Lithology constrains the origination of autochthonous carbon in river ecosystems.

Dissolved inorganic carbon (DIC) provides a substrate for primary production in the lotic ecosystems, yet carbon's biogeochemical origination in the lotic food webs is still poorly constrained. Here, we assembled a global dataset of isotopic composition (i.e., 13C/12C or δ13C) of DIC and periphyton (algae being the primary producers) in river waters, and carried out a field study in two catchments respectively with carbonate and silicate dominated lithologies on the Tibetan Plateau. A two-endmember mixing model based on the datasets indicated that δ13C and concentrations of DIC in the river waters were largely determined by the catchment-scale chemical weathering of different lithologies. Meanwhile, a significant correlation was obtained between δ13C-DIC and δ13C-periphyton in the datasets, strongly implying that the origination of periphyton carbon was largely regulated by the catchment lithologies. The δ13C-periphyton compositions are also affected by isotopic fractionations during algal primary production, which, in turn, were closely related to the relationships between primary productivity and DIC availability in the rivers. The study advances our understanding of the origination and transfer of carbon biogeochemically bridging the geosphere and biosphere in the lotic ecosystems.

Remittance from migrants reinforces forest recovery for China's reforestation policy.

Forests play a key role in the mitigation of global warming and provide many other vital ecosystem goods and services. However, as forest continues to vanish at an alarming rate from the surface of the planet, the world desperately needs knowledge on what contributes to forest preservation and restoration. Migration, a hallmark of globalization, is widely recognized as a main driver of forest recovery and poverty alleviation. Here, we show that remittance from migrants reinforces forest recovery that would otherwise be unlikely with mere migration, realizing the additionality of payments for ecosystem services for China's largest reforestation policy, the Conversion of Cropland to Forest Program (CCFP). Guided by the framework that integrates telecoupling and coupled natural and human systems, we investigate forest-livelihood dynamics under the CCFP through the lens of rural out-migration and remittance using both satellite remote sensing imagery and household survey data in two representative sites of rural China. Results show that payments from the CCFP significantly increases the probability of sending remittance by out-migrants to their origin households. We observe substantial forest regeneration and greening surrounding households receiving remittance but forest decline and browning in proximity to households with migrants but not receiving remittance, as measured by forest coverage and the Enhanced Vegetation Index derived from space-borne remotely sensed data. The primary mechanism is that remittance reduces the reliance of households on natural capital from forests, particularly fuelwood, allowing forests near the households to recover. The shares of the estimated ecological and economic additionality induced by remittance are 2.0% (1.4%∼3.8%) and 9.7% (5.0%∼15.2%), respectively, to the baseline of the reforested areas enrolled in CCFP and the payments received by the participating households. Remittance-facilitated forest regeneration amounts to 12.7% (6.0%∼18.0%) of the total new forest gained during the 2003-2013 in China. Our results demonstrate that remittance constitutes a telecoupling mechanism between rural areas and cities over long distances, influencing the local social-ecological gains that the forest policy intended to stimulate. Thus, supporting remittance-sending migrants in cities can be an effective global warming mitigation strategy.

Meta-analysis shows the impacts of ecological restoration on greenhouse gas emissions.

International initiatives set ambitious targets for ecological restoration, which is considered a promising greenhouse gas mitigation strategy. Here, we conduct a meta-analysis to quantify the impacts of ecological restoration on greenhouse gas emissions using a dataset compiled from 253 articles. Our findings reveal that forest and grassland restoration increase CH4 uptake by 90.0% and 30.8%, respectively, mainly due to changes in soil properties. Conversely, wetland restoration increases CH4 emissions by 544.4%, primarily attributable to elevated water table depth. Forest and grassland restoration have no significant effect on N2O emissions, while wetland restoration reduces N2O emissions by 68.6%. Wetland restoration enhances net CO2 uptake, and the transition from net CO2 sources to net sinks takes approximately 4 years following restoration. The net ecosystem CO2 exchange of the restored forests decreases with restoration age, and the transition from net CO2 sources to net sinks takes about 3-5 years for afforestation and reforestation sites, and 6-13 years for clear-cutting and post-fire sites. Overall, forest, grassland and wetland restoration decrease the global warming potentials by 327.7%, 157.7% and 62.0% compared with their paired control ecosystems, respectively. Our findings suggest that afforestation, reforestation, rewetting drained wetlands, and restoring degraded grasslands through grazing exclusion, reducing grazing intensity, or converting croplands to grasslands can effectively mitigate greenhouse gas emissions.

