Effects of experimental warming and increased precipitation on soil respiration in an alpine meadow in the Northern Tibetan Plateau

Whole-soil warming leads to substantial soil carbon emission in an alpine grassland

The sensitivity of soil organic carbon (SOC) decomposition in seasonally frozen soils, such as alpine ecosystems, to climate warming is a major uncertainty in global carbon cycling. Here we measure soil CO2 emission during four years (2018-2021) from the whole-soil warming experiment (4 °C for the top 1 m) in an alpine grassland ecosystem. We find that whole-soil warming stimulates total and SOC-derived CO2 efflux by 26% and 37%, respectively, but has a minor effect on root-derived CO2 efflux. Moreover, experimental warming only promotes total soil CO2 efflux by 7-8% on average in the meta-analysis across all grasslands or alpine grasslands globally (none of these experiments were whole-soil warming). We show that whole-soil warming has a much stronger effect on soil carbon emission in the alpine grassland ecosystem than what was reported in previous warming experiments, most of which only heat surface soils.

Rainfall effect on soil respiration depends on antecedent soil moisture

Climate change has a discernible influence on rainfall patterns, thus potentially affecting the intricate dynamics of soil respiration (Rs) and soil carbon storage. However, we still lack a profound understanding of the determinants of Rs response to rainfall events. Here, utilizing a comprehensive 10-year dataset (2004-2013), we explored the direction and magnitude of Rs response to rainfall events and the underlying determinants in a temperate forest. Based on the identified 368 rainfall events over the study period, we demonstrate that rainfall suppresses Rs when the soil moisture is optimal and moist in the growing season, whereas its effect on Rs during the non-growing season is minimal. Notably, antecedent soil moisture, rather than rainfall amount, shows a substantial impact on Rs during the growing season (coefficient of determination (R2) = 0.37 for antecedent soil moisture, and R2 < 0.01 for rainfall amount). Incorporating antecedent soil moisture significantly enhances the explanatory power (R2) from 0.09 to 0.45 regarding the relative changes in Rs following rainfall events. Our results highlight the environmental dependency of Rs response to rainfall events and suggest that incorporating the role of antecedent soil moisture could enhance predictability and reduce uncertainty in ecosystem modeling.

Warming affects leaf light use efficiency and functional traits in alpine plants: evidence from a 4-year in-situ field experiment

Introduction

Light use efficiency (LUE) is a crucial determinant of plant productivity, while leaf functional traits directly affect ecosystem functions. However, it remains unclear how climate warming affects LUE and leaf functional traits of dominant species in alpine meadows.

Methods

We conducted a 4-year in-situ field warming experiment to investigate the eco-physiological characteristics for a dominant species (Elymus nutans) and a common species (Potentilla anserina) on the Tibetan Plateau. The leaf traits, photosynthesis and fluorescence characteristics were measured, along with the soil physical-chemical properties associated with the two species.

Results and discussions

Experimental warming increased the leaf LUE, maximum photochemical efficiency, non-photochemical quenching, relative water content and specific leaf area for both species. However, there was a decrease in leaf and soil element content. Different species exhibit varying adaptability to warming. Increasing temperature significantly increased the photosynthetic rate, stomatal conductance, transpiration rate, total water content, and specific leaf volume of E. nutans; however, all these traits exhibited an opposite trend in P. anserina. Warming has a direct negative impact on leaf LUE and an indirectly enhances LUE through its effects on leaf traits. The impact of warming on plant photosynthetic capacity is primarily mediated by soil nutrients and leaf traits. These results indicate that the two different species employ distinct adaptive strategies in response to climate change, which are related to their species-specific variations. Such changes can confer an adaptive advantage for plant to cope with environmental change and potentially lead to alterations to ecosystem structure and functioning.

