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The terrestrial methane budget varies between different vegetation types and soil conditions and is highly uncertain for alpine grasslands. This work used eddy covariance techniques to continuously measure CH4 flux (NEEm) over a semiarid alpine meadow on the northeastern Qinghai-Tibetan Plateau from January 2017 to August 2019. The diel NEEm averaged 0.14 ± 0.98 nmol CH4 m−2 s−1 (mean ± S.D.), with a rough pattern of daytime release and nocturnal uptake. The 8-day NEEm exhibited a similar sinusoid variation, with a peak of 6.8 mg CH4 m−2 d−1 at the end of April and a minimum of −1.5 mg CH4 m−2 d−1 at the end of August. The maximum release probably coincided with the thawing of frozen soil in the root zone, and the peak uptake may be related to high soil temperature. Monthly CH4 uptake was highest from June to September and consumed 51.7 mg CH4 m−2 from the atmosphere. CH4 production in the other months totaled 647.6 mg CH4 m−2. The semiarid alpine meadow thus acted as a weak net CH4 source, releasing ca 0.6 g CH4 m−2 year−1 to the atmosphere. The boosted regression trees analysis shows that the sensible heat flux (H) is positively related to half-hour NEEm and accounted for 34% of its variability. The piecewise structural equation models reveal that the magnitude of the effects from soil temperature and vapor pressure deficit on 8-day and monthly NEEm were almost equal, but acted in opposite directions. Vegetation growth and soil moisture exerted little direct influence on NEEm variability at half-hour, 8-day, or monthly scales. Our results show that CH4 emissions of the nongrowing season dominate the annual methane budget for this alpine meadow area. Methane consumption during the growing season was significantly constrained by low soil temperature and high soil water content. These findings imply that semiarid alpine meadows may consume more methane during the growing season if soil temperatures increase and soil moisture levels decrease as projected by future warming scenarios, thus constituting a climate change negative feedback. [ABSTRACT FROM AUTHOR]

Biomass temporal stability plays a key role in maintaining sustainable ecosystem functions and services of grasslands, and climate change has exerted a profound impact on plant biomass. However, it remains unclear how the community biomass stability in alpine meadows responds to changes in some climate factors (e.g., temperature and precipitation). Long‐term field aboveground biomass monitoring was conducted in four alpine meadows (Haiyan [HY], Henan [HN], Gande [GD], and Qumalai [QML]) on the Qinghai‐Tibet Plateau. We found that climate factors and ecological factors together affected the community biomass stability and only the stability of HY had a significant decrease over the study period. The community biomass stability at each site was positively correlated with both the stability of the dominant functional group and functional groups asynchrony. The effect of dominant functional groups on community stability decreased with the increase of the effect of functional groups asynchrony on community stability and there may be a 'trade‐off' relationship between the effects of these two factors on community stability. Climatic factors directly or indirectly affect community biomass stability by influencing the stability of the dominant functional group or functional groups asynchrony. Air temperature and precipitation indirectly affected the community stability of HY and HN, but air temperature in the growing season and nongrowing season had direct negative and direct positive effects on the community stability of GD and QML, respectively. The underlying mechanisms varied between community composition and local climate conditions. Our findings highlighted the role of dominant functional group and functional groups asynchrony in maintaining community biomass stability in alpine meadows and we highlighted the importance of the environmental context when exploring the stability influence mechanism. Studies of community stability in alpine meadows along with different precipitation and temperature gradients are needed to improve our comprehensive understanding of the mechanisms controlling alpine meadow stability. [ABSTRACT FROM AUTHOR]