Linking ecosystem multifunctionality to microbial community features in rivers along a latitudinal gradient.

Microorganisms regulate numerous ecosystem functions and show considerable differences along a latitudinal gradient. Although studies have revealed the latitudinal patterns of microbial community structure and single ecosystem function, the latitudinal patterns of ecosystem multifunctionality (EMF) and how microbial communities affect EMF along a latitudinal gradient remain unclear. Here, we collected channel sediments, riparian rhizosphere soils, and riparian bulk soils from 30 rivers across China and calculated EMF using 18 variables related to nitrogen cycling, nutrient pool, plant productivity, and water quality. We also determined microbial diversity (taxonomic and functional) and microbial network complexity using metagenomic sequencing. The results showed that EMF significantly decreased with increasing latitude in riparian rhizosphere and bulk soils but not in channel sediments. Microbial taxonomic and functional richness (observed species) in channel sediments were significantly higher in the low-latitude group than in the high-latitude group. However, microbial co-occurrence networks were more complex in the high-latitude group compared with the low-latitude group. Abiotic factors, primarily geographic and climatic factors, contributed more to EMF than microbial diversity and network complexity parameters in which only betweenness centralization had a significant relationship with EMF. Together, this study provides insight into the latitudinal pattern of EMF in rivers and highlights the importance of large-scale factors in explaining such latitudinal patterns.IMPORTANCEEcosystem multifunctionality (EMF) is the capacity of an ecosystem to provide multiple functions simultaneously. Microorganisms, as dominant drivers of belowground processes, have a profound effect on ecosystem functions. Although studies have revealed the latitudinal patterns of microbial community structure and single ecosystem function, the latitudinal patterns of EMF and how microbial communities affect EMF along a latitudinal gradient remain unclear. We collected channel sediments, riparian rhizosphere soils, and riparian bulk soils from 30 rivers along a latitudinal gradient across China and calculated EMF using 18 variables related to nitrogen cycling, nutrient pool, plant productivity, and water quality. This study fills a critical knowledge gap regarding the latitudinal patterns and drivers of EMF in river ecosystems and gives new insights into how microbial diversity and network complexity affect EMF from a metagenomic perspective.

Multiomics analyses reveal the mechanisms of the responses of subalpine treeline trees to phenology and winter low-temperature stress.

Withstanding extreme cold stress is a prerequisite for alpine treeline trees to persist and survive. However, the underlying mechanism by which treeline trees sense phenological changes and survive hard winters has not been fully elucidated. Here, we investigated the physiology, transcriptome, and metabolome of the subalpine treeline species Larix chinensis to identify the molecular mechanism of phenological and cold resistance. Calcium and antioxidant enzyme activities (e.g., superoxide dismutase and glutathione peroxidase) are essential for coping with winter cold stress in L. chinensis. Transcriptome analysis revealed that circadian rhythm and phytohormone signalling transduction played important roles in regulating L. chinensis phenological changes and cold stress responses. The variations in the transcriptome identified were accompanied by the specific accumulation of flavones, flavonols, and monosaccharides. The flavonoid biosynthesis and phenylpropanoid biosynthesis pathways played important roles in the adaptation of L. chinensis to the extreme winter environment, and flavone and flavonol biosynthesis was an important pathway involved in bud burst. In addition, temperature and photoperiod had synergistic influences on the formation and release of bud dormancy. Thus, our findings provided new insights into the mechanism of subalpine treeline formation.

Space-time cube uncovers spatiotemporal patterns of basin ecological quality and their relationship with water eutrophication.

