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• Ecosystem C exchange in China's coastal wetlands were evaluated. • C sink rate of coastal wetlands exceeds terrestrial and marine ecosystems. • C fluxes exhibited significant spatial variability with the latitude gradient. • Climate controlled their spatial patterns by impacting vegetation and soil. Coastal wetlands are of great importance for global carbon cycle and climate mitigation because of their strong carbon uptake capacity. However, the national-scale ecosystem carbon exchange dynamics in coastal wetlands, including magnitudes, spatial patterns, and controlling mechanisms, remains poorly understood. In this study, we utilized eddy covariance measurements from China's coastal wetlands to quantify carbon fluxes, assess their spatial patterns, and explore the controlling mechanisms. Integrating climate, vegetation, and soil factors, we constructed a cascaded relationship network to reveal the potential controlling mechanisms of spatial variations in carbon fluxes. The results revealed that gross primary production (GPP) and ecosystem respiration (ER) were 1405 ± 656 (mean ± sd) gC m−2 yr−1and 893 ± 465, respectively. Net ecosystem production (NEP) in coastal wetlands (567 ± 348 gC m−2 yr−1) exceeded China's terrestrial and marine ecosystems by 2–8 times, highlighting their significant carbon sink capacity. The carbon sink capacity in mangroves was significantly higher than salt marsh, exhibiting a twofold difference in NEP. Spatially, carbon fluxes displayed negative correlations with latitude, indicating the influence of climate features. Although tropical climate zone exhibited significantly higher carbon fluxes than temperate zone, no differences were observed between subtropics zone with others due to the distribution of mixed plants and largest area. The structural equation model (SEM) showed that the climate factors, including temperature, precipitation, and net radiation indirectly promoted GPP and ER through regulating the physiological process of mangrove and salt marsh, as well as soil carbon production and consumption. The cascaded relationship of climate-vegetation-soil showed in SEM explained 71–85 % of the spatial variations in GPP and ER, and ultimately accounted for 85 % of NEP. The carbon consumption efficiency (ER/GPP) in coastal vegetations was of 0.6, lower than that of global and China's terrestrial ecosystems, suggesting their strong efficiency in carbon fixation in coastal wetlands. Lastly, the scenario simulation results implied the importance of coastal vegetation restoration on enhancing blue carbon benefits. [ABSTRACT FROM AUTHOR]

• WUE was evaluated using 2003–2020 data from 83 observed sites. • WUE in China has high spatial variability, decreasing from southeast to northwest. • The multi-year measured average WUE value in China was 2.13 ± 0.88 g C/kg H 2 O. • South China has the largest WUE value in theory, whereas exists oversaturation. • Biological factors, especially productivity, dominated spatial variability of WUE. Water use efficiency (WUE) in terrestrial ecosystems is a crucial indicator of regional resource use efficiency and the water-carbon balance relationship. However, spatial variability casts uncertainties in evaluating the water-carbon coupling, leaving the spatial patterns and underlying driving mechanisms of WUE obscure. To address this issue, our study conducted a comprehensive analysis of China's WUE from 2003 to 2020, utilizing 83 observed ecosystem fluxes and micrometeorological data from ChinaFLUX. We discovered the multi-year measured average WUE value was 2.13 ± 0.88 g C/kg H 2 O, with forest and cropland ecosystems demonstrating superior WUE (2.59 and 2.58 g C/kg H 2 O, respectively) compared to other ecosystem types. China's WUE exhibited marked spatial heterogeneity, characterized by geographic and climatic gradients, mainly decreasing from southeast towards northwest. Intriguingly, South China potentially exhibits maximal WUE theoretically. Yet, we observed an "oversaturation" effect in its WUE concerning gross primary productivity (GPP), peaking when GPP surpassed approximately 2200 g C m−2 yr−1. A clear dividing line between high- and low- WUE regions in China was observed at 400–500 mm isohyets. Our structural equation model, based on the "geography-climate-biology-flux coupling-WUE" cascade relationship, explained 93 % of WUE spatial variability. Here, GPP emerges as the primary direct factor of WUE variation, with climate conditions primarily affecting WUE through GPP. Our study provides valuable insights into the regional pattern of resource use efficiency and enriches the understanding of the integrated water-carbon cycle and carbon sinks. [ABSTRACT FROM AUTHOR]

Temporal dynamics in aboveground net primary productivity (ANPP) are significant for environmental protection and economic development in the Mongolian Plateau grasslands (MPGs). Climate control of ANPP's temporal variations has intensified in response to local meteorological conditions, while the contributions of large‐scale atmospheric circulations to ANPP variations are poorly understood in the MPGs. First, we explored the temporal dynamics in annual ANPP, and then, we analyzed correlations among annual ANPP, local meteorological variables, and three different teleconnections during 1982–2015 in this study. Our analytical results indicated that (a) ANPP exhibited an increasing trend during the entire study period. However, the increasing trend did not persist, and two reversals occurred in 1998 and 2007. (b) ANPP variations were primarily influenced by summer precipitation in most regions and by summer precipitation and temperature together in other regions of the MPGs. (c) The Atlantic Multidecadal Oscillation (AMO) in winter negatively dominated annual ANPP variations by influencing summer precipitation and summer temperature in central Mongolia. The North Atlantic Oscillation (NAO) in winter exerted strong positive impacts on ANPP variations by influencing summer precipitation in Inner Mongolia, China. The East Atlantic Western Russia (EAWR) in spring controlled annual ANPP variations by influencing summer precipitation in western and eastern Mongolia. (d) The winter AMO, winter NAO and spring EAWR significantly dominated the annual ANPP variations in 49.78% of the MPGs, with values of 18.90, 9.81 and 21.07%, respectively. (e) Reversals of the winter AMO, winter NAO and spring EAWR between negative and positive phases resulted in two reversals of ANPP trends. This study will improve our understanding of the role of large‐scale atmospheric circulations in regulating ANPP variations and will be valuable in guiding the development of grazing land management in Mongolia and Inner Mongolia, China. [ABSTRACT FROM AUTHOR]

• A comprehensive ecosystem-level dataset of ET was constructed. • The average ET of terrestrial ecosystems in China was 551.00 ± 204.12 mm. • The ET showed a general increasing trend from the North to South in China. • Precipitation played a dominant role in the spatial variation of ET. Evapotranspiration (ET) is generally affected by climate and vegetation, but the main factor controlling the spatial variation of ET largely varied with different regions. Clarifying the characteristics and dominant factor of ET in China is essential for understanding the regional ecosystem water balance. This study quantitatively assessed the magnitude and spatial variations of ET in China's terrestrial ecosystems using data measured by eddy covariance from 150 observation sites and investigated its underlying direct and indirect influencing factors. The results demonstrated that the ET of China's terrestrial ecosystems varied from 146.95 to 1125.33 mm, with an average value of 551.00 ± 204.12 mm. The ET showed a general increasing trend from the North to South, aligning with the latitudinal patterns of water availability (mean annual precipitation (MAP)), energy conditions (mean annual net radiation (MARn), and mean annual temperature (MAT)). Compared to regions at the same latitude, the North China Plain exhibited relatively higher ET because of the extensive cropland sites in the area. Furthermore, significant positive linear correlations were found between ET and MAT in the majority of ecosystems. However, a negative correlation of ET with MAT was observed in grasslands, which may be attributed to inconsistent responses of ET to MAT on the Tibetan Plateau and Inner Mongolia. Fractional vegetation coverage (FVC) exhibited a more significant direct influence on ET than climate factors (MARn, MAT, and MAP). Nevertheless, climate factors not only exert a direct influence on ET but also have cascade effects that affect FVC, thereby indirectly influencing the ET. When considering these cascade effects, MAP emerged as the dominant driver of spatial variations in ET of China's terrestrial ecosystems. This study provides essential information on variations of ET in China's terrestrial ecosystem and valuable insights for future explorations into the adaptive mechanisms underlying spatial variations of ET. [ABSTRACT FROM AUTHOR]

• The β of terrestrial ecosystems in China varied among ecosystem types. • The β of terrestrial ecosystems in China increased linearly with latitude. • Climate-shaped vegetation factors dominated the spatial variation in β. • Lower hydrothermal conditions limit the importance of vegetation factors to β. The Bowen ratio (β), which is the ratio of sensible heat (H) to latent heat (LE), reflects the energy balance and partitioning processes among soil, vegetation, and the atmosphere. Although the spatial patterns of β have been clearly delineated, the importance of vegetation in the spatial variation of β is frequently underestimated. Revealing the spatial patterns of β would improve the understanding of the variation in energy partitioning in terrestrial ecosystems and its reciprocal relationship with environmental change. Here, we calculated β by integrating H and LE flux values from 80 flux observation sites based on the eddy-covariance method in ChinaFLUX to analyze the spatial pattern and mechanism of β in China. Terrestrial ecosystems in China had an average β of 0.64 ± 0.47. β varied significantly among ecosystem types. Deserts had the highest β (2.08 ± 0.17), while wetlands had the lowest β (0.37 ± 0.11). The β values of terrestrial ecosystems exhibited a significant latitudinal pattern, increasing linearly with latitude. This pattern also existed in forest and cropland ecosystems. The spatial pattern of β was dominated by climate-shaped vegetation factors, including leaf area index (LAI) and fractional vegetation cover (FVC). Nevertheless, as water and thermal conditions decline, the contribution of vegetation factors gradually wanes. These findings demonstrated the spatial variations and driving mechanisms of terrestrial ecosystem β and provided insights into the mitigation of future climate change by vegetation. [ABSTRACT FROM AUTHOR]