Aims Variations in vegetation spring phenology are widely attributed to temperature in temperate and cold regions. However, temperature effect on phenology remains elusive in cold and arid/semiarid ecosystems because soil water condition also plays an important role in mediating phenology. Methods We used growing degree day (GDD) model and growing season index (GSI) model, coupling minimum temperature (T min) with soil moisture (SM) to explore the influence of heat requirement and hydroclimatic interaction on the start of carbon uptake period (SCUP) and net ecosystem productivity (NEP) in two alpine meadows with different precipitation regimes on the Qinghai-Tibet Plateau (QTP). One is the water-limited alpine steppe-meadow, and the other is the temperature-limited alpine shrub-meadow. Important Findings We observed two clear patterns linking GDD and GSI to SCUP: SCUP was similarly sensitive to variations in preseason GDD and GSI in the humid alpine shrub-meadow, while SCUP was more sensitive to the variability in preseason GSI than GDD in the semiarid alpine steppe-meadow. The divergent patterns indicated a balance of the limiting climatic factors between temperature and water availability. In the humid meadow, higher temperature sensitivity of SCUP could maximize thermal benefit without drought stress, as evidenced by the stronger linear correlation coefficient (R 2) and Akaike's information criterion (AIC) between observed SCUPs and those of simulated by GDD model. However, greater water sensitivity of SCUP could maximize the benefit of water in semiarid steppe-meadow, which is indicated by the stronger R 2 and AIC between observed SCUPs and those of simulated by GSI model. Additionally, although SCUPs were determined by GDD in the alpine shrub-meadow ecosystem, NEP was both controlled by accumulative GSI in two alpine meadows. Our study highlights the impacts of hydroclimatic interaction on spring carbon flux phenology and vegetation productivity in the humid and semiarid alpine ecosystems. The results also suggest that water, together with temperature should be included in the models of phenology and carbon budget for alpine ecosystems in semiarid regions. These findings have important implications for improving vegetation phenology models, thus advancing our understanding of the interplay between vegetation phenology, productivity and climate change in future. [ABSTRACT FROM AUTHOR]

Aims The Qinghai-Tibetan Plateau has a mean altitude exceeding 4000 m and covers about 2.5 million km2. More than 60% of this area is alpine grassland. Exclosures have been widely used in this region to study the sustainable use of grassland resources. We used patterns of functional trait diversity to infer the effects of exclosures on community assembly in alpine meadows. Methods We studied functional diversity using five traits under grazing and three enclosed (exclosure) plots (3, 8, and 18 years old) in an alpine meadow on the Qinghai-Tibetan Plateau. We quantified the strength of the community assembly processes by comparing the observed functional trait diversity with a null model that assumes random community assembly. Important findings We found evidence for deterministic assembly processes for plant communities in exclosures. The changes in CWM of the five traits from grazing land to 18-year exclosure indicated that environmental filtering occurred due to the exclosures. Multivariate functional diversity (MFDis and MPDses), and functional diversity of individual traits, including that of leaf area, leaf weight and aboveground biomass (FDis of leaf area, leaf weight, and aboveground biomass), increased gradually from grazing land to the 18-year exclosure, and the values of the 18-year exclosure were significantly greater than null expectation. This can be interpreted to indicate that exclosures resulted in greater competitive interaction between species. These results suggest that the effect of exclosures on community assembly is more deterministic than stochastic in this meadow. [ABSTRACT FROM AUTHOR]

Plants' pattern of compensatory growth is often used to intuitively estimate their grazing tolerance. However, this tolerance is sometimes measured by the overall grazing tolerance index (overall GTI), which assumes that tolerance is a multivariate linear function of various underlying mechanisms. Because the interaction among mechanisms is not independent, the grazing tolerance expression based on overall GTI may be inconsistent with that based on compensatory growth. Through a manipulative field experiment from 2007 to 2012, we measured the responses of 12 traits of Elymus nutans to clipping under different resource availabilities in an alpine meadow and explored the compensatory aboveground biomass and the overall GTI to assess the possible differences between the two expressions of tolerance. Our results showed that these two expressions of tolerance were completely opposite. The expression based on overall GTI was over‐compensatory and did not vary with clipping and resource availability, while the expression based on compensatory aboveground biomass was under‐compensatory and altered to over‐compensation after fertilization. The over‐expression of highly variable traits with extremely high negative mean GTI to defoliation damage, the influence of random errors contained in traits considered, and the doubling weight of functional redundant traits greatly inflated the overall GTI, which leads to the inconsistency of the two tolerance expressions. This inconsistency is also associated with the different determining mechanisms of the two tolerance expressions. Our data suggest that plants' grazing tolerance is not a multivariate linear function of traits or mechanisms that determine grazing tolerance; the overall GTI is only a measure of traits' variability to defoliation damage. Our findings highlight that the tolerance of E. nutans mainly depends on the response of traits with lower variability to defoliation, and the overall GTI is not an ideal predictor for describing a single‐species tolerance to grazing. [ABSTRACT FROM AUTHOR]

