance and stronger aggregate protection were coexisting mechanisms underlying the lower Q 10 in the subsoil. Of them, microbial communities were the main determinant of Q 10 in the active pool, whereas aggregate protection exerted more important control in the slow pool. These results highlight the crucial role of soil C stabilization mechanisms in regulating temperature response of SOM decomposition, potentially attenuating the terrestrial C-climate feedback.

Higher soil capacity of intercepting heavy rainfall in mixed stands than in pure stands in riparian forests.

Changes in global precipitation patterns would make wet regions more humid and extreme precipitation events occur frequently, followed by widespread flooding. Riparian forests are more capable of withstanding floods than inland forests because they are frequently exposed to short-term flooding events. Although many previous studies have investigated the soil water dynamics of terrestrial forests, little is known about how the soil water of riparian forests responds to different amounts of rainfall and which factors mainly regulate the soil water-holding capacity. Here, we employed stable hydrogen isotope to explore the contribution of different magnitudes of rainfall (7.9, 18.6 and 34.1 mm) to the soil water in two types of riparian forests (pure vs. mixed stand of Populus deltoides) in the middle-lower reaches of the Yangtze River, China. We further used structure equation modelling to determine the relative importance of soil properties and vegetation biomass in affecting the contribution of different magnitudes of rainfall to soil water. Our results revealed that there was no significant difference between these two stand types in the contributions of light and moderate rainfall to soil water, while the contribution of heavy rainfall to soil water (CHRSW) in mixed stand was significantly higher than that in pure stand (74.3% vs. 62.9%), suggesting that mixed stand soil has higher water-holding capacity than pure stand soil. Furthermore, soil properties were the best predictor affecting CHRSW, which explained 68% and 59% of the variation in the CHRSW on the 1st and 8th days after rainfall, respectively. Moreover, the root biomass could indirectly affect the CHRSW. Overall, mixed stand soil had a greater capacity in intercepting and storing rainwater than pure stand soil, implying that the mixed stand plantation, rather than the pure stand, should be recommended in riparian forest restoration projects that aim to improve their capacity for alleviating floods.

Nitrogen availability regulates topsoil carbon dynamics after permafrost thaw by altering microbial metabolic efficiency.

Input of labile carbon may accelerate the decomposition of existing soil organic matter (priming effect), with the priming intensity depending on changes in soil nitrogen availability after permafrost thaw. However, experimental evidence for the linkage between the priming effect and post-thaw nitrogen availability is unavailable. Here we test the hypothesis that elevated nitrogen availability after permafrost collapse inhibits the priming effect by increasing microbial metabolic efficiency based on a combination of thermokarst-induced natural nitrogen gradient and nitrogen addition experiment. We find a negative correlation between the priming intensity and soil total dissolved nitrogen concentration along the thaw sequence. The negative effect is confirmed by the reduced priming effect after nitrogen addition. In contrast to the prevailing view, this nitrogen-regulated priming intensity is independent of extracellular enzyme activities but associated with microbial metabolic efficiency. These findings demonstrate that post-thaw nitrogen availability regulates topsoil carbon dynamics through its modification of microbial metabolic efficiency.

Ultraviolet radiation rather than inorganic nitrogen increases dissolved organic carbon biodegradability in a typical thermo-erosion gully on the Tibetan Plateau.

Permafrost thaw could lead to frozen carbon (C) being laterally transferred to aquatic systems as dissolved organic carbon (DOC). If this part of DOC has high biodegradability, it could be decomposed during the delivery process, release greenhouse gases to the atmosphere and trigger positive C-climate feedback. Thermokarst is an abrupt permafrost thaw process that can enhance DOC export and also impact DOC processing through increased inorganic nitrogen (N) and ultraviolet (UV) light exposure. Especially on the Tibetan Plateau, where thermokarst develops widely and suffers from serious UV radiation and N limitation. However, it remains unclear how thermokarst-impacted biodegradable DOC (BDOC) responds to inorganic N addition and UV radiation. Here, we explored the responses of DOC concentration, composition and its biodegradability to inorganic N and UV amendments in a typical thermokarst on the Tibetan Plateau, by using laboratory incubations with spectral analyses (UV-visible absorption and three-dimensional fluorescence spectra) and parallel factor analyses. Our results showed that BDOC in thermokarst outflows was significantly higher than in reference water. Our results also revealed that inorganic N addition had no influence on thermokarst-impacted BDOC, whereas exposure to UV light significantly increased BDOC by as much as 2.3 times higher than the dark-control. Moreover, N addition and UV radiation did not generate additive effects on BDOC. Our results further illustrated that dissolved organic matter (DOM) composition explained more of the variability in BDOC, while the nutrients and other physicochemical properties played a minor role. Overall, these results imply that UV light rather than inorganic N significantly increases thermokarst-derived BDOC, potentially strengthening the positive permafrost C-climate feedback.

Warming effects on permafrost ecosystem carbon fluxes associated with plant nutrients.

Large uncertainties exist in carbon (C)-climate feedback in permafrost regions, partly due to an insufficient understanding of warming effects on nutrient availabilities and their subsequent impacts on vegetation C sequestration. Although a warming climate may promote a substantial release of soil C to the atmosphere, a warming-induced increase in soil nutrient availability may enhance plant productivity, thus offsetting C loss from microbial respiration. Here, we present evidence that the positive temperature effect on carbon dioxide (CO2 ) fluxes may be weakened by reduced plant nitrogen (N) and phosphorous (P) concentrations in a Tibetan permafrost ecosystem. Although experimental warming initially enhanced ecosystem CO2 uptake, the increased rate disappeared after the period of peak plant growth during the early growing season, even though soil moisture was not a limiting factor in this swamp meadow ecosystem. We observed that warming did not significantly affect soil extractable N or P during the period of peak growth, but decreased both N and P concentrations in the leaves of dominant plant species, likely caused by accelerated plant senescence in the warmed plots. The attenuated warming effect on CO2 assimilation during the late growing season was associated with lowered leaf N and P concentrations. These findings suggest that warming-mediated nutrient changes may not always benefit ecosystem C uptake in permafrost regions, making our ability to predict the C balance in these warming-sensitive ecosystems more challenging than previously thought.