Nutrient allocation is an important aspect of plant resource uptake and use, which is related to life‐history strategies. Although to date considerable attention has focused on plant allocation of nitrogen and phosphorus, comparatively little information is available on the allocation of various other nutrients and their up‐scaling from the species to community level.We measured 10 nutrient elements in the leaves, branches and fine roots of 551 plant species growing in eight forest ecosystems in China, ranging from cold temperate to subtropical forests. We estimated the scaling relationship of multiple nutrients among plant organs at the species level and scaled‐up the relationship to the community level by combining this information with that of community structure.Nutrient allocation among plant organs was conserved in different functional groups and biomes across broad environmental gradients. Nutrient partitioning between organs with similar function tended to be isometric, whereas partitioning between organs with distinct functions tended to be allometric. The scaling relationship between above‐ and below‐ground organs remained consistent, whereas the scaling relationship within above‐ground organs changed after scaling up from the species to the community level, with the relative change in nutrients being consistently smaller in the more active organs.Synthesis. The pattern of multiple nutrient allocation among organs showed a degree of conservatism across plant functional groups and biomes, with disproportional changes in nutrient content between functionally distinct organs and a lower relative change in more active organs. This conservative strategy implies the existence of general rules that constrain plant nutrient allocation. [ABSTRACT FROM AUTHOR]

Carbon use efficiency (CUE), one of the most important eco-physiological parameters, represents the capacity of plants to transform carbon into new biomass. Understanding the variations and controls of CUE is crucial for regional carbon assessment. Here, we used 15-years of continuous remote sensing data to examine the variations of CUE across broad geographic and climatic gradients in China. The results showed that the vegetation CUE was averaged to 0.54 ± 0.11 with minor interannual variation. However, the CUE greatly varied with geographic gradients and ecosystem types. Forests have a lower CUE than grasslands and croplands. Evergreen needleleaf forests have a higher CUE than other forest types. Climate factors (mean annual temperature (MAT), precipitation (MAP) and the index of water availability (IWA)) dominantly regulated the spatial variations of CUE. The CUE exhibited a linear decrease with enhanced MAT and MAP and a parabolic response to the IWA. Furthermore, the responses of CUE to environmental change varied with individual ecosystem type. In contrast, precipitation exerted strong control on CUE in grassland, while in forest and cropland, the CUE was mainly controlled by the available water. This study identifies the variations and response of CUE to environmental drivers in China, which will be valuable for the regional assessment of carbon cycling dynamics under future climate change. [ABSTRACT FROM AUTHOR]

• Dominant factor for the IAVs of ET varied among the four grassland ecosystems. • Net radiation dominated the IAV of ET at the alpine marsh meadow. • Water availability mainly regulated the IAVs of ET for relatively arid sites. • Environmental responses of surface conductance varied across the grasslands. Understanding the long-term variation in evapotranspiration (ET) for the spatially distributed grasslands is crucial for the accurate prediction of ET response to climate change. In this study, we analyzed the interannual variability (IAV) of ET and its responses to environmental conditions at four grassland ecosystems across a wide range of climatic and biome conditions based on the long-term (9–11 years) eddy-covariance measurements. The four ecosystems encompassed the most prevalent grassland vegetations in China, containing a typical temperate steppe, an alpine meadow-steppe, an alpine shrubland meadow, and an alpine marsh meadow. The IAVs of annual ET at the typical temperate steppe and the alpine meadow-steppe were primarily affected by either change in precipitation (P) or relative humidity (RH). Leaf area index (LAI) was the dominant factor controlling the IAVs of annual and growing-season ET at the alpine shrubland meadow, and the IAV of LAI was significantly correlated with P variation. As to the alpine marsh meadow, net radiation turned to be the dominant factor for the IAV of annual ET, additionally with significant effects from water supply condition (P and RH) on the IAV of growing-season ET. Similar environmental responses were also found for the IAVs of mean surface conductance (g s) across the sites. Specifically, annual and growing-season mean g s significantly increased with increases in LAI at the alpine shrubland meadow, and appeared to be more sensitive to changes in water availability and VPD at the typical temperate steppe and the alpine meadow-steppe. Significant linear relationships were also observed among the IAVs of mean Priestley-Taylor coefficient (α = ET/ET eq , where ET eq is the equilibrium evaporation), decoupling coefficient (Ω), and g s on both the annual and growing-season basis in this study. Moreover, the variabilities of annual mean g s , Ω , and α further demonstrated the energy-limited conditions at the alpine marsh meadow, and the overall water-limited conditions at the other three grasslands. This study reveals the divergent environmental responses of long-term ET variations over grassland ecosystems, and contributes to the comprehensive understanding on the ET process and modeling efforts as well. [ABSTRACT FROM AUTHOR]

Highlights • Higher soil free-living N 2 fixation in temperate site than in subtropical site. • Different strategies for regulating soil free-living N 2 fixation these two sites. • N input decreased N 2 fixation by decreasing nifH abundance in temperate soils. • P input increased N 2 fixation at the physiological level in subtropical soils. Abstract Biological nitrogen (N 2) fixation by free-living diazotrophs is an important source of bioavailable N in forest ecosystems. However, little is known about how these free-living diazotrophs are affected by exogenous nitrogen (N) and phosphorus (P) additions. Set in the context of increases in both N deposition and P deficits in forest soils, we examined how the abundance, composition, and activity of free-living microbes from the diazotrophic functional group changed in response to N and/or P additions, and identified the main factors that drove free-living N 2 fixation in nutrient-amended subtropical and temperate forest soils. We found that the free-living N 2 fixation was higher in the temperate soils than in the subtropical soils, which was mainly explained by the soil pH. The compositions and richness of the free-living diazotrophs were stable, but different strategies were used to regulate the N 2 fixation in these two sites. While the free-living N 2 fixation did not change much when P was added, it decreased when N was added to the temperate soils, reflecting a decrease in the abundance of the nifH gene. In the subtropical soils, the free-living N 2 fixation was mainly stimulated at the physiological level by P additions and did not change when N was added. The nifH gene abundances and soil available P concentrations were the main predictors on N 2 fixation by free-living diazotrophs in nutrient-amended temperate and subtropical forest soils, respectively, and so should be considered when modelling N cycling in these ecosystems under scenarios of increased N deposition and P deficiency. [ABSTRACT FROM AUTHOR]

Abstract Because of increases in atmospheric nitrogen (N) deposition worldwide, nutrient imbalances and phosphorus (P) limitations in soil are aggravated, with the result that P fertilizer applications to terrestrial ecosystems worldwide may increase. Nitrification and denitrification in soil are major sources of nitrous oxide emissions, especially in soils treated with fertilizers. However, few researchers have studied how forest soils respond to nutrient additions, so we are not sure how the potential nitrification and denitrification activities (PNA and PDA, respectively) and microbial communities involved in these processes might respond when N and P are added to temperate and subtropical forest soils. We investigated how the PNA, PDA, the abundances and community compositions of nitrifiers and denitrifiers, and environmental properties, including soil pH, soil total and dissolved organic carbon, total and available N and phosphorus P, changed when N and/or P were added to subtropical and temperate forest soils. We quantified the abundance, and analyzed the composition, of functional marker genes of nitrifiers (ammonia-oxidizing bacteria and archaea amoA) and denitrifiers (nirK and nirS) using quantitative PCR and sequencing, respectively. We found that the PNA and PDA in the subtropical soil increased when P was added and PNA in the temperate forest soil increased when either N or P was added. The PNA and PDA were positively correlated with the abundance of ammonia-oxidizing bacteria and nirK -denitrifiers, respectively, in the subtropical forest soil but were not correlated with changes in corresponding community compositions in either of the forest soils. The soil total N to total P ratio explained most of the variabilities in the PNA and PDA in the subtropical forest soils, and the soil exchangeable ammonium concentrations and pH were the main controls on the PNA and PDA, respectively, in the temperate forest soils. Our results indicate that soil environmental conditions have more influence on variations in the PNA and PDA in forest soils fertilized with N and P than the corresponding microbial properties. Graphical abstract Unlabelled Image Highlights • Both PNA and PDA increased when P was added to subtropical forest soils. • PNA increased and PDA decreased when N was added to temperate forest soils. • The composition of functional groups had minimal effects on PNA or PDA. • PNA and PDA in subtropical forest soils were mainly controlled by the soil TN/TP. • Overall, NH 4 + and pH were the main drivers of PNA and PDA, respectively. [ABSTRACT FROM AUTHOR]