The artificial grassland is one of the most important practices to restore degraded alpine meadow. However, it is still not clear how the water conservation capacity respond along the restoration duration. We conducted the experiment for soil water holding capacity along the restoration duration of 0 year (extremely degradation, CK), 2 years (2Y), 4 years (4Y), 6 years (6Y) and 12 years (12Y), as well as light degradation (CK1). Along the restoration duration, aboveground biomass, soil porosity, soil organic carbon, and total nitrogen increased while soil bulk density decreased. The saturated soil water holding capacity increased and plateaued at 6Y restoration duration. The total porosity and soil bulk density played a predominated role in the soil water-holding capacity variations. Our results revealed that the asymptotic response of soil water-holding capacity to restoration duration in artificial grasslands and indicated that 6-year restoration would be appropriate in rehabilitating water conservation for extremely degraded alpine meadow. [ABSTRACT FROM AUTHOR]

Abstract: Accurately quantifying evapotranspiration (ET) is crucial to fully understanding regional water resource management and potential feedbacks to climate change in alpine grasslands. The quantitative relationships between ET and environmental controls were investigated by a continuous eddy covariance dataset from June 2014 to December 2016 over an alpine Kobresia meadow on the northeastern Qinghai‐Tibetan Plateau. The results showed that daily ET averaged 1.7± 1.5 mm/day (Mean ± 1 S.D.), with values of 2.9 ± 1.3, 1.6 ± 1.0, and 0.7± 0.6 mm/day during the growing season, seasonal transition period, and nongrowing season, respectively. Cumulative growing season ET was 63% of annual ET with little annual variability (349.9 ± 12.1 mm). Paired‐samples t‐test analysis indicated that monthly ET was larger than maximum potential ET derived from the FAO‐56 reference crop ET by 17% (p < .001, N = 12) in the growing season, likely because of high aerodynamic conductance, but was less than the minimum equilibrium ET by 19% (p < .001, N = 14) during the nongrowing season owing to limited surface moisture availability from the frozen soil. The structural equation models revealed that daily ET was mostly dominated by net radiation (the standardized coefficient of the total effect was 0.78). Soil surface moisture and leaf area index played secondary roles in daily ET variability during the nongrowing season and growing season, respectively. At an annual scale, the bulk surface conductance (8.25–10.65 mm/s), decoupling coefficient (0.43–0.48, 0.61 in the growing season), and the ratio of ET to equilibrium ET (1.08–1.33) were consistent with the strongly energy‐limited conditions in the alpine meadow. This study indicated that initial vegetation rehabilitation on the severely degraded meadow would be at the risk of rapid water consumption in humid alpine regions. [ABSTRACT FROM AUTHOR]