Maintaining an optimal eco-environment is important for sustainable regional development. However, existing methods are inadequate for examining both spatial and temporal dimensions. Here, we propose a systematic procedure for spatiotemporal examination of the eco-environment using the space-time cube (STC) model and describe a preliminary investigation of the coupling relationships between basin ecological quality and water eutrophication in upstream of the Han River basin between 2000 and 2020. The STC model considers the temporal dimension as the third dimension in calculations. We first categorized the basin into three sub-watershed types: forest, cultivated land, and artificial surface. Subsequently, the ecological quality and driving factors were assessed and identified using the remote sensing ecological index (RSEI) and Geodetector method, respectively. The findings indicated that the forest basin and artificial surface basin had the highest and lowest ecological quality, respectively. The spatiotemporal cold spots of ecological quality during the past 20 years were mostly located in the vicinity of reservoirs, rivers, and artificial surface areas. Human activity, precipitation, and the percentage of cultivated land were other important driving factors in the artificial surface, forest, and cultivated land sub-watersheds, respectively, in addition to the dominant factors of elevation and temperature. The results also indicated that when the ecological quality degraded to a certain extent, water eutrophication was significantly coupled with the ecological quality of the catchments. The findings of this study are useful for ecological restoration and sustainable river basin development.

Eutrophication and loss of riparian shading influence food quality and trophic relation in stream food webs.

Eutrophication induced by excessive inputs of nutrient is one of the main stressors in aquatic ecosystems. Deforestation in riparian zones alter riparian shading, which together with eutrophication is expected to exert a complex control over stream food webs. We manipulated two levels of riparian shading (open canopy vs. shading canopy) and nutrient supply (ambient vs. nutrient addition) in three headwater streams to investigate the individual and combined effects of eutrophication and loss of riparian shading on carbon sources and nutritional quality of biofilms, and the subsequent trophic effects on macroinvertebrate grazers. Nutrient enrichment increased the autochthonous carbon (i.e., algae especially diatoms) indicated by fatty acid (FA) biomarkers within biofilms and grazers. The nutritional quality indicated by eicosapentaenoic acid (EPA) content of biofilms was increased with nutrient enrichment and more so with the combined effect of an increase in riparian shading, consequently leading to an increase in the nutritional quality, density, and biomass of grazers. In particular, the trophic linkages between biofilms and grazers were mainly influenced by EPA concentration in the biofilms, and strengthened with the combined effects of riparian shading and additional nutrients. Our study emphasizes the nutritional significance of EPA for consumers at higher trophic levels and proposes its potential as an indicator for monitoring the health of aquatic ecosystems.

Climate warming could free cold-adapted trees from C-conservative allocation strategy of storage over growth.

Carbon allocation has been fundamental for long-lived trees to survive cold stress at their upper elevation range limit. Although carbon allocation between non-structural carbohydrate (NSC) storage and structural growth is well-documented, it still remains unclear how ongoing climate warming influences these processes, particularly whether these two processes will shift in parallel or respond divergently to warming. Using a combination of an in situ downward-transplant warming experiment and an ex situ chamber warming treatment, we investigated how subalpine fir trees at their upper elevation limit coordinated carbon allocation priority among different sinks (e.g., NSC storage and structural growth) at whole-tree level in response to elevated temperature. We found that transplanted individuals from the upper elevation limit to lower elevations generally induced an increase in specific leaf area, but there was no detected evidence of warming effect on leaf-level saturated photosynthetic rates. Additionally, our results challenged the expectation that climate warming will accelerate structural carbon accumulation while maintaining NSC constant. Instead, individuals favored allocating available carbon to NSC storage over structural growth after 1 year of warming, despite the amplification in total biomass encouraged by both in situ and ex situ experimental warming. Unexpectedly, continued warming drove a regime shift in carbon allocation priority, which was manifested in the increase of NSC storage in synchrony to structural growth enhancement. These findings imply that climate warming would release trees at their cold edge from C-conservative allocation strategy of storage over structural growth. Thus, understanding the strategical regulation of the carbon allocation priority and the distinctive function of carbon sink components is of great implication for predicting tree fate in the future climate warming.

Divergent effects of warming on nonstructural carbohydrates in woody plants: a meta-analysis.