Abstract Forest carbon exchange and use efficiency (the capacity of transferring gross primary production to net ecosystem production) play an important role in regulating the atmospheric carbon balance and thus affecting the climate. Here, we conducted a synthesis of eddy covariance fluxes measurement across China's typical forests to examine the magnitude, pattern and controls of forest carbon fluxes and carbon use efficiency. The results showed that China's forests had strong carbon sink capacity and high carbon use efficiency. The average annual gross primary production (GPP), ecosystem respiration (RE) and net ecosystem production (NEP) of forests was 1541 ± 92, 1155 ± 91 and 382 ± 37 g C m−2 a−1, and the average NEP:GPP, RE:GPP and NEP:RE was 0.25 ± 0.02, 0.74 ± 0.02 and 0.36 ± 0.04, respectively. Both carbon fluxes and use efficiency significantly varied with biomes, presenting higher values in subtropical and temperate forests than tropical and boreal forests. GPP and RE geographically decreased linearly with latitude increasing, while the NEP and its ratios to GPP and RE peaked at the mid-latitude. Large variances of carbon fluxes and use efficiency were mainly attributable to the regulation of mean annual temperature (MAT). MAT explained 40–54% of the variation in carbon fluxes, and 21–47% of variation in carbon use efficiency. Our result also indicated that carbon use efficiency was affected by stand age that presented an apparent decreasing trend from young to old-growth forests. The results advance our understanding about the variances and controls of carbon exchange and use efficiency and are valuable for regional carbon budget assessment. Highlights • Carbon fluxes and use efficiency significantly varied with biomes. • GPP and RE linearly decreased with latitude, while NEP and its ratios to GPP and RE peaked at the mid-latitude. • Temperature was the dominant driver for spatial variances in annual carbon fluxes and carbon use efficiency. • Carbon use efficiency decreased from young to old-growth forests across sites. [ABSTRACT FROM AUTHOR]

Ecosystem carbon use efficiency (CUEe), one of the most important ecological indicators of ecosystem, represents the capacity of transferring carbon from atmosphere to potential carbon sink. Understanding the variation of CUEe and its controlling factors is paramount for regional carbon budget evaluation. In this study, we conducted a synthesis of 50 field measurements on CUEe to examine the variations of CUEe across China’s terrestrial ecosystems. The results showed that the CUEe values of China’s ecosystems varied from −0.44 to 0.53 with the mean value of 0.19. Grassland had a lower CUEe than forest, cropland and wetland. However, the apparent effects of vegetation types on CUEe were eliminated by accounting for the covariates of climate. Variations of CUEe were correlated with temperature (MAT), precipitation (MAP) and nitrogen deposition (Ndep) in different relationships depend on climate and Ndep condition. CUEe was increased with the augment in MAT only for wet and low Ndep ecosystems. Ndep facilitated the increase of CUEe for wet ecosystems while oppositely restrained the CUEe for dry ecosystems. The results clarify the integrated roles of climate and nitrogen deposition on variations of CUEe that would be valuable for regional ecosystem function evaluation. [ABSTRACT FROM AUTHOR]

Soil microorganisms form an important component of the Earth’s biosphere and play an integral role in carbon, nitrogen and phosphorus cycling. Several biotic and abiotic factors affect the biogeographical distributions of soil microbial biomass (MB) and communities on geographical scales; however, the extent to which soil microbial communities are influenced by these factors is not yet clear. We examined and compared the biomass and structure of soil microbial communities within and between nine mature undisturbed forest ecosystems along the 3700-km North-South Transect in Eastern China (NSTEC). The results showed that soil MB and phospholipid fatty acids (PLFAs) increased with latitude. The structure of the microbial community in boreal forest soils was comparable with that in temperate forests but was significantly different from the microbial communities in warm temperate, subtropical and tropical forests. The mean annual temperature, soil organic carbon, soil total nitrogen and soil easily soluble phosphorus were the main predictors of latitudinal variance in the soil microbial communities. Soils within the same climatic types had similar properties, and soil MB and PLFAs seemed to change along gradients in the various forest types along the NSTEC. Microbial communities showed spatial variation and were dependent on soil properties and climate but were relatively independent of plant functional traits and litter quality. The results suggested that soil microbes could improve the ecosystem models so that they simulate the microbial mechanisms of carbon (C) and nutrient cycling. [ABSTRACT FROM AUTHOR]

The soil nitrogen (N) cycle, as a network of interlinked processes, is mainly driven by microbial communities. However, the patterns and factors that influence the abundance of the different N-cycling microbial communities in temperate and subtropical forest soils are still poorly understood. Moreover, recent studies have tended to focus on surface soils, and have neglected the fact that there may be steep gradients in nutrient concentrations in forest soil profiles. The objective of our study was to determine whether the distributions and compositions of N-cycling microbial communities changed with depth in subtropical and temperate forest soils. We collected soil samples from the surface to 80 cm deep from 2 forests, a native conifer mixed broadleaf temperate forest in northeast China and a native evergreen broadleaf subtropical forest in south eastern China. We used high-throughput sequencing to fingerprint the soil microbial community structures and real-time quantitative PCR to measure the abundances of N-cycling functional genes (NFGs), specifically those involved in organic N decomposition ( chiA ), N fixation ( nifH ), archaeal and bacterial ammonia oxidation ( amoA ; AOA and AOB, respectively), and nitrite reduction ( nirK and nirS ). The composition and diversity of N-cycling microbes shifted between the upper and lower soils. Because of changes in soil nutrient concentrations and soil conditions with soil depth, N-cycling microbial taxa searched for different ecological niches. The absolute abundances of NFGs decreased with depth in both forest soils, indicating that N transformations mainly occurred in surface soil. The relative abundances of NFGs differed between the two forest soils. Because the soil C/N ratios declined with depth in the soil profile, N-cycling microbes invested less energy in N-supplying mineralization and fixation processes that were C-intensive and invested more energy in ammonia oxidization to balance the soil stoichiometry in the subtropical forest subsoils. The nifH , which dominated the soil profiles of the temperate forest were mainly controlled by the soil N concentrations, indicating the high microbial demand for N. We also found that the microbial N storage potentials were lower, and the NO 3 − -N leaching potentials were higher, in the N-rich subtropical forest surface soils and subsoils than in the temperate forest subsoils and surface soils, and that these potentials were significantly correlated with the soil N/P ratios. This suggests that N-cycling microbes change their energy investments at the molecular scale depending on the level of N saturation in the soil. [ABSTRACT FROM AUTHOR]

Nitrogen (N) has great ecological importance, but the biogeographic pattern across forest biomes in China has only recently been explored. Here we conducted a systematic census of leaf C and N following the same protocol to explore the variations of leaf traits, and their possible responses to plant functional types (PFTs) and environmental factors. Results showed that leaf traits varied substantially across biomes, and the relationships of PFTs to climatic factors were stronger than those of PFTs versus soil nutrient proxies, indicating that plant species composition might be a better predictor of plant species distribution with climate than leaf traits. Soil nutrient proxies explained more variation of leaf traits than climate, which demonstrates that leaf traits reflect important aspects of plant responses to soil nutrients. Importantly, partial general linear models analyses found that PFTs showed the greatest direct influence for leaf traits, and climate and soil affected leaf traits mainly through the change in plant species composition rather than having direct impacts. Hence, we concluded that leaf traits were largely controlled by PFTs rather than climate or soil at the biome scale. The results favored the species composition hypothesis, indicating that leaf nutrient concentration is mainly determined by PFTs. [ABSTRACT FROM AUTHOR]

Estimates of atmospheric evaporative demand have been widely required for a variety of hydrological analyses, with potential evapotranspiration (PET) being an important measure representing evaporative demand of actual vegetated surfaces under given metrological conditions. In this study, we assessed the ability of various PET models in capturing long-term (typically 2003–2011) dynamics of evaporative demand at eight ecosystems across various biomes and climatic regimes in China. Prior to assessing PET dynamics, we first examined the reasonability of fourteen PET models in representing the magnitudes of evaporative demand using eddy-covariance actual evapotranspiration (AET) as an indicator. Results showed that the robustness of the fourteen PET models differed somewhat across the sites, and only three PET models could produce reasonable magnitudes of evaporative demand (i.e., PET ≥ AET on average) for all eight sites, including the: (i) Penman; (ii) Priestly-Taylor and (iii) Linacre models. Then, we assessed the ability of these three PET models in capturing dynamics of evaporative demand by comparing the annual and seasonal trends in PET against the equivalent trends in AET and precipitation ( P ) for particular sites. Results indicated that nearly all the three PET models could faithfully reproduce the dynamics in evaporative demand for the energy-limited conditions at both annual and seasonal scales, while only the Penman and Linacre models could represent dynamics in evaporative demand for the water-limited conditions. However, the Linacre model was unable to reproduce the seasonal switches between water- and energy-limited states for some sites. Our findings demonstrated that the choice of PET models would be essential for the evaporative demand analyses and other related hydrological analyses at different temporal and spatial scales. [ABSTRACT FROM AUTHOR]

To explore the importance of the Eurasian steppe region (EASR) in global carbon cycling, we analyzed the spatiotemporal dynamics of the aboveground net primary productivity (ANPP) of the entire EASR from 1982 to 2013. The ANPP in the EASR was estimated from the Integrated ANPPNDVI model, which is an empirical model developed based on field-observed ANPP and long-term normalized difference vegetation index (NDVI) data. The optimal composite period of NDVI data was identified by considering spatial heterogeneities across the study area in the Integrated ANPPNDVI model. EASR's ANPP had apparent zonal patterns along hydrothermal gradients, and the mean annual value was 43.78 g C m−2 yr−1, which was lower than the global grasslands average. Compared to other important natural grasslands, EASR's ANPP was lower than the North American, South American, and African grasslands. The total aboveground net primary productivity (TANPP) was found to be 378.97 Tg C yr−1, which accounted for 8.18%-36.03% of the TANPP for all grasslands. In addition, EASR's TANPP was higher than that of the grasslands in North America, South America, and Africa. The EASR's TANPP increased in a fluctuating manner throughout the entire period of 1982-2013. The increasing trend was greater than that for North American and South American and was lower than that for African grasslands over the same period. The years 1995 and 2007 were two turning points at which trends in EASR's TANPP significantly changed. Our analysis demonstrated that the EASR has been playing a substantial and progressively more important role in global carbon sequestration. In addition, in the development of empirical NDVI-based ANPP models, the early-middle growing season averaged NDVI, the middle-late growing season averaged NDVI and the annual maximum NDVI are recommended for use for semi-humid regions, semi-arid regions, and desert vegetation in semi-arid regions, respectively. [ABSTRACT FROM AUTHOR]