Response of plant diversity and soil microbial diversity to warming and increased precipitation in alpine grasslands on the Qinghai-Xizang Plateau - A review

Plant diversity and soil microbial diversity are closely related, and they maintain the health and stability of terrestrial ecosystems. As a hotspot region of global biodiversity research, both air temperature and precipitation of the Qinghai-Xizang Plateau tend to increase in future. Based on an overview of the responses of grassland/alpine ecosystems to seasonal asymmetric warming and increased precipitation worldwide, we elaborated the advancements and uncertainties on the responses of plant diversity and soil microbial diversity to warming and increased precipitation in alpine grasslands on the Qinghai-Xizang Plateau. The future research focus of plant diversity and soil microbial diversity in the alpine grasslands of the Qinghai-Xizang Plateau under climate warming and increased precipitation was proposed. Generally, previous studies found that the responses of plant species diversity and soil microbial species diversity to warming and increased precipitation differed between alpine meadows and alpine steppes, but few studies focused on their responses to warming and increased precipitation in alpine desert steppes. Previous studies mainly focused on species diversity, although phylogenetic and functional diversities are also important aspects of biodiversity. Previous studies mainly explained responses of plant diversity and soil microbial diversity to warming and increased precipitation based on niche theory, although neutral theory is also the other important mechanism in regulating biodiversity. Moreover, previous studies almost ignored the coupling relationship between plant diversity and soil microbial diversity. Therefore, the following four aspects need to be strengthened, including the responses of plant diversity and soil microbial diversity to warming and increased precipitation in alpine desert steppes, the responses of plant and soil microbial phylogenetic diversity and functional diversity to warming and increased precipitation, combining the niche theory and neutral theory to examining the mechanism of biodiversity, and the coupling relationships between plant diversity and soil microbial diversity under warming and increased precipitation.

Response of Aboveground Net Primary Production, Species and Phylogenetic Diversity to Warming and Increased Precipitation in an Alpine Meadow

The uncertain responses of aboveground net primary productivity (ANPP) and plant diversity to climate warming and increased precipitation will limit our ability to predict changes in vegetation productivity and plant diversity under future climate change and further constrain our ability to protect biodiversity and ecosystems. A long-term experiment was conducted to explore the responses of ANPP, plant species, phylogenetic α-diversity, and community composition to warming and increased precipitation in an alpine meadow of the Northern Tibet from 2014 to 2019. Coverage, height, and species name were obtained by conventional community investigation methods, and ANPP was obtained using observed height and coverage. Open-top chambers with two different heights were used to simulate low- and high-level climate warming. The low- and high-level increased precipitation treatments were achieved by using two kinds of surface area funnel devices. The high-level warming reduced sedge ANPP (ANPPsedge) by 62.81%, species richness (SR) by 21.05%, Shannon by 13.06%, and phylogenetic diversity (PD) by 14.48%, but increased forb ANPP (ANPPforb) by 56.65% and mean nearest taxon distance (MNTD) by 33.88%. Species richness, Shannon, and PD of the high-level warming were 19.64%, 9.67%, and 14.66% lower than those of the low-level warming, respectively. The high-level warming-induced dissimilarity magnitudes of species and phylogenetic composition were greater than those caused by low-level warming. The low- rather than high-level increased precipitation altered species and phylogenetic composition. There were significant inter-annual variations of ANPP, plant species, phylogenetic α-diversity and community composition. Therefore, climate warming and increased precipitation had non-linear effects on ANPP and plant diversity, which were due to non-linear changes in temperature, water availability, and/or soil nutrition caused by warming and increased precipitation. The inter-annual variations of ANPP and plant diversity were stronger than the effects of warming and especially increased precipitation on ANPP and plant diversity. In terms of plant diversity conservation and related policy formulation, we should pay more attention to regions with greater warming, at least for the northern Tibet grasslands. Besides paying attention to the responses of ANPP and plant diversity to climate change, the large inter-annual changes of ANPP and plant diversity should be given great attention because the large inter-annual variation indicates the low temporal stability of ANPP and plant diversity and thus produces great uncertainty for the development of animal husbandry.

Impacts of Anthropogenic Activities and Climate Change on Forage Nutrition Storage in Tibetan Grasslands

Uncertainties about the impacts of anthropogenic activities and climate change on forage nutrition storage of grasslands can limit the adaptive management of grasslands across the whole Tibetan Plateau. The main objective was to investigate the impacts of anthropogenic activities and climate change on the forage nutrition storage of grasslands on the Tibetan Plateau. Based on random forest models, we quantified the responses of forage nutrition storage to anthropogenic activities and climate change across the whole Tibetan grasslands from 2000 to 2020. Warming and increased precipitation did not always increase forage nutrition storage, and cooling and decreased precipitation did not always reduce forage nutrition storage. Compared to temperature and precipitation changes, radiation change had stronger contributions to potential and actual forage nutrition storage. Humankind's activities altered the impacts of climate change on forage nutrition storage. The impacts of anthropogenic activities on forage nutrition storage increased linearly with increasing mean annual temperature and decreasing elevation but showed quadratic relationships with longitude, mean annual precipitation and radiation. The change in the impacts of humankind's activities on forage nutrition storage was more closely related to radiation change than temperature and precipitation changes. The findings observed by this study caution that the impacts of radiation change on forage nutrition forage should be taken seriously under global change. Both climate change and humankind activities cannot always increase forage nutrition storage but may cause the degradation of forage nutrition storage.