Nonstructural carbohydrates (NSC, including soluble sugars and starch) are essential for supporting growth and survival of woody plants, and play multifunctional roles in various ecophysiological processes that are being rapidly changed by climate warming. However, it still remains unclear whether there is a consistent response pattern of NSC dynamics in woody plants to climate warming across organ types and species taxa. Here, based on a compiled database of 52 woody plant species worldwide, we conducted a meta-analysis to investigate the effects of experimental warming on NSC dynamics. Our results indicated that the responses of NSC dynamics to warming were primarily driven by the fluctuations of starch, while soluble sugars did not undergo significant changes. The effects of warming on NSC shifted from negative to positive with the extension of warming duration, while the negative warming effects on NSC became more pronounced as warming magnitude increased. Overall, our study showed the divergent responses of NSC and its components in different organs of woody plants to experimental warming, suggesting a potentially changed carbon (C) balance in woody plants in future global warming. Thus, our findings highlight that predicting future changes in plant functions and terrestrial C cycle requires a mechanism understanding of how NSC is linked to a specific global change driver.

Effects of environmental factors on the river water quality on the Tibetan Plateau: a case study of the Xoirong River, China.

Climate, topography, and landscape patterns affect river water quality through processes that influence non-point source pollution. However, little is known about the response of the water quality of rivers on China's Tibetan Plateau to these environmental factors. Based on the water quality parameters data of the Xoirong River on the Tibetan Plateau in western China, the redundancy analysis and variation partitioning analysis were adopted to determine the main influencing factors affecting river water quality and their spatial scale effects. The major water pollutants were further analyzed using the partial least square-structural equation modeling (PLS-SEM). Another mountainous river with a similar latitude, the same stream order, and low anthropogenic disturbance in central China, the Jinshui River, was also selected for comparative discussion. The results indicated that the overall river water quality on the Tibetan Plateau was superior to that of the Jinshui River. At the catchment scale, the cumulative explanatory powers of the influencing factors of both rivers were greatest. Landscape composition and configuration were the determinant factors for the overall water quality of the two rivers, while the river on the Tibetan Plateau was also significantly affected by climatic and topographical factors. Regarding the main water quality issue, i.e., total nitrogen, agricultural production activities might be the main cause of the river on the Tibetan Plateau. This study unveiled that the river water quality on the Tibetan Plateau is sensitive to climate and topography through comparative studies.

Unexpectedly high nitrate levels in a pristine forest river on the Southeastern Qinghai-Tibet Plateau.

River nitrate (NO3-) pollution is a global environmental issue. Recently, high NO3- levels in some pristine or minimally-disturbed rivers were reported, but their drivers remain unclear. This study integrated river isotopes (δ18O/δ15N-NO3- and δD/18O-H2O), 15N pairing experiments, and qPCR to reveal the processes driving the high NO3- levels in a nearly pristine forest river on the Qinghai-Tibet Plateau. The river isotopes suggested that, at the catchment scale, NO3- removal was prevalent in summer, but weak in winter. The pristine forest soils contributed more than 90 % of the riverine NO3-, indicating the high NO3- backgrounds. The release of soil NO3- to the river was "transport-limited" in both seasons, i.e., the NO3- production/stock in the soils exceeded the capacity of hydrological NO3- leaching. In summer, this regime and the NO3--plentiful conditions in the soils associated with the strong NO3- nitrification led to the high riverine NO3- levels. While the in-soil nitrification was weak in winter, the leaching of legacy NO3- resulted in the consistently high NO3- levels. This study provides insights into the reasons for high NO3- levels in pristine or minimally-disturbed rivers worldwide and highlights the necessity to consider NO3- backgrounds when evaluating anthropogenic NO3- pollution in rivers.

From Soil to River: Revealing the Mechanisms Underlying the High Riverine Nitrate Levels in a Forest Dominated Catchment.