Soil enzymes, as indicators of microbial metabolism, play an important role in nutrient biogeochemistry at the ecosystem level. In this study, we present information from a comprehensive analysis of the latitudinal variations in, and stoichiometric relationships between, soil β-glucosidase (BG), N -acetylglucosaminidase (NAG), leucine aminopeptidase (LAP), and acid phosphatase (AP) in nine forest ecosystems along the North-South Transect in eastern China. The results showed that soil BG and NAG activities were higher in temperate forests than in subtropical and tropical forests. Soil AP activities were the opposite, which indicates that microbial nutrient demand in tropical forests was limited by the nutrient phosphorus (P). Soil BG and NAG activities were significantly and negatively correlated with mean annual temperature (MAT), mean annual precipitation (MAP), the soil carbon (C):P and soil nitrogen (N):P ratios, but not with the soil C:N ratio. Soil NAG and AP activities were inversely correlated with soil pH, and soil AP activity increased as soil pH decreased. The latitudinal variations in the C:N, C:P, and N:P acquisition ratios represented by ln(BG):ln(LAP + NAG):ln(AP) activities across ecosystems were significantly and negatively related to MAP and MAT. The C:P and N:P acquisition ratios were positively related to soil pH but negatively related to the soil C:P and N:P ratios. The C:N and C:P acquisition ratios were also negatively correlated with leaf C:N, C:P, and N:P ratios. This study provides useful information about environmental controls on enzyme stoichiometry, and also highlights the stoichiometric and energy limitations on the metabolism of soil microbes. [ABSTRACT FROM AUTHOR]

The C sequestration in coastal blue carbon (C b ) ecosystems, including mangroves, seagrasses and saltmarshes, was discovered to be useful in mitigating the increasing trend of carbon dioxide (CO 2 ) emission due to climate change. In this study, we systematically estimate traditional C b ecosystem distribution and the associated C b sequestration rate, and then further quantify the C b sinks fishery contribution to C b ecosystem due to human activity in coastal ecosystem. The results show that the global C b ecosystem is able to store 10.8 PgC, wherein biomass and soil are able to store 2.13 and 8.68 PgC, respectively. In China, the C b pools are 162 TgC in mangroves, 67 TgC in saltmarshes and 75 TgC in seagrass. The human activity induced global C b sink fishery on C b ecosystem is about 26.58–37.6 TgC yr − 1 , accounting for 30.7%–43.4% of the world's traditional C b sequestration ecosystem.The global C b sequestration potential reaches up to 86.59 Tg yr − 1 , while China can explain 1.70% of the world's total C b sequestration. However, in China, the C b sequestration due to human activity reaches up to 6.32–7.89 TgC yr − 1 , accounting for 20.9%–23.7% of global C b sink fishery. Therefore, it is very important to build the C b sink fisheries measure and monitor system to scientifically valuate C b sink fisheries and associated development potential. [ABSTRACT FROM AUTHOR]

Nitrogen (N) to phosphorus (P) allocation in plant organs is of particular interest, as both elements are important to regulate plant growth. We analyzed the scaling relationship of N and P in leaves, stems and fine roots of 224 plant species along an altitudinal transect (500-2,300 m) on the northern slope of Changbai Mountain, China. We tested whether the scaling relationships of N and P were conserved in response to environmental variations. We found that the N and P concentrations of the leaves, stems and fine roots decreased, whereas the N:P ratios increased with increasing altitude. Allometric scaling relationships of N and P were found in the leaves, stems and fine roots, with allometric exponents of 0.78, 0.71 and 0.87, respectively. An invariant allometric scaling of N and P in the leaves, stems and fine roots was detected for woody plants along the altitudinal gradient. These results may advance our understanding of plant responses to climate change, and provide a basis for practical implication of various ecological models. [ABSTRACT FROM AUTHOR]

Aim The elemental composition of plants is of fundamental importance for plant physiology and biogeochemical cycling. Knowledge about how the pattern of multi-element variability is coordinated between above- and below-ground organs remains limited. Here, we quantify multi-element variability in the leaves and roots of terrestrial plants, in addition to trying to understand its taxonomic and environmental regulation at large scales. Location China. Methods Sixteen elements in the leaves and fine roots of 792 plant species across nine forests located along the north-south transect of eastern China were measured. General linear mixed models were used to partition taxonomic and environmental variation. Canonical discriminant analyses were conducted to identify elements with the highest discriminatory power for different plant orders. Results Elemental composition differed significantly between the leaves and roots, with the roots containing higher concentrations of trace metal elements (aluminium, iron, sodium, zinc, copper, lead, nickel and cobalt). A coordinated pattern of multi-element variability and similar taxonomic regulation was observed between the leaves and roots of terrestrial plants. That is, elements with higher internal concentrations were less variable, with most of the variability being attributed to taxonomic effects rather than the environment. Main conclusions Taxonomic and environmental regulation differed for different elements. Compared with microelements, macroelements exhibited a narrow range of internal concentrations, less environmental control and stronger taxonomic conservatism. The coordinated pattern of multi-element variability and similar taxonomic effects in the leaves and roots implies that above- and below-ground ecological processes are tightly linked. [ABSTRACT FROM AUTHOR]

The decomposition of soil organic matter (SOM) can be described by a set of kinetic principles, environmental constraints, and substrate supply. Here, we hypothesized that SOM decomposition rates ( R) and its temperature sensitivity ( Q10) would increase steadily with the N:C ratios of added substrates by alleviating N limitation on microbial growth. We tested this hypothesis by investigating SOM decomposition in both grassland and forest soils after addition of substrates with a range of N:C ratios. The results showed that Michaelis-Menten equations well fit the response of R to the N:C ratio variations of added substrates, and their coefficients of determination ( R2) ranged from 0.65 to 0.89 ( P < 0.01). Moreover, the maximal R, Q10, and cumulative C emission of SOM decomposition increased exponentially with the N:C ratios of added substrates, and were controlled interactively by incubation temperature and the N:C ratios of the added substrates. We demonstrated that SOM decomposition rate and temperature sensitivity were exponentially correlated to substrate stoichiometry (N:C ratio) in both grassland and forest soils. Therefore, these correlations should be incorporated into the models for the prediction of SOM decomposition rate under warmer climatic scenarios. [ABSTRACT FROM AUTHOR]

Characterization of the vertical distribution of soil organic carbon (C), nitrogen (N), and phosphorus (P) may improve our ability to accurately estimate soil C, N, and P storage. Based on a database of 21 354 records in 74 long-term monitoring plots from 2004 to 2013 in the Chinese Ecosystem Research Network (CERN), we built fitting functions to quantify the vertical distribution of soil C, N, and P (up to 100 cm depth) in the typical Chinese terrestrial ecosystems. The decrease of soil C, N, and P content with depth can be well fitted with various mathematical functions. The fitting functions differed greatly between artificial (agriculture) and natural (desert, forest, and grassland) ecosystems, and also differed with respect to soil C, N, and P content. In both the artificial and natural ecosystems, the best fitting functions were exponential functions for C, quadratic functions for N, and quadratic functions for P. Furthermore, the stoichiometric ratios of soil C, N, and P were ranked in descending order: grassland > forest > agriculture > desert, and were also associated with climate. This study is the first to build the fitting functions for the profile distribution of soil C, N, and P in China at a national scale. Our findings provide a scientific basis to accurately assess the storage of C, N, and P in soils at a large scale, especially for the integrative analysis of historical data. [ABSTRACT FROM AUTHOR]

Ecosystem respiration ( R e ) is rarely quantified from remote sensing data because satellite technique is incapable of observing the key processes associated with soil respiration. In this study, we develop a Remote Sensing Model for R e (ReRSM) by assuming that one part of R e is derived from current photosynthate with the respiratory rate coupling closely with gross primary production (GPP), and the other part of R e is derived from reserved ecosystem organic matter (including plant biomass, plant residues and soil organic matter) with the respiratory rate responding strongly to temperature change. The ReRSM is solely driven by the Enhanced Vegetation Index (EVI), the Land Surface Water Index (LSWI) and the Land Surface Temperature (LST) from MODIS data. Multi-year eddy CO 2 flux data of five vegetation types in Northern China and the Tibetan Plateau (including temperate mixed forest, temperate steppe, alpine shrubland, alpine marsh and alpine meadow-steppe) were used for model parameterization and validation. In most cases, the simulated R e agreed well with the observed R e in terms of seasonal and interannual variation irrespective of vegetation types. The ReRSM could explain approximately 93% of the variation in the observed R e across five vegetation types, with the root mean square error (RMSE) of 0.04 mol C m −2 d −1 and the modeling efficiency (EF) of 0.93. Model comparison showed that the performance of the ReRSM was comparable with that of the RECO in the studied five vegetation types, while the former had much fewer parameters than the latter. The ReRSM parameters showed good linear relationships with the mean annual satellite indices. With these linear functions, the ReRSM could explain approximately 90% of the variation in the observed R e across five vegetation types, with the RMSE of 0.05 mol C m −2 d −1 and the EF of 0.89. These analyses indicated that the ReRSM is a simple and alternative approach in R e estimation and has the potential of estimating spatial R e . However, the performance of ReRSM in other vegetation types or regions still needs a further study. [ABSTRACT FROM AUTHOR]