Nitrogen availability mediates soil carbon cycling response to climate warming: A meta-analysis

Global climate warming may induce a positive feedback through increasing soil carbon (C) release to the atmosphere. Although warming can affect both C input to and output from soil, direct and convincing evidence illustrating that warming induces a net change in soil C is still lacking. We synthesized the results from field warming experiments at 165 sites across the globe and found that climate warming had no significant effect on soil C stock. On average, warming significantly increased root biomass and soil respiration, but warming effects on root biomass and soil respiration strongly depended on soil nitrogen (N) availability. Under high N availability (soil C:N ratio < 15), warming had no significant effect on root biomass, but promoted the coupling between effect sizes of root biomass and soil C stock. Under relative N limitation (soil C:N ratio > 15), warming significantly enhanced root biomass. However, the enhancement of root biomass did not induce a corresponding C accumulation in soil, possibly because warming promoted microbial CO₂ release that offset the increased root C input. Also, reactive N input alleviated warming-induced C loss from soil, but elevated atmospheric CO₂ or precipitation increase/reduction did not. Together, our findings indicate that the relative availability of soil C to N (i.e., soil C:N ratio) critically mediates warming effects on soil C dynamics, suggesting that its incorporation into C-climate models may improve the prediction of soil C cycling under future global warming scenarios.

Ambient precipitation determines the sensitivity of soil respiration to precipitation treatments in a marsh

The effects in field manipulation experiments are strongly influenced by amplified interannual variation in ambient climate as the experimental duration increases. Soil respiration (SR), as an important part of the carbon cycle in terrestrial ecosystems, is sensitive to climate changes such as temperature and precipitation changes. A growing body of evidence has indicated that ambient climate affects the temperature sensitivity of SR, which benchmarks the strength of terrestrial soil carbon-climate feedbacks. However, whether SR sensitivity to precipitation changes is influenced by ambient climate is still not clear. In addition, the mechanism driving the above phenomenon is still poorly understood. Here, a long-term field manipulation experiment with five precipitation treatments (-60%, -40%, +0%, +40%, and +60% of annual precipitation) was conducted in a marsh in the Yellow River Delta, China, which is sensitive to soil drying-wetting cycle caused by precipitation changes. Results showed that SR increased exponentially along the experimental precipitation gradient each year and the sensitivity of SR (standardized by per 100 mm change in precipitation under precipitation treatments) exhibited significant interannual variation from 2016 to 2021. In addition, temperature, net radiation, and ambient precipitation all exhibited dramatic interannual variability; however, only ambient precipitation had a significant negative correlation with SR sensitivity. Moreover, the sensitivity of SR was significantly positively related to the sensitivity of belowground biomass (BGB) across 6 years. Structural equation modeling and regression analysis also showed that precipitation treatments significantly affected SR and its autotrophic and heterotrophic components by altering BGB. Our study demonstrated that ambient precipitation determines the sensitivity of SR to precipitation treatments in marshes. The findings underscore the importance of ambient climate in regulating ecosystem responses in long-term field manipulation experiments.

Significant effects of precipitation frequency on soil respiration and its components-A global synthesis

Global warming intensifies the hydrological cycle, which results in changes in precipitation regime (frequency and amount), and will likely have significant impacts on soil respiration (R_s ). Although the responses of R_s to changes in precipitation amount have been extensively studied, there is little consensus on how R_s will be affected by changes in precipitation frequency (PF) across the globe. Here, we synthesized the field observations from 296 published papers to quantify the effects of PF on R_s and its components using meta-analysis. Our results indicated that the effects of PF on R_s decreased with an increase in background mean annual precipitation. When the data were grouped by climate conditions, increased PF showed positive effects on R_s under the arid condition but not under the semi-humid or humid conditions, whereas decreased PF suppressed R_s across all the climate conditions. The positive effects of increased PF mainly resulted from the positive response of heterotrophic respiration under the arid condition while the negative effects of decreased PF were mainly attributed to the reductions in root biomass and respiration. Overall, our global synthesis provided for the first time a comprehensive analysis of the divergent effects of PF on R_s and its components across climate regions. This study also provided a framework for understanding and modeling responses of ecosystem carbon cycling to global precipitation change.