Fencing is an important method for restoring and reconstructing degraded grasslands on the Tibetan Plateau. Understanding the characteristics of soil water percolation and soil dissolved organic carbon (DOC) leaching and their response to long-term fencing (10 years) could provide a scientific basis for the management of soil water resources and carbon sinks in alpine grasslands. In this study, grazing plots and plots fenced for 10 years were selected in an alpine meadow, and soil bulk density, soil organic carbon (SOC) density, vegetative carbon (VC) density, soil water percolation and DOC leaching were monitored regularly in the two treatments. The results were as follows: (1) Long-term fencing reduced the soil bulk density and improved carbon sequestration in the alpine meadow. The soil bulk density at a depth of 0-20 cm in the fenced plots was significantly lower than that of the grazing plots ( p < 0.05), and SOC density and VC density at a depth of 0-40 cm were higher than those of the grazing plots to a different degree. (2) During the non-frozen period (from May to September), total soil water leakage at a depth of 40 cm in the grazing plots was 9.6 mm, which accounted for 2.2% of the total rainfall in this period. After fencing for 10 years, soil water leakage was increased by 53.1%. (3) From May to September, total DOC leaching from the grazing plots was 34.6 gC m, and total DOC leaching from the fenced plots was increased by 55.5% compared to the grazing plots. Our results demonstrated obvious soil water percolation and DOC leaching in the alpine meadow. The DOC entered into the groundwater system with the percolation water and finally flowed into lakes and rivers, which represents an important soil carbon loss pathway in alpine meadows. [ABSTRACT FROM AUTHOR]

Aims: To assess the effects of atmospheric N deposition on the C budget of an alpine meadow ecosystem on the Qinghai-Tibetan Plateau, it is necessary to explore the responses of soil-atmosphere carbon dioxide (CO) exchange to N addition. Methods: Based on a multi-form, low-level N addition experiment, soil CO effluxes were monitored weekly using the static chamber and gas chromatograph technique. Soil variables and aboveground biomass were measured monthly to examine the key driving factors of soil CO efflux. Results: The results showed that low-level N input tended to decrease soil moisture, whereas medium-level N input maintained soil moisture. Three-year N additions slightly increased soil inorganic N pools, especially the soil NH-N pool. N applications significantly increased aboveground biomass and soil CO efflux; moreover, this effect was more significant from NH-N than from NO-N fertilizer. In addition, the soil CO efflux was mainly driven by soil temperature, followed by aboveground biomass and NH-N pool. Conclusions: These results suggest that chronic atmospheric N deposition will stimulate soil CO efflux in the alpine meadow on the Qinghai-Tibetan Plateau by increasing available N content and promoting plant growth. [ABSTRACT FROM AUTHOR]

The meadow ecosystem on the Qinghai-Tibetan Plateau is considered to be sensitive to climate change. An understanding of the alpine meadow ecosystem is therefore important for predicting the response of ecosystems to climate change. In this study, we use the coefficients of variation (Cv) and stability (E) obtained from the Haibei Alpine Meadow Ecosystem Research Station to characterize the ecosystem stability. The results suggest that the net primary production of the alpine meadow ecosystem was more stable (Cv = 13.18%) than annual precipitation (Cv = 16.55%) and annual mean air temperature (Cv = 28.82%). The net primary production was insensitive to either the precipitation (E = 0.0782) or air temperature (E= 0.1113). In summary, the alpine meadow ecosystem on the Qinghai- Tibetan Plateau is much stable. Comparison of alpine meadow ecosystem stability with other five natural grassland ecosystems in Israel and southern African indicates that the alpine meadow ecosystem on the Qinghai-Tibetan Plateau is the most stable ecosystem. The alpine meadow ecosystem with relatively simple structure has high stability, which indicates that community stability is not only correlated with biodiversity and community complicity but also with environmental stability. An average oscillation cycles of 3-4 years existed in annual precipitation, annual mean air temperature, net primary production and the population size of consumers at the Haibei natural ecosystem. The high stability of the alpine meadow ecosystem may be resulting also from the adaptation of the ecosystem to the alpine environment. [ABSTRACT FROM AUTHOR]