Gross primary production (GPP) and ecosystem respiration (RE) are two important processes in the terrestrial carbon cycle. Understanding the relationships between GPP and RE across space, as well as the underlying mechanisms, is helpful for understanding the terrestrial carbon cycle and predicting the global carbon budget. In this study, we investigated the correlation between the spatial variations in GPP and RE by compiling carbon flux data from 264 sites across the Asian, European, North American, South American, African, and Oceanian regions. The results indicated that GPP and RE covaried across space regionally and globally ( P < 0.001). The spatial variations in GPP explained 66–98% of the variations in RE in the six regions (approximately explained 60–76% when considered the effects of self-correlation caused by current flux partitioning algorithm), and it explained 90% of RE variations at the global scale (about 70% when considered the effects of self-correlation). RE/GPP values were not significantly different among the six regions or between the two hemispheres. RE/GPP values consistently averaged at 0.87 ± 0.04 along the spatial variations in climate and vegetation index. This covariation between GPP and RE across space is largely attributed to the parallel responses of GPP and RE to the common climatic and vegetation factors, but the underlying mechanism lies in productivity as the primary and direct substrate supplier for respiration which fundamentally constrains RE. These results suggest that the variation in photosynthate availability is the dominant driver for respiration across space and that this process must be fully considered in the cross-site RE comparisons. [ABSTRACT FROM AUTHOR]

Climate, vegetation, and soil characteristics play important roles in regulating the spatial variation in carbon dioxide fluxes, but their relative influence is still uncertain. In this study, we compiled data from 241 eddy covariance flux sites in the Northern Hemisphere and used Classification and Regression Trees and Redundancy Analysis to assess how climate, vegetation, and soil affect the spatial variations in three carbon dioxide fluxes (annual gross primary production (AGPP), annual ecosystem respiration (ARE), and annual net ecosystem production (ANEP)). Our results showed that the spatial variations in AGPP, ARE, and ANEP were significantly related to the climate and vegetation factors (correlation coefficients, R = 0.22 to 0.69, P < 0.01) while they were not related to the soil factors (R = -0.11 to 0.14, P > 0.05) in the Northern Hemisphere. The climate and vegetation together explained 60 % and 58 % of the spatial variations in AGPP and ARE, respectively. Climate factors (mean annual temperature and precipitation) could account for 45 - 47 % of the spatial variations in AGPP and ARE, but the climate constraint on the vegetation index explained approximately 75 %. Our findings suggest that climate factors affect the spatial variations in AGPP and ARE mainly by regulating vegetation properties, while soil factors exert a minor effect. To more accurately assess global carbon balance and predict ecosystem responses to climate change, these discrepant roles of climate, vegetation, and soil are required to be fully considered in the future land surface models. Moreover, our results showed that climate and vegetation factors failed to capture the spatial variation in ANEP and suggest that to reveal the underlying mechanism for variation in ANEP, taking into account the effects of other factors (such as climate change and disturbances) is necessary. [ABSTRACT FROM AUTHOR]

We present a comprehensive analysis of the changes in the structure and functioning of the soil microbial communities, as well as their driving factors along the northern slope of Changbai Mountain by analyzing soil phospholipid fatty acids (PLFAs) profiles and enzymatic activities. The four vertical zones of vegetation selected in this study were as follows: (1) mixed coniferous and broad-leaved forest (MCB) located below 1100 m (site A and B); (2) dark-coniferous spruce-fir forest (DCF) located between 1100–1700 m (site C); (3) Ermans birch forest (EB) located between 1700 and 2000 m (site D); and (4) alpine tundra (AT) located above 2000 m (sites E and F). The results showed that the total viable microbial PLFAs varied with altitude, with the highest value detected in the EB (site D) (76.8 nmol g −1 ). The fungal/bacterial (F/B) and the Gram positive/Gram negative (G + /G − ) ratios increased with elevation. The lower values of F/B obtained in the MCB (site A) and in DCF (site C) for G + /G － . The soil microbial community structure observed in the MCB (sites A and B) was different from that observed in the other four sites, with lower fungal biomass. The soil microbial activities such as β -glucosidase (βG), N -acetylglucosaminidase (NAG), acid phosphatase (AP) and leucine aminopeptidase (LAP) in the MCB (sites A and B) were significantly higher than those documented in the other four sites. The substantial differences in the soil microbial community composition were significantly correlated with the mean annual temperature (MAT), the mean annual precipitation (MAP) and soil temperature, silt and clay fraction, total phosphorus, nitrate nitrogen. However, the variations in soil enzyme activities were significantly correlated with soil nutrients. The MAP and nitrate nitrogen were the significant predictors of the variance in the soil microbial community and enzyme activity, respectively. The soil organic matter (SOM) decomposition rate showed a significant positive relationship with the total microbial, bacterial, actinomycetes PLFAs and the soil enzyme activities. Our results suggest that studies incorporating the microbial community structure and soil enzyme activity with the SOM decomposition may enhance our understanding of the mechanisms of biogeochemical processes. [ABSTRACT FROM AUTHOR]

Understanding the variation in stomatal characteristics in relation to climatic gradients can reveal the adaptation strategies of plants, and help us to predict their responses to future climate changes. In this study, we investigated stomatal density (SD) and stomatal length (SL) in 150 plant species along an elevation gradient (540–2357 m) in Changbai Mountain, China, and explored the patterns and drivers of stomatal characteristics across species and plant functional types (PFTs: trees, shrubs, and herbs). The average values of SD and SL for all species combined were 156 mm–2 and 35 µm, respectively. SD was higher in trees (224 mm–2) than in shrubs (156 mm–2) or herbs (124 mm–2), and SL was largest in herbs (37 µm). SD was negatively correlated with SL in all species and PFTs (P<0.01). The relationship between stomatal characteristics and elevation differed among PFTs. In trees, SD decreased and SL increased with elevation; in shrubs and herbs, SD initially increased and then decreased. Elevation-related differences in SL were not significant. PFT explained 7.20–17.6% of the total variation in SD and SL; the contributions of CO2 partial pressure (), precipitation, and soil water content (SWC) were weak (0.02–2.28%). Our findings suggest that elevation-related patterns of stomatal characteristics in leaves are primarily a function of PFT, and highlight the importance of differences among PFTs in modeling gas exchange in terrestrial ecosystems under global climate change. [ABSTRACT FROM AUTHOR]

Abstract: Accurate quantification of the spatio-temporal variation of gross primary production (GPP) for terrestrial ecosystems is significant for ecosystem management and the study of the global carbon cycle. In this study, we propose a MODIS-based Photosynthetic Capacity Model (PCM) to estimate GPP in Northern China and the Tibetan Plateau. The PCM follows the logic of the light use efficiency model and is only driven by the Enhanced Vegetation Index (EVI) and the Land Surface Water Index (LSWI). Multi-year eddy CO2 flux data from five vegetation types in North China (temperate mixed forest, temperate steppe) and the Tibetan Plateau (alpine shrubland, alpine marsh and alpine meadow-steppe) were used for model parameterization and validation. In most cases, the seasonal and interannual variation in the simulated GPP agreed well with the observed GPP. Model comparisons showed that the predictive accuracy of the PCM was higher than that of the MODIS GPP products and was comparable with that of the Vegetation Photosynthesis Model (VPM) and the potential PAR-based GPP models. The model parameter (PC max) of the PCM represents the maximum photosynthetic capacity, which showed a good linear relationship with the mean annual nighttime Land Surface Temperature (LSTan). With this linear function, the PCM-simulated GPP can explain approximately 93% of the variation in the flux-observed GPP across all five vegetation types. These analyses demonstrated the potential of the PCM as an alternative tool for regional GPP estimation. [Copyright &y& Elsevier]

A continuous three-year observation (from May 2008 to April 2011) was conducted to characterize the spatial variation of dissolved inorganic nitrogen (DIN) deposition at eight main forest ecosystems along the north-south transect of eastern China (NSTEC). The results show that both throughfall DIN deposition and bulk DIN deposition increase from north to south along the NSTEC. Throughfall DIN deposition varies greatly from 2.7 kg N/(ha·yr) to 33.0 kg N/(ha·yr), with an average of 10.6 kg N/(ha·yr), and bulk DIN deposition ranges from 4.1 kg N/(ha·yr) to 25.4 kg N/(ha·yr), with an average of 9.8 kg N/(ha·yr). NH-N is the dominant form of DIN deposition at most sampling sites. Additionally, the spatial variation of DIN deposition is controlled mainly by precipitation. Moreover, in the northern part of the NSTEC, bulk DIN deposition is 17% higher than throughfall DIN deposition, whereas the trend is opposite in the southern part of the NSTEC. The results demonstrate that DIN deposition would likely threaten the forest ecosystems along the NSTEC, compared with the critical loads (CL) of N deposition, and DIN deposition in this region is mostly controlled by agricultural activities rather than industrial activities or transportation. [ABSTRACT FROM AUTHOR]