Non-growing/growing season non-uniform-warming increases precipitation use efficiency but reduces its temporal stability in an alpine meadow

There are still uncertainties on the impacts of season-non-uniform-warming on plant precipitation use efficiency (PUE) and its temporal stability (PUE_stability) in alpine areas. Here, we examined the changes of PUE and PUE_stability under two scenes of non-growing/growing season non-uniform-warming (i.e., GLNG: growing-season-warming lower than non-growing-season-warming; GHNG: growing-season-warming higher than non-growing-season-warming) based on a five-year non-uniform-warming of non-growing/growing season experiment. The GLNG treatment increased PUE by 38.70% and reduced PUE_stability by 50.47%, but the GHNG treatment did not change PUE and PUE_stability. This finding was mainly due to the fact that the GLNG treatment had stronger influences on aboveground biomass (AGB), non-growing-season soil moisture (SM_NG), temporal stability of AGB (AGB_stability), temporal stability of non-growing-season air temperature (T _{a_NG_stability}), temporal stability of growing-season vapor pressure deficit (VPD_{G_stability}) and temporal stability of start of growing-season (SGS_stability). Therefore, the warming scene with a higher non-growing-season-warming can have greater influences on PUE and PUE_stability than the warming scene with a higher growing-season-warming, and there were possibly trade-offs between plant PUE and PUE_stability under season-non-uniform-warming scenes in the alpine meadow.

Nitrogen and water availability control plant carbon storage with warming

Plants may slow global warming through enhanced growth, because increased levels of photosynthesis stimulate the land carbon (C) sink. However, how climate warming affects plant C storage globally and key drivers determining the response of plant C storage to climate warming remains unclear, causing uncertainty in climate projections. We performed a comprehensive meta-analysis, compiling 393 observations from 99 warming studies to examine the global patterns of plant C storage responses to climate warming and explore the key drivers. Warming significantly increased total biomass (+8.4 %), aboveground biomass (+12.6 %) and belowground biomass (+10.1 %). The effect of experimental warming on plant biomass was best explained by the availability of soil nitrogen (N) and water. Across the entire dataset, warming-induced changes in total, aboveground and belowground biomass all positively correlated with soil C:N ratio, an indicator of soil N availability. In addition, warming stimulated plant biomass more strongly in humid than in dry ecosystems, and warming tended to decrease root:shoot ratios at high soil C:N ratios. Together, these results suggest dual controls of warming effects on plant C storage; warming increases plant growth in ecosystems where N is limiting plant growth, but it reduces plant growth where water availability is limiting plant growth.

Geo-Distribution Patterns of Soil Fungal Community of Pennisetum flaccidum in Tibet

Pennisetum flaccidum can be used as a pioneer species for the restoration of degraded grasslands and as a high-quality forage for local yak and sheep in alpine regions. The geographical distribution pattern of soil fungal community can modify that of P. flaccidum. A field survey along 32 sampling sites was conducted to explore the geo-distribution patterns of soil fungal community of P. flaccidum in Tibet. Soil fungal species, phylogenetic and function diversity generally had a closer correlation with longitude/elevation than latitude. The geo-distribution patterns of soil fungal species, phylogenetic and function diversity varied with soil depth. Soil fungal species, phylogenetic and function diversity had dissimilar geo-distribution patterns. Precipitation had stronger impacts on total abundance, species α-diversity, phylogenetic α-diversity, and function β-diversity than temperature for both topsoil (0-10 cm depth) and subtopsoil (10-20 cm depth). Furthermore, precipitation had stronger impacts on function α-diversity for topsoil, species β-diversity for topsoil, and phylogenetic β-diversity for subtopsoil than temperature. The combination of species, phylogenetic and function diversity can better reflect geo-distribution patterns of soil fungal community. Compared to global warming, the impact of precipitation change on the variation in soil fungal community of P. flaccidum should be given more attention.