Elevated riverine nitrate (NO3-) levels have led to increased eutrophication and other ecological implications. While high riverine NO3- levels were generally ascribed to anthropogenic activities, high NO3- levels in some pristine or minimally disturbed rivers were reported. The drivers of these unexpectedly high NO3- levels remain unclear. This study combined natural abundance isotopes, 15N-labeling techniques, and molecular techniques to reveal the processes driving the high NO3- levels in a sparsely populated forest river. The natural abundance isotopes revealed that the NO3- was mainly from soil sources and that NO3- removal processes were insignificant. The 15N-labeling experiments also quantitatively showed that the biological NO3- removal processes, i.e., denitrification, dissimilatory NO3- reduction to ammonium (DNRA), and anaerobic ammonia oxidation (anammox), in the soils and sediments were weak relative to nitrification in summer. While nitrification was minor in winter, the NO3- removal was insignificant relative to the large NO3- stock in the catchment. Stepwise multiple regression analyses and structural equation models revealed that in summer, nitrification in the soils was regulated by the amoA-AOB gene abundances and NH4+-N contents. Low temperature constrained nitrification in winter. Denitrification was largely controlled by moisture content in both seasons, and anammox and DNRA could be explained by the competition with nitrification and denitrification on their substrate (nitrite-NO2-). We also revealed the strong hydrological control on the transport of soil NO3- to the river. This study effectively revealed the mechanisms underlying the high NO3- levels in a nearly pristine river, which has implications for the understanding of riverine NO3- levels worldwide.

[Hydrochemical Evolution in the Yarlung Zangbo River Basin].

In order to explore the hydro-chemical evolution law of the Yarlung Zangbo River Basin from 1973 to 2020, the hydro-chemical characteristics and major ion sources were studied using a Piper diagram, Gibbs diagram, ion ratio, and correlation analysis, and the irrigation applicability of the Yarlung Zangbo River was evaluated using the sodium adsorption ratio (SAR), sodium percentage (Na+%), and permeability index (PI). The results showed that the mean value of TDS was (208.30±58.26) mg·L-1, which increased with time. Ca2+ was the dominant cation, accounting for (65.49±7.67)% of the total cations. HCO-3 and SO2-4 were the dominant anions, accounting for (68.56±9.84)% and (26.85±9.82)% of the main anions, respectively. The annual growth rates of Ca2+, HCO-3, and SO2-4 were respectively 2.07, 3.19, and 4.70 mg·(L·10 a)-1. The hydro-chemical type of the Yarlung Zangbo River was HCO3-Ca type, and the main ionic chemistry was controlled by the chemical weathering of carbonate rocks. The weathering of carbonate rocks during the period of 1973 to 1990 was mainly controlled by carbonation, whereas from 2001 to 2020 it was mainly controlled by both carbonation and sulfuric acid. The main ion concentrations in the mainstream of Yarlung Zangbo River were within the range of drinking water standards, with SAR between 0.11-0.93, Na+% between 8.00-36.73, and PI values between 0.39-0.87, demonstrating that the waters were suitable for drinking and irrigation. The results were of great significance to the protection and sustainable development of water resources in the Yarlung Zangbo River Basin.

Open riparian canopy and nutrient pollution interactively decrease trophic redundancy and allochthonous resource in streams.

Riparian deforestation, which leads to increase in light intensity and excessive nutrient loading in waterways, are two pervasive environmental stressors in the stream ecosystems. Both have been found to alter basal resource availability and consequently stream food webs. However, their interactive effects on trophic structure in stream food webs are unclear. Here, we manipulated light intensity and nutrient availability in three headwater streams to evaluate their effects on consumer diet composition and food web characteristics (i.e., trophic diversity and redundancy) with stable isotope analysis. Dietary analysis revealed that the relative contribution of stream periphyton to the diets of macroinvertebrates increased, while that of allochthonous resources, specifically leaf litter from the terrestrial ecosystems in the catchment, decreased in response to open canopy and nutrient enrichment in the streams. The trophic diversity also increased with the elevated light intensity and nutrient availability, while the trophic redundancy decreased, suggesting a reduced ability of the stream ecosystems to resist environmental changes. Nutrient enrichment also increased the δ15N ratios of periphyton and macroinvertebrates, indicating potential δ15N enrichment of stream benthos by nitrogen pollution. Our results suggested that an increase in light intensity due to riparian canopy openness and stream water nutrient enrichment primarily from human activities have interactive effects on resource flow and trophic structure in stream food webs.

Coupling geochemical and microbial molecular techniques to reveal catchment-scale nitrate yield and fluvial export dynamics.