Aim To understand: (1) how temperature, precipitation and stand age control the above-ground biomass carbon density ( BCDa) of mature forests and its macroecology patterns across latitudes; (2) the age threshold for old-growth forests at a global scale. Location Global forests. Methods We compiled a database (897 sites) of mature forests between 80 and 1200 years old. The site data include latitude, longitude, mean annual temperature, mean annual precipitation, forest type, stand age, BCDa, living biomass (above- and below-ground biomass) carbon density and total (living plus dead) biomass carbon density. Based on the site data, we performed regression analyses to show how BCDa changes with climate and forest stand age. Results At a global scale, the highest BCDa of mature forests occurred mainly in the mid-latitude regions where mean annual temperatures were 8-10 °C and mean annual precipitation was between 1000 and 2500 mm. The average BCDa of forests in the stand age class of 450-500 years was higher than those in the other stand age classes. For forests between 80 and 450 years old, which form the majority of mature forests, carbon accumulation was faster in dead biomass than in living biomass. Main conclusions The highest BCDa of mature forests is located in mid-latitude regions with cool temperatures and moderate precipitation. The age threshold for old-growth forests at a global scale should be 450-500 years, which is much older than the previously documented age of 100-200 years. This older age threshold for old-growth forests is probably one of the primary reasons why recent works have concluded that old-growth forests are still carbon sinks. [ABSTRACT FROM AUTHOR]

We added the stable isotope 15N in the form of (15NH4)2SO4 and K15NO3 to forest ecosystems in eastern China under two different N deposition levels to study the fate of the different forms of deposited N. Prior to the addition of the 15N tracers, the natural 15N abundance ranging from −3.4‰ to +10.9‰ in the forest under heavy N deposition at Dinghushan (DHS), and from −3.92‰ to +7.25‰ in the forest under light N deposition at Daxinganling (DXAL). Four months after the tracer application, the total 15N recovery from the major ecosystem compartments ranged from 55.3% to 90.5%. The total 15N recoveries were similar under the (15NH4)2SO4 tracer treatment in both two forest ecosystems, whereas the total 15N recovery was significantly lower in the subtropical forest ecosystem at DHS than in the boreal forest ecosystem at DXAL under the K15NO3 tracer treatment. The 15N assimilated into the tree biomass represented only 8.8% to 33.7% of the 15N added to the forest ecosystems. In both of the tracer application treatments, more 15N was recovered from the tree biomass in the subtropical forest ecosystem at DHS than the boreal forest ecosystem at DXAL. The amount of 15N assimilated into tree biomass was greater under the K15NO3 tracer treatment than that of the (15NH4)2SO4 treatment in both forest ecosystems. This study suggests that, although less N was immobilized in the forest ecosystems under more intensive N deposition conditions, forest ecosystems in China strongly retain N deposition, even in areas under heavy N deposition intensity or in ecosystems undergoing spring freezing and thawing melts. Compared to ammonium deposition, deposited nitrate is released from the forest ecosystem more easily. However, nitrate deposition could be retained mostly in the plant N pool, which might lead to more C sequestration in these ecosystems. [ABSTRACT FROM AUTHOR]

Aim Carbon use efficiency [net primary production ( NPP)/gross primary production ( GPP) ratio] is a parameter related to the allocation of photosynthesized products by plants and is commonly used in many biogeochemical cycling models. But how this parameter changes with climates is still unknown. Faced by an aggravated global warming, there is a heightened necessity in unravelling the dependence of the NPP/ GPP ratio on climates. The objective of this study was to examine how ongoing climate change is regulating global patterns of change in the NPP/ GPP ratio. The study finding would elucidate whether the global vegetation ecosystem is becoming more or less efficient in terms of carbon storage under climatic fluctuation. Location The global planetary ecosystem. Methods The annual NPP/ GPP ratio of the global terrestrial ecosystem was calculated over a 10-year period based on Moderate Resolution Imaging Spectroradiometer data and an ecosystem productivity model. The temporal dynamics of the global NPP/ GPP ratio and their dependence on climate were investigated. Results The global NPP/ GPP ratio exhibited a decreasing trend from 2000 to 2009 due to decreasing NPP and stable GPP over this period. The temporal dynamics of the NPP/ GPP ratio were strongly controlled by temperature and precipitation. Increased temperature lowered the NPP/ GPP ratio, and increased precipitation led to a higher NPP/ GPP ratio. Conclusions The NPP/ GPP ratio exhibits a clear temporal pattern associated with climatic fluctuations at a global scale. The associations of the NPP/ GPP ratio with climatic variability challenge the conventional assumption that the NPP/ GPP ratio should be consistent independent of environmental conditions. More importantly, the findings of this study have fundamental significance for our understanding of ongoing global climatic change. In regions and time periods experiencing drought or increased temperatures, plant ecosystems would suffer a higher ecosystem respiration cost and their net productivity would shrink. [ABSTRACT FROM AUTHOR]

Highlights: [•] The majority of Asian terrestrial ecosystems are currently large carbon sinks. [•] Temperature and precipitation were the main controlling factors in the spatial variation of GPP, Re in the Asian region. [•] The spatial variation of NEP was coordinately regulated by GPP and Re. [•] Climate-driven theory applied to the Asian region. [ABSTRACT FROM AUTHOR]

Abstract: The world has long neglected the negative impacts that carbon (C) sequestration has on ecosystem health. Accordingly, the aims of this study were to advocate a conceptual C health threshold model devised for terrestrial ecosystems while proposing a method by which to qualify the C health threshold of ecosystems. Since coupling relationships between C, nitrogen (N), and water can shape the response of ecosystems to conditions of global climate change, this study concentrated on C sequestration, N input, and water erosion impacts on ecosystem health. If C storage exceeds the terrestrial ecosystem C health threshold, ecological degradation will either take place or ecosystems will fall into a sub-health state in accordance with the C health threshold model. Additionally, C sequestration engineering approaches, excess N inputs, and water erosion destroy the balance of C cycling processes and may consequently have an effect on the C health threshold. Therefore, analysis related to the interannual variability of C cycles and their potential future behavior must take into account mechanisms driven through the coupling of water, C, and N cycles. Defining ecosystem health will help familiarize and eventually lead to proficiency in understanding C health threshold awareness. This will aid in determining the appropriate ecological restoration measures to take when dealing with climate change impacts, leading to the preservation of biogeochemical C cycling native to terrestrial ecosystems. [Copyright &y& Elsevier]

Abstract: Nitrogen (N) and phosphorus (P) resorption from senescing organs are important plant nutrient conservation mechanisms. However, whether nutrient resorption can indicate plant adaptation to nutrient-poor environment remains a controversy. We quantified nutrient resorption efficiency of major species in an alpine meadow of the Qinghai-Tibetan Plateau. N and P resorption for these species averaged 65.2% and 67.4%, respectively, which was at the high end of the values of global scale. Among three life forms, sedges contained least N and P and had highest resorption efficiency, indicating sedges had a competitive advantage over grasses and forbs in this nutrient-poor environment. This result was consistent with the fact that sedge was the typically dominant plant functional group in Qinghai-Tibetan Plateau. By fitting the allometric equations ([nutrient]senescent = A([nutrient]live) B ) separately for N and P, we found much smaller B (N) than B (P), which suggested that relative to the resorption from senescent leaves, the cost of N uptake from soils increased more quickly than that of P as resource availability decreased. This phenomenon was contrary to the corresponding N and P acquisition pattern of the tropical ecosystem, which matched the large geographical gradients of N vs. P limitation (N for alpine ecosystem, while P for tropical ecosystem). This study suggests that surveying leaf nutrient concentration and resorption could provide indicative information about plant adaptation to nutrient-poor soil within and across ecosystems. Our finding offers insights to nutrient management and ecosystem restoration in nutrient-poor environment and delivers information for upcoming meta-studies and model simulation of global leaf nutrient resorption. [Copyright &y& Elsevier]

Understanding forest carbon cycling responses to atmospheric N deposition is critical to evaluating ecosystem N dynamics. The natural abundance of N (δN) has been suggested as an efficient and non-invasive tool to monitor N pools and fluxes. In this study, three successional forests in southern China were treated with four levels of N addition. In each treatment, we measured rates of soil N mineralization, nitrification, NO emission and inorganic N leaching as well as N concentration and δN of leaves, litters and soils. We found that foliar N concentration and δN were higher in the mature broadleaf forest than in the successional pine or mixed forests. Three-year continuous N addition did not change foliar N concentration, but significantly increased foliar δN ( p < 0.05). Also, N addition decreased the δN of top soil in the N-poor pine and mixed forests and significantly increased that of organic and mineral soils in N-rich broadleaf forests ( p < 0.05). In addition, the soil NO emission flux and inorganic N leaching rate increased with increasing N addition and were positively correlated with the N enrichment factor ( ε) of forest ecosystems. Our study indicates that δN of leaf, litter and soil integrates various information on plant species, forest stand age, exogenous N input and soil N transformation and loss, which can be used to monitor N availability and N dynamics in forest ecosystems caused by increasing N deposition in the future. [ABSTRACT FROM AUTHOR]