Using the eddy covariance method, from 1 July 2003 to 30 June 2004, we conducted the observation and analysis of ecosystem CO2 flux in 3 types of alpine meadow vegetation (Kobresia humilis, Potentilla fruticosa shrub and Kobresia tibetica swamp meadows) on the Qinghai-Tibetan Plateau. The results show that the Kobresia humilis meadow, the shrub meadow and the swamp meadow's highest CO2 uptake rates are 16.78, 10.42 and 16.57 µmol·m-2·s-1 respectively, while their highest CO2 release rates are 8.22, 7.73 and 18.67 µmol·m-2·s-1 respectively. The Kobresia humilis meadow and shrub meadow's annual atmospheric uptakes are 282 g CO2/m² and 53 g CO2/m², respectively, while swamp meadow's annual atmospheric release is 478 g CO2/m². This proves that the Kobresia humilis meadow and the shrub meadow on the Qinghai-Tibetan Plateau have relatively low potential for CO2 uptake and release compared to C4 grasslands, a number of lowland grasslands, and forests. Moreover, swamp meadow has relatively high release potential. This, in turn, reveals clear differences in carbon source/sink between different types of vegetation in the Qinghai-Tibetan Plateau alpine meadow ecosystem. These differences are mainly brought by differences in the vegetations' photosynthetic capacity and soil respiration. [ABSTRACT FROM AUTHOR]

Soil seepage is an important component used for quantifying hydrological processes that remains unclear in high-altitude alpine meadows. Shallow soil seepage was continuously measured using an auto-logged micro-lysimeter (diameter = 30 cm, depth = 30 cm) from July 2018 to June 2019 in a piedmont summer pasture of alpine meadow on the Northeastern Qinghai–Tibetan Plateau. The results showed that all the shallow soil seepage events occurred during the non-frozen period from April to September and that the cumulative amount was 106.8 mm, representing about 1/5 of the annual precipitation. The maximum and minimum monthly soil seepage were 30.7 mm in September and 1.0 mm in April, respectively. The boosted regression trees (BRT) model's area under the curve averaged 0.92 and revealed that the daily half-hour rainfall frequency, volumetric soil water content, and air temperature played significant roles in the daily soil seepage probability, with the cumulative relative contribution of 68%. The stepwise linear regression analysis showed that the rainfall amount accounted for 59% of the variation in the daily amount of soil seepage. The monthly soil seepage was found to be significantly correlated with the monthly rainfall frequency (r = 0.86, p = 0.005). Our results highlighted that rainfall, including its amount and frequency, was the key determinant of the probability and amount of shallow soil seepage in the piedmont summer pasture of alpine meadows. These findings will be helpful for improving predictions of the water budgets of piedmont alpine meadows. [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]

Shifts in soil microbial community composition along elevational gradients have been extensively studied; yet, the assembly process and co-occurrence network of soil microbes underlying these shifts remain to be explored, especially for diazotrophs that contribute to the most nitrogen input in natural ecosystems. In this study, we examined how assembly processes and co-occurrence networks of soil diazotrophs changed along an elevational gradient with four elevation levels ranging from 3200 m to 3800 m using high-throughput sequencing of the nifH gene in Tibetan alpine meadows. Our analyses showed that α-diversity of soil diazotrophs was lower at low elevations (i.e., 3200 m and 3400 m) relative to high elevations (i.e., 3600 m and 3800 m). Moreover, dominant diazotrophic phyla shifted from Actinobacteria at low elevations to Proteobacteria at high elevations. The change in diazotrophic community composition along the elevation gradient was related to the differences in soil temperature, moisture, NH 4 +-N concentration and pH. Species rank-abundance distribution models showed that soil diazotrophic communities were mainly assembled by deterministic processes at 3200 m, 3600 m and 3800 m elevations, and by stochastic processes at the 3400 elevation. Additionally, the complexity of diazotrophic co-occurrence networks increased with increasing elevations. Collectively, our findings highlight the change in soil diazotrophic communities along the elevation gradient and the high environmental sensitivity of soil diazotrophs, and suggest that global change could have consequences for soil diazotrophic community composition and structure in montane ecosystems. [Display omitted] • Soil diazotrophic diversity was lower at low elevations than high elevations. • Dominant phyla shifted from Actinobacteria to Proteobacteria with increasing elevations. • Soil temperature explained 42% of variation in diazotrophic community composition. • Diazotrophic communities were mainly assembled by deterministic processes at high elevations. • Diazotrophic co-occurrence networks became more complex with increasing elevations. [ABSTRACT FROM AUTHOR]