The disturbance of reactive nitrogen (N) on ecosystems and biogeochemical cycles is now one of the most severe environmental problems worldwide. Nitrate (NO3-) is usually a dominant reactive N species in river ecosystems. Excessive NO3- concentrations in rivers have led to eutrophication and consequent ecological and environmental damages. Quantifying catchment-scale NO3- yield and export dynamics is crucial for effective remediation of river NO3- pollution. Frequently, natural abundance isotopes of NO3- in a river (δ15N/δ18O-NO3-) are applied to identify sources and potential transformations of NO3- at a catchment scale, while microbial molecular techniques and 15N pairing experiments are employed to reveal the NO3- production and removal processes and their underlying mechanisms in microenvironments (e.g., sediments and soils). In this study, we developed a novel protocol that couples these complementary geochemical and molecular techniques to quantify catchment-scale NO3- yield and fluvial export dynamics. The protocol links microscopic processes with catchment-scale geochemical characteristics to explicitly describe the NO3- cycling processes and their underlying abiotic and biotic mechanisms within a catchment. We applied the protocol to the Dadu and Jiazela catchments on the Qinghai-Tibet Plateau, and demonstrated the effectiveness of the protocol in determining NO3- yield and export dynamics in the catchments.

Global analysis of the perturbation effects of metal-based nanoparticles on soil nitrogen cycling.

Although studies have investigated the effects of metal-based nanoparticles (MNPs) on soil biogeochemical processes, the results obtained thus far are highly variable. Moreover, we do not yet understand how the impact of MNPs is affected by experimental design and environmental conditions. Herein, we conducted a global analysis to synthesize the effects of MNPs on 17 variables associated with soil nitrogen (N) cycling from 62 studies. Our results showed that MNPs generally exerted inhibitory effects on N-cycling process rates, N-related enzyme activities, and microbial variables. The response of soil N cycling varied with MNP type, and exposure dose was the most decisive factor for the variations in the responses of N-cycling process rates and enzyme activities. Notably, Ag/Ag2 S and CuO had dose-dependent inhibitory effects on ammonia oxidation rates, while CuO and Zn/ZnO showed hormetic effects on nitrification and denitrification rates, respectively. Other experimental design factors (e.g., MNP size and exposure duration) also regulated the effect of MNPs on soil N cycling, and specific MNPs, such as Ag/Ag2 S, exerted stronger effects during long-term (>28 days) exposure. Environmental conditions, including soil pH, organic carbon, texture, and presence/absence of plants, significantly influenced MNP toxicity. For instance, the effects of Ag/Ag2 S on the ammonia oxidation rate and the activity of leucine aminopeptidase were more potent in acid (pH <6), organic matter-limited (organic carbon content ≤10 g kg-1 ), and coarser soils. Overall, these results provide new insights into the general mechanisms by which MNPs alter soil N processes in different environments and underscore the urgent need to perform multivariate and long-term in situ trials in simulated natural environments.

Nanoplastics alter ecosystem multifunctionality and may increase global warming potential.

Although the presence of nanoplastics in aquatic and terrestrial ecosystems has received increasing attention, little is known about its potential effect on ecosystem processes and functions. Here, we evaluated if differentially charged polystyrene (PS) nanoplastics (PS-NH2 and PS-SO3 H) exhibit distinct influences on microbial community structure, nitrogen removal processes (denitrification and anammox), emissions of greenhouse gases (CO2 , CH4 , and N2 O), and ecosystem multifunctionality in soils with and without earthworms through a 42-day microcosm experiment. Our results indicated that nanoplastics significantly altered soil microbial community structure and potential functions, with more pronounced effects for positively charged PS-NH2 than for negatively charged PS-SO3 H. Ecologically relevant concentration (3 g kg-1 ) of nanoplastics inhibited both soil denitrification and anammox rates, while environmentally realistic concentration (0.3 g kg-1 ) of nanoplastics decreased the denitrification rate and enhanced the anammox rate. The soil N2 O flux was always inhibited 6%-51% by both types of nanoplastics, whereas emissions of CO2 and CH4 were enhanced by nanoplastics in most cases. Significantly, although N2 O emissions were decreased by nanoplastics, the global warming potential of total greenhouse gases was increased 21%-75% by nanoplastics in soils without earthworms. Moreover, ecosystem multifunctionality was increased 4%-12% by 0.3 g kg-1 of nanoplastics but decreased 4%-11% by 3 g kg-1 of nanoplastics. Our findings provide the only evidence to date that the rapid increase in nanoplastics is altering not only ecosystem structure and processes but also ecosystem multifunctionality, and it may increase the emission of CO2 and CH4 and their global warming potential to some extent.