Aims Clarifying the spatiotemporal variations in precipitation-use efficiency (PUE), the ratio of vegetation above-ground productivity to annual precipitation, will advance our understanding of how ecosystems' carbon and water cycles respond to climate change. Our goal is to investigate the variations in PUE at both regional and site scales along a 4500-km climate-related grassland transect. Location The Inner Mongolian Plateau in northern China and the Qinghai-Tibetan Plateau. Methods We collected data on 580 sites from four data sources. The data were acquired through field surveys and long-term in situ observations. We investigated the relationships between precipitation and PUE at both regional and site scales, and we evaluated the effects of the main biotic and climatic factors on PUE at both spatial scales. Results PUE decreased with decreasing mean annual precipitation (MAP), except for a slight rise toward the dry end of the gradient. The maximum PUE showed large site-to-site variation along the transect. Vegetation cover significantly affected the spatial variations in PUE, and this probably accounts for the positive relationship between PUE and MAP. However, there was no significant relationship between inter-annual variations in precipitation or vegetation cover and PUE within given ecosystems along the transect. Conclusions The findings of this research contradict the prevailing view that a convergent maximum PUE exists among diverse ecosystems, as presented in previous reports. Our findings also suggest the action of distinct mechanisms in controlling PUE at different spatial scales. We propose the use of a conceptual model for predicting vegetation productivity at continental and global scales with a sigmoid function, which illustrates an increasing PUE with MAP in arid regions. Our approach may represent an improvement over use of the popular Miami model. [ABSTRACT FROM AUTHOR]

Abstract: An increasing nitrogen deposition experiment (2 g N m−2 year−1) was initiated in an alpine meadow on the Qinghai-Tibetan Plateau in May 2007. The greenhouse gases (GHGs), including CO2, CH4 and N2O, was observed in the growing season (from May to September) of 2008 using static chamber and gas chromatography techniques. The CO2 emission and CH4 uptake rate showed a seasonal fluctuation, reaching the maximum in the middle of July. We found soil temperature and water-filled pore space (WFPS) were the dominant factors that controlled seasonal variation of CO2 and CH4 respectively and lacks of correlation between N2O fluxes and environmental variables. The temperature sensitivity (Q 10) of CO2 emission and CH4 uptake were relatively higher (3.79 for CO2, 3.29 for CH4) than that of warmer region ecosystems, indicating the increase of temperature in the future will exert great impacts on CO2 emission and CH4 uptake in the alpine meadow. In the entire growing season, nitrogen deposition tended to increase N2O emission, to reduce CH4 uptake and to decrease CO2 emission, and the differences caused by nitrogen deposition were all not significant (p < 0.05). However, we still found significant difference (p < 0.05) between the control and nitrogen deposition treatment at some observation dates for CH4 rather than for CO2 and N2O, implying CH4 is most susceptible in response to increased nitrogen availability among the three greenhouse gases. In addition, we found short-term nitrogen deposition treatment had very limited impacts on net global warming potential (GWP) of the three GHGs together in term of CO2-equivalents. Overall, the research suggests that longer study periods are needed to verify the cumulative effects of increasing nitrogen deposition on GHG fluxes in the alpine meadow. [Copyright &y& Elsevier]

Abstract: Quantifying the partitioning of evapotranspiration (ET) and its controls are particularly important for accurate prediction of the climatic response of ecosystem carbon, water, and energy budgets. In this study, we employed the Shuttleworth–Wallace model to partition ET into soil water evaporation (E) and vegetation transpiration (T) at four grassland ecosystems in China. Two to three years (2003–2005) of continuous measurements of ET with the eddy covariance technique were used to test the long-term performance of the model. Monte Carlo simulations were performed to estimate the key parameters in the model and to evaluate the accuracy in model partitioning (i.e. E/ET). Results indicated that the simulated ET at the four ecosystems was in good agreement with the measurements at both the diurnal and seasonal timescales, but the model tended to underestimate ET by 3–11% on rainy days, probably due to the lack of model representation of rainfall interception. In general, E accounted for a large proportion of ET at these grasslands. The monthly E/ET ranged from 12% to 56% in the peak growing seasons and the annual E/ET ranged from 51% to 67% across the four ecosystems. Canopy stomatal conductance controlled E/ET at the diurnal timescale, and the variations and magnitude of leaf area index (LAI) explained most of the seasonal, annual, and site-to-site variations in E/ET. A simple linear relationship between growing season LAI and E/ET explained ca. 80% of the variation observed at the four sites for the 10 modeled site-years. Our work indicated that the daily E/ET decreased to a minimum value of ca. 10% for values of LAI greater than 3m2 m−2 at the ecosystem with a dense canopy. The sensitivities of E/ET to changes in LAI increased with the decline in water and vegetation conditions at both the seasonal and the annual time scales, i.e., the variations in LAI could cause stronger effects on E/ET in the sparse-canopy ecosystems than in the dense-canopy ecosystems. It implies that the hydrological processes and vegetation productivity for ecosystems in arid environments might be more vulnerable to projected climate change than those in humid environments. [Copyright &y& Elsevier]

Through 2–3-year (2003–2005) continuous eddy covariance measurements of carbon dioxide and water vapor fluxes, we examined the seasonal, inter-annual, and inter-ecosystem variations in the ecosystem-level water use efficiency (WUE, defined as the ratio of gross primary production, GPP, to evapotranspiration, ET) at four Chinese grassland ecosystems in the Qinghai-Tibet Plateau and North China. Representing the most prevalent grassland types in China, the four ecosystems are an alpine swamp meadow ecosystem, an alpine shrub-meadow ecosystem, an alpine meadow-steppe ecosystem, and a temperate steppe ecosystem, which illustrate a water availability gradient and thus provide us an opportunity to quantify environmental and biological controls on ecosystem WUE at different spatiotemporal scales. Seasonally, WUE tracked closely with GPP at the four ecosystems, being low at the beginning and the end of the growing seasons and high during the active periods of plant growth. Such consistent correspondence between WUE and GPP suggested that photosynthetic processes were the dominant regulator of the seasonal variations in WUE. Further investigation indicated that the regulations were mainly due to the effect of leaf area index (LAI) on carbon assimilation and on the ratio of transpiration to ET ( T/ET). Besides, except for the swamp meadow, LAI also controlled the year-to-year and site-to-site variations in WUE in the same way, resulting in the years or sites with high productivity being accompanied by high WUE. The general good correlation between LAI and ecosystem WUE indicates that it may be possible to predict grassland ecosystem WUE simply with LAI. Our results also imply that climate change-induced shifts in vegetation structure, and consequently LAI may have a significant impact on the relationship between ecosystem carbon and water cycles in grasslands. [ABSTRACT FROM AUTHOR]

The eddy covariance technique provides measurements of net ecosystem exchange (NEE) of CO2 between the atmosphere and terrestrial ecosystems, which is widely used to estimate ecosystem respiration and gross primary production (GPP) at a number of CO2 eddy flux tower sites. In this paper, canopy-level maximum light use efficiency, a key parameter in the satellite-based Vegetation Photosynthesis Model (VPM), was estimated by using the observed CO2 flux data and photosynthetically active radiation (PAR) data from eddy flux tower sites in an alpine swamp ecosystem, an alpine shrub ecosystem and an alpine meadow ecosystem in Qinghai–Tibetan Plateau, China. The VPM model uses two improved vegetation indices (Enhanced Vegetation Index (EVI), Land Surface Water Index (LSWI)) derived from the Moderate Resolution Imaging Spectral radiometer (MODIS) data and climate data at the flux tower sites, and estimated the seasonal dynamics of GPP of the three alpine grassland ecosystems in Qinghai–Tibetan Plateau. The seasonal dynamics of GPP predicted by the VPM model agreed well with estimated GPP from eddy flux towers. These results demonstrated the potential of the satellite-driven VPM model for scaling-up GPP of alpine grassland ecosystems, a key component for the study of the carbon cycle at regional and global scales. [Copyright &y& Elsevier]

Quantification of areal evapotranspiration from remote sensing data requires the determination of surface energy balance components with support of field observations. Much attention should be given to spatial resolution sensitivity to the physics of surface heterogeneity. Using the Priestley–Taylor model, we generated evapotranspiration maps at several spatial resolutions for a heterogeneous area at Haibei, and validated the evapotranspiration maps with the flux tower data. The results suggested that the mean values for all evapotranspiration maps were quite similar but their standard deviations decreased with the coarsening of spatial resolution. When the resolution transcended about 480 m, the standard deviations drastically decreased, indicating a loss of spatial structure information of the original resolution evapotranspiration map. The absolute values of relative errors of the points for evapotranspiration maps showed a fluctuant trend as spatial resolution of input parameter data layers coarsening, and the absolute value of relative errors reached minimum when pixel size of map matched up to measuring scale of eddy covariance system. Finally, based on the analyses of the semi‐variogram of the original resolution evapotranspiration map and the shapes of spatial autocorrelation indices of Moran and Geary for evapotranspiration maps at different resolutions, an appropriate resolution was suggested for the areal evapotranspiration simulation in this study area. [ABSTRACT FROM AUTHOR]