The impact of global climate change on precipitation regimes may bring about significant consequences to the productivity of grassland ecosystems. This study reports the effects of the characteristics of precipitation events (PEC, e.g., size, frequency, interval, and seasonal distribution of precipitation events) on gross primary productivity (GPP) based on long-term measurements of net ecosystem CO 2 exchange and evapotranspiration by the eddy covariance technique in a water-limited temperate steppe in Inner Mongolia and a humid but temperature-limited alpine meadow in Tibet, China. We determined that the predominant characteristics of precipitation events that affected GPP differed remarkably between these two ecosystems. The number of heavy precipitation events (>10 mm day −1 ) was the most important PEC to impact GPP in the temperate steppe. Years with more frequent heavy precipitation events favored higher GPP. However, in contrast with the case in the temperate steppe, precipitation interval was the factor that affected GPP most in the alpine meadow. GPP was higher as the precipitation intervals lengthened or their distribution shifted to fewer but longer intervals (concentrated distribution). Additionally, the mechanisms for the effects of PEC on GPP also differed between these two ecosystems. Compared with small events, heavy precipitation events recharged deeper soil layers in the temperate steppe, which was biologically more meaningful for plant transpiration (i.e., an increase in the ratio of transpiration to evapotranspiration), relief from drought stress (an increase in the duration of high soil water content), and thereby higher GPP. In contrast, mean air temperature was higher in the alpine meadow when the precipitation intervals increased or their distribution was more concentrated, leading to a longer period of higher temperature that was favorable to higher GPP. Our results imply that soil water is not the exclusive means of revealing the mechanism of precipitation affecting productivity; the corresponding changes in temperature along with precipitation events may also play a substantial role in temperature-limited grasslands. [ABSTRACT FROM AUTHOR]

Highlights: [•] N dose rather than N form significantly changed quantity and quality of soil DOC. [•] The responses of soil DOC amount and composition to N addition depend on soil layer. [•] Changes in microbial decomposition and humification processes dominate DOC dynamics. [•] Increasing atmospheric N deposition could decrease soil C storage in the alpine meadow. [Copyright &y& Elsevier]

Abstract: It is crucial to understand the effects of enhanced nitrogen (N) deposition on soil methane (CH4) uptake to develop a better comprehension of carbon (C) dynamics in terrestrial ecosystems. A two-year field study was conducted to assess the effects of various forms of N (NH4 + and NO3 −) and associated N deposition rates (0, 10, 20 and 40kgNha−1 yr−1) on alpine meadow soil CH4 fluxes on the Qinghai–Tibetan Plateau, China. Soil CH4 fluxes, soil temperature, and soil moisture were monitored weekly using the static chamber technique and gas chromatography. Soil inorganic N pools, soil pH and aboveground biomass were measured monthly to examine the key controlling factors of soil CH4 flux. Our results showed that N addition significantly promoted plant growth and changed soil water-filled pore space (WFPS), but did not alter soil inorganic N storages over the short term. Low rates of N addition significantly decreased the seasonal amount of CH4 uptake by 8.6% compared with the control. Soil CH4 fluxes were mainly determined by soil WFPS, followed by inorganic N availability. N addition increased the contribution of soil WFPS, pH and soil NO3 − storage. The observed reduction in CH4 uptake caused by N addition may be largely due to a decrease in physical diffusion, as the biochemical inhibition effects on methanotrophic bacteria are minor. These results suggest that soil inorganic N is a regulatory factor of soil CH4 uptake, and its promotion or inhibition to soil CH4 uptake depends on the N status in terrestrial ecosystems. [Copyright &y& Elsevier]