Asymmetric effects of daytime and nighttime warming on alpine treeline recruitment.

Trees at their upper range limits are highly sensitive to climate change, and thus alpine treelines worldwide have changed their recruitment patterns in response to climate warming. However, previous studies focused only on daily mean temperature, neglecting the asymmetric influences of daytime and nighttime warming on recruitments in alpine treelines. Here, based on the compiled dataset of tree recruitment series from 172 alpine treelines across the Northern Hemisphere, we quantified and compared the different effects of daytime and nighttime warming on treeline recruitment using four indices of temperature sensitivity, and assessed the responses of treeline recruitment to warming-induced drought stress. Our analyses demonstrated that even in different environmental regions, both daytime and nighttime warming could significantly promote treeline recruitment, and however, treeline recruitment was much more sensitive to nighttime warming than to daytime warming, which could be attributable to the presence of drought stress. The increasing drought stress primarily driven by daytime warming rather than by nighttime warming would likely constrain the responses of treeline recruitment to daytime warming. Our findings provided compelling evidence that nighttime warming rather than daytime warming could play a primary role in promoting the recruitment in alpine treelines, which was related to the daytime warming-induced drought stress. Thus, daytime and nighttime warming should be considered separately to improve future projections of global change impacts across alpine ecosystems.

Anthropogenic impacts on the nitrate pollution in an urban river: Insights from a combination of natural-abundance and paired isotopes.

Urban rivers are often characterized by high nitrate (NO3-) loadings. High NO3- loadings cause water quality and ecological damages, which undermines the sustainable development of cities. To date, the drivers of these high NO3- loadings remain unclear. This study, for the first time, integrated natural-abundance isotopes (δ15 N/δ18O-NO3- and δD/δ18O-H2O) and 15N-pairing techniques to comprehensively reveal the anthropogenic impacts on the NO3- pollution in an urban river. Natural-abundance isotopes suggested that in both the wet and dry seasons, the NO3- was predominantly from the conservative mixing of different sources, and biological NO3- removal was minor. The 15N-pairing experiments supported the natural-abundance isotope data, quantitatively showing that in-soil nitrification was prevailing, while NO3- removal processes (denitrification, anammox, and dissimilatory NO3- reduction to ammonium) were weak. A Bayesian isotope-mixing model showed that soil sources (soil organic nitrogen and chemical fertilizer) dominated the NO3- in the upper reaches, while in the lower reaches, the impermeable riparian zone short-circuited the access of soils to the river. Here, the wastewater treatment plants became a significant source of NO3-. This study quantitatively revealed the drivers of high NO3- loadings in an urban river, and generated important clues for effective NO3- pollution control and remediation in urban rivers.

Effect of multiple spatial scale characterization of land use on water quality.

Land use in uplands is an important factor affecting water quality in its respective catchment, and its influences at the different spatial scales and configurations warrant further investigation. Here, we selected 26 catchments in the upper Han River (China) and sampled the surface water at the outlet of each catchment in four seasons during 2019. Multivariate statistics were used to identify the relationships between land use characteristics in uplands and water quality in river system. The results indicated that chemical oxygen demand (CODMn); pH; dissolved oxygen; electrical conductivity; nutrient, i.e., NH4+-N, NO3--N; and dissolved phosphorus (DP) in rivers displayed significant seasonal variations. Stepwise regression revealed that landscape metrics such as patch density, landscape shape index, and splitting index were important factors influencing water quality in rivers regardless of their spatiality and seasonality. Urban was the most frequently chosen land-use type in the best prediction models, and forest area showed a negative correlation with water quality parameters in most cases for example, DP. Overall, the influence of land use on river water quality was slightly stronger at reach scale than at catchment and riparian scales. Also, nutrients (i.e., NH4+-N, NO3--N, and DP) in rivers were primarily impacted by the land use characteristic at catchment and riparian scales. Our results suggested that multi-scale explorations would help to achieve a fully understanding on the impacts of land use on river water quality.