Elucidating aboveground net primary production (ANPP) and precipitation-use efficiency (PUE) spatial variations and mechanisms are essential for predicting how ecosystem functioning will respond to future climate change. However, a comprehensive recognition of spatial patterns of ANPP and PUE across continental scale is still lacking. Here, we integrated long-term GIMMS NDVI remote sensing, field survey ANPP and meteorological datasets to reveal the spatial variations and controls of ANPP and PUE across Eurasian grasslands for the first time. The results showed that the mean value of ANPP and PUE of Eurasian grasslands were 40.20 ± 0.40 g C m−2 yr−1 and 0.15 ± 0.01 g C m−2 mm−1, respectively. At the continental scale, the ANPP and PUE showed unimodal patterns along mean annual precipitation (MAP) and hydrothermal index (HT) gradients, while a piecewise linear pattern along mean annual temperature (MAT) gradients. The MAP exerted positive effect on the ANPP in desert and temperate grasslands, while negative effect on the ANPP in alpine grasslands. Conversely, the MAT negatively affected the ANPP in desert and temperate grasslands, while positively affected the ANPP in alpine grasslands. The results indicated that the hydrothermal conditions coupled with the transition of vegetation types and its different responses combinedly shaped the spatial patterns of ANPP and PUE in Eurasian grasslands. This study advanced our knowledge of the spatial variations of ANPP and PUE at continental scale, providing theoretical information for predicting productivity and water use changes of arid and semi-arid grasslands under climate change in the future. Unlabelled Image • Mean ANPP was 40.2 g C m−2 yr−1 and PUE was 0.15 g C m−2 mm−1 in Eurasian grasslands. • ANPP and PUE showed unimodal pattern with MAP and HT, and piecewise linear with MAT. • MAP and MAT dominated ANPP and PUE in desert and alpine grasslands, respectively. • Hydrothermal conditions coupled with vegetation shaped ANPP and PUE spatial pattern. [ABSTRACT FROM AUTHOR]

• α GPP , integrated index of GPP seasonality, was ratio of mean to maximum daily GPP. • α GPP in Northern Hemisphere ranged from 0.47 to 0.85 with an average of 0.62. • α GPP decreased as latitude increased, and was stable between years. • Spatial pattern of astronomical radiation seasonality affected that of α GPP. • Spatiotemporal variation of α GPP provided a new approach to assess annual GPP. The seasonal dynamic of gross primary productivity (GPP) has influences on the annual GPP (AGPP) of the terrestrial ecosystem. However, the spatiotemporal variation of the seasonal dynamic of GPP and its effects on spatial and temporal variations of AGPP are still poorly addressed. In this study, we developed a parameter, α GPP , defined as the ratio of mean daily GPP (GPP mean) to the maximum daily GPP (GPP max) during the growing season, to analyze the seasonal dynamic of GPP based on Weibull function. The α GPP was a comprehensive parameter characterizing the shape, scale, and location of the seasonal dynamic curve of GPP. We calculated α GPP based on the data of GPP for 942 site-years from 115 flux sites in the Northern Hemisphere, and analyzed the spatiotemporal variation and influencing factors of the α GPP. We found that the α GPP of terrestrial ecosystems in the Northern Hemisphere ranged from 0.47 to 0.85, with an average of 0.62 ± 0.06. The α GPP varied significantly both among different climatic zones and different ecosystem types. The α GPP was stable on the interannual scale, while decreased as latitude increased, which was consistent across different ecosystem types. The spatial pattern of the seasonal dynamic of astronomical radiation was the dominating factor of the spatial pattern of α GPP , that was, the spatial pattern of the seasonal dynamic of astronomical radiation determined that of the seasonal dynamic of GPP by controlling that of seasonal dynamics of total radiation and temperature. In addition, we assessed the spatial variation of AGPP preliminarily based on α GPP and other seasonal dynamic parameters of GPP, indicating that the understanding of the spatiotemporal variation of α GPP could provide a new approach for studying the spatial and temporal variations of AGPP and estimating AGPP based on the seasonal dynamic of GPP. [ABSTRACT FROM AUTHOR]

Measurements of greenhouse gas fluxes over many ecosystems have been made as part of the attempt to quantify global carbon and nitrogen cycles. In particular, annual flux observations are of great value for regional flux assessments, as well as model development and optimization. The chamber method is a popular approach for soil/ecosystem respiration and CH4 flux observations of terrestrial ecosystems. However, in situ flux chamber measurements are usually made with non-continuous sampling. To date, efficient methods for the application of such sporadic data to upscale temporally and obtain annual cumulative fluxes have not yet been determined. To address this issue, we tested the adequacy of non-continuous sampling using multi-source data aggregation. We collected 330 site-years monthly soil/ecosystem respiration and 154 site-years monthly CH4 flux data in China, all obtained using the chamber method. The data were randomly divided into a training group and verification group. Fluxes of all possible sampling months of a year, i.e., 4094 different month combinations were used to obtain the annual cumulative flux. The results showed a good linear relationship between the monthly flux and the annual cumulative flux. The flux obtained during the warm season from May to October generally played a more important role in annual flux estimations, as compared to other months. An independent verification analysis showed that the monthly flux of 1 to 4 months explained up to 67%, 89%, 94%, and 97% of the variability of the annual cumulative soil/ecosystem respiration and 92%, 99%, 99%, and 99% of the variability of the annual cumulative CH4 flux. This study supports the use of chamber-observed sporadic flux data, which remains the most commonly-used method for annual flux estimating. The flux estimation method used in this study can be used as a guide for designing sampling programs with the intention of estimating the annual cumulative flux. [ABSTRACT FROM AUTHOR]

• Light response parameters showed significant spatial heterogeneity over China. • Spatial pattern of GPP was significantly regulated by light response parameters. • The large spatial variation of A max in China was dominated by moisture conditions. • α is more stable and minor hydrothermal controls than A max , and mainly controlled by LAI and PAR. • Dominant factors of light response parameters varied with ecosystem types. Light response parameters are used to model the photosynthetic processes of ecosystems, but they are usually considered as fixed or empirical values for specific vegetation type and their geographical variation and influence mechanisms at ecosystem scale remain unclear. In this study, we analyzed the light response parameters (photosynthetic quantum yields, α ; maximum gross photosynthetic rate, A max) at 64 typical ecosystems of ChinaFLUX over 20 years' measurement, to explore their spatial patterns and controlling factors. The results showed that the light response parameters significantly varied with ecosystem types: in grassland and desert ecosystems, both α and A max were significantly lower than other ecosystems, and in cropland ecosystems, the A max was significantly higher than that of other ecosystems. The light response parameters significantly decreased with increasing altitude. Vegetation index and photosynthetically active radiation were the dominant factor for the spatial variation of α , while the spatial variation of A max was mainly controlled by moisture conditions. Compared to A max , α was more stable which varied less with hydrothermal conditions. The dominant factors of light response parameters also differed among ecosystem types. Hydrothermal conditions regulated vegetation indexes and further modulate light response parameters and GPP. This study suggested that the quantitative analysis and simulation prediction of photosynthesis should consider the spatial heterogeneity of light response parameters and their different environmental responses at ecosystem scale in future studies. [ABSTRACT FROM AUTHOR]

Global warming has significantly altered the distribution and productivity of vegetation owing to shifts in plant functional traits. However, chlorophyll adaptations—good representative of plant production—in grasslands have not been investigated on a large scale, hindering ecological predictions of climate change. Three grassland transects with a natural temperature gradient were designed in the Tibetan, Mongolian, and Loess Plateau to describe the changes in chlorophyll under different warming scenarios for 475 species. In the three plateaus, variations and distributions of species chlorophyll concentration and composition were compared. The results showed that the means of chlorophyll concentration and composition (chlorophyll a/b) increased with the mean annual temperature. Still, their distributions shifted in opposite manners: chlorophyll concentration was distributed in a broader but more differential manner, while chlorophyll composition was distributed in a narrower but more uniform manner. Compared to chlorophyll concentration, chlorophyll composition was more conservative, with a slight shift in distribution. At the regional level, the chlorophyll concentration and composition depend on the limitations of the local climate or resources. The results implied that warming might drive shifts in grassland chlorophyll distribution mainly by alternations in species composition. Large-scale chlorophyll investigations will be useful for developing prediction techniques. [ABSTRACT FROM AUTHOR]

• New expanded dataset was used to assess ecosystem soil respiration of China. • Increased grassland Rs contributed the most to the overall rise of Rs in China. • Increasing precipitation caused an increase in soil respiration in grasslands. • The temporal response of Rs to environment differed from that on spatial scale. In the context of increasing CO 2 concentrations and climate warming, there are large uncertainties regarding the regulatory mechanisms of soil respiration and its accurate assessment. As an area sensitive to global climate change, China covers a broad range of longitudes, latitudes, and altitudes, and it is of great significance to study the spatial and temporal patterns of soil respiration and its environmental response in China's terrestrial ecosystems to clarify the mechanisms of regional and global carbon balance processes. Based on the expanded annual soil respiration dataset from 1993 to 2020, we evaluated soil respiration in China's terrestrial ecosystems and its responses to environmental factors. The mean annual soil respiration in China's terrestrial ecosystems was 777.90 ± 489.43 g C m −2 yr−1 by arithmetic averaging and 595.52 g C m −2 yr−1 by area weighting, with soil respiration in wetlands being higher than that of the forest, grassland, shrubland, cropland, and desert. Soil respiration showed a significant upward trend from 2000 to 2017, which was mainly caused by a significant increase in soil respiration in grasslands due to increasing precipitation. In addition, we found differences in the environmental factors influencing the interannual and spatial variability of soil respiration. The response sensitivities of soil respiration to environmental changes differed significantly among the various ecosystem types; however, these differences were not significant when comparing interannual and spatial scales. This study provides information on soil respiration in China's terrestrial ecosystem and highlights the importance of considering the two-dimensional responses of soil respiration to environmental changes in both time and space. [ABSTRACT FROM AUTHOR]