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: To assess the impacts of yak excreta patches on greenhouse gas (GHG) fluxes in the alpine meadow of the Qinghai-Tibetan plateau, methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) fluxes were measured for the first time from experimental excreta patches placed on the meadow during the summer grazing seasons in 2005 and 2006. Dung patches were CH4 sources (average 586μgm−2 h−1 in 2005 and 199μgm−2 h−1 in 2006) during the investigation period of two years, while urine patches (average −31μgm−2 h−1 in 2005 and −33μgm−2 h−1 in 2006) and control plots (average −28μgm−2 h−1 in 2005 and −30μgm−2 h−1 in 2006) consumed CH4. The cumulative CO2 emission for dung patches was about 36–50% higher than control plots during the experimental period in 2005 and 2006. The cumulative N2O emissions for both urine and dung patches were 2.1–3.7 and 1.8–3.5 times greater than control plots in 2005 and 2006, respectively. Soil water-filled pore space (WFPS) explained 35% and 36% of CH4 flux variation for urine patches and control plots, respectively. Soil temperature explained 40–75% of temporal variation of CO2 emissions for all treatments. Temporal N2O flux variation in urine patches (34%), dung patches (48%), and control (56%) plots was mainly driven by the simultaneous effect of soil temperature and WFPS. Although yak excreta patches significantly affected GHG fluxes, their contributions to the whole grazing alpine meadow in terms of CO2 equivalents are limited under the moderate grazing intensity (1.45yakha−1). However, the contributions of excreta patches to N2O emissions are not negligible when estimating N2O emissions in the grazing meadow. In this study, the N2O emission factor of yak excreta patches varied with year (about 0.9–1.0%, and 0.1–0.2% in 2005 and 2006, respectively), which was lower than IPCC default value of 2%. [Copyright &y& Elsevier]

• The CO 2 sink was strong in the alpine meadows located in east of the plateau (∼200 g cm−2 y−1). • The carbon fluxes of the water-adequate ecosystems were mainly controlled by soil temperature. • Surface soil moisture explained 48% of the variations in spatial variations NEE. The magnitude and spatial patterns of carbon fluxes in alpine grasslands determine the regional terrestrial carbon balance of the Tibetan Plateau. However, the patterns and controlling factors of carbon fluxes on the plateau remain unclear, hampering the understanding of the carbon cycle of these vulnerable ecosystems. In this study, we compared the spatial variations of carbon fluxes of ten alpine ecosystems with diverse grassland types and explored their environmental controls across these different ecosystems. Our results show that the mean annual net ecosystem exchange (NEE) of carbon dioxide (CO 2) varied from -284 to 31 g C m−2 across sites. The alpine meadow ecosystems in the northeast and east of the plateau were strong CO 2 sinks (∼200 g C m−2 y−1), while the western alpine grasslands were weak CO 2 sinks or even sources. During the growing season, soil temperature generally played the dominant role in regulating the daily variations of the carbon fluxes for the alpine meadow ecosystems in the cold and humid northeastern areas, while soil moisture was the main con-trolling factor for the alpine grassland ecosystems in the dry western areas. Annual gross primary productivity (GPP), ecosystem respiration (Re) and the carbon sink capacity linearly increased with the increasing longitude but linearly decreased with elevation. The spatial pattern of annual NEE was primarily controlled by surface soil moisture, and higher soil water content (SWC) led to greater carbon sink capacity. SWC, vapor pressure deficit (VPD) also had favorable effects on the annual GPP and Re. The spatial variations of carbon fluxes resulted primarily from the longitudinal or altitudinal variations of the dominant environmental factors. This study provides guidance for the assessment of carbon fluxes on the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

• The CO 2 sink was strong in the alpine meadows located in east of the plateau (∼x223C 200 g C m-2 y-1). • The carbon fluxes of the water-adequate ecosystems were mainly controlled by soil temperature. • Surface soil moisture explained 48% of the variations in spatial variations NEE. The magnitude and spatial patterns of carbon fluxes in alpine grasslands determine the regional terrestrial carbon balance of the Tibetan Plateau. However, the patterns and controlling factors of carbon fluxes on the plateau remain unclear, hampering the understanding of the carbon cycle of these vulnerable ecosystems. In this study, we compared the spatial variations of carbon fluxes of ten alpine ecosystems with diverse grassland types and explored their environmental controls across these different ecosystems. Our results show that the mean annual net ecosystem exchange (NEE) of carbon dioxide (CO 2) varied from -284 to 31 g C m−2 across sites. The alpine meadow ecosystems in the northeast and east of the plateau were strong CO 2 sinks (∼x223C 200 g C m−2 y−1), while the western alpine grasslands were weak CO 2 sinks or even sources. During the growing season, soil temperature generally played the dominant role in regulating the daily variations of the carbon fluxes for the alpine meadow ecosystems in the cold and humid northeastern areas, while soil moisture was the main controlling factor for the alpine grassland ecosystems in the dry western areas. Annual gross primary productivity (GPP), ecosystem respiration (Re) and the carbon sink capacity linearly increased with the increasing longitude but linearly decreased with elevation. The spatial pattern of annual NEE was primarily controlled by surface soil moisture, and higher soil water content (SWC) led to greater carbon sink capacity. SWC, vapor pressure deficit (VPD) also had favorable effects on the annual GPP and Re. The spatial variations of carbon fluxes resulted primarily from the longitudinal or altitudinal variations of the dominant environmental factors. This study provides guidance for the assessment of carbon fluxes on the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

• ET was primarily radiatively driven at the two humid alpine sites. • Water supply was both controlled by precipitation with a unity slope. • Higher bulk surface resistance accounted for lower ET and higher water yields at the upper shrub. Alpine regions are generally referred to as water towers for the lowlands, yet the water balance has not been quantified for alpine catchments with different vegetation types. This paper presented a multi-year time series of evapotranspiration (ET) and water supply (precipitation minus ET) during the warm-season (June to September), measured using eddy covariance techniques for an upper alpine shrub (3400 m) and for a lower alpine meadow (3200 m), on the northeastern Qinghai-Tibetan Plateau. The monthly ET for the shrub averaged 71.8 ± 10.1 mm (Mean ± SD), which was 22.7% lower than for the meadow. ET peaked at 89.0 ± 3.0 mm for the shrub and at 113.9 ± 2.6 mm for the meadow both in July. The monthly water supply was nearly neutral from June to July and represented a surplus in August and September at both sites. The mean warm-season ET and water supply were 287.0 ± 17.6 mm and 59.6 ± 16.3 mm for the shrub, and 352.2 ± 15.7 mm and 22.5 ± 32.5 mm for the meadow, respectively. Piecewise structural equation models showed that variability of daily ET was dominated more by net radiation (R n) than by vapor pressure deficit (VPD) at both sites. However, net radiation was 32.6% higher for the shrub than for the meadow, but this did not drive higher ET for the shrub. This was explained by the difference in the energy partitioning strategy (Bowen ratio), which was attributable to the difference in the bulk surface resistance, rather than the difference in the climatological resistance (which is proportional to VPD/ R n). The seasonal and annual variability of water supply was determined by precipitation rather than by ET. The linear slopes of water supply with precipitation were all close to one, regardless of vegetation type. This suggested that the water yield efficiency was consistent at the two sites. Our results highlighted the different effects of vegetation type on ET and water supply for humid alpine regions. The lower ET loss and consequent higher water yield, but with less digestible forage in the shrub, would present a dilemma for the balance between water supply and forage production under current shrub expansion in alpine rangeland. [ABSTRACT FROM AUTHOR]