Precipitation increase counteracts warming effects on plant and soil C:N:P stoichiometry in an alpine meadow

Temperature and precipitation are expected to increase in the forthcoming decades in the northeastern Qinghai-Tibetan Plateau, with uncertain effects of their interaction on plant and soil carbon:nitrogen:phosphorus (C:N:P) stoichiometry in alpine ecosystems. A two-year field experiment was conducted to examine the effects of warming, precipitation increase, and their interaction on soil and plant C:N:P stoichiometry at functional groups and community level in an alpine meadow. Warming increased aboveground biomass of legumes and N:P ratios of grasses and community, but did not affect soil C:N:P stoichiometry. The piecewise structural equation model (SEM) indicated that the positive effect of warming on community N:P ratio was mainly resulted from its positive influence on the aboveground biomass of functional groups. Precipitation increase reduced C:N ratios of soil, grasses, and community, indicating the alleviation in soil N-limitation and the reduction in N use efficiency of plant. SEM also demonstrated the decisive role of grasses C:N:P stoichiometry on the response of community C:N:P stoichiometry to precipitation increase. The interaction of warming and precipitation increase did not alter plant community and soil, N:P and C:P ratios, which was resulting from their antagonistic effects. The stable soil and plant community C:N:P stoichiometry raised important implications that the effect of warming was offset by precipitation increase. Our study highlights the importance of considering the interaction between warming and precipitation increase when predicting the impacts of climate change on biogeochemical cycles in alpine meadow ecosystems.

Response of forage nutritional quality to climate change and human activities in alpine grasslands

The impacts of climate change and human activities on forage nutritional quality will affect nutrient capacity, livestock development and wildlife conservation in alpine regions. However, the response of forage nutritional quality to climate change and human activities remains indistinguishable across the whole Tibet. Here, six forage variables (i.e., crude protein, CP; ether extract, EE; crude ash, Ash; acid detergent fiber, ADF; neutral detergent fiber, NDF; water-soluble carbohydrates, WSC) together represented forage nutritional quality. We estimated potential forage CP, EE, Ash, ADF, NDF and WSC contents using growing mean air temperature, total precipitation and total radiation based on random forest models. We also estimated actual forage CP, EE, Ash, ADF, NDF and WSC contents using growing mean air temperature, total precipitation and total radiation, and maximum normalized difference vegetation index based on random forest models. Climate change had nonlinear effects on potential forage CP, EE, Ash, ADF, NDF and WSC contents. Radiation change predominated the variations of potential forage nutritional quality. Human activities altered the sensitivities of forage nutritional quality to climate change. The effects of human activities on forage nutritional quality increased with increasing longitude and precipitation, and decreasing elevation and radiation. Consequently, we should pay attention to the radiation change besides climate warming and precipitation change, at least for forage nutritional quality in alpine grasslands. The effects of human activities on forage nutritional quality can vary with longitude, elevation, precipitation and radiation in alpine grasslands.

Spatial and Temporal Variability and Driving Factors of Carbon Dioxide and Nitrous Oxide Fluxes in Alpine Wetland Ecosystems

Plants regulate greenhouse gas (GHG) fluxes in wetland ecosystems, but the mechanisms of plant removal and plant species that contribute to GHG emissions remain unclear. In this study, the fluxes of carbon dioxide (CO2) and nitrous oxide (N2O) were measured using the static chamber method from an island forest dominated by two different species, namely Betula platyphylla (BP) and Larix gmelinii (LG), in a marsh wetland in the Great Xing’an Mountains. Four sub-plots were established in this study: (1) bare soil after removing vegetation under BP (SBP); (2) bare soil after removing vegetation under LG (SLG); (3) soil with vegetation under BP (VSBP); and (4) soil with vegetation under LG (VSLG). Additionally, the contributions of the dark respiration from plant aerial parts under BP (VBP) and LG (VLG) to GHG fluxes were calculated. We found that the substantial spatial variability of CO2 fluxes ranged from −25.32 ± 15.45 to 187.20 ± 74.76 mg m−2 h−1 during the study period. The CO2 fluxes decreased in the order of SBP > VSLG > VSBP > SLG > VLG > VBP, indicating that vegetation species had a great impact on CO2 emissions. Particularly, the absence of vegetation promoted CO2 emission in both BP and LG. Additionally, CO2 fluxes showed dramatically seasonal variations, with high CO2 fluxes in late spring (May) and summer (June, July, and August), but low fluxes in late summer (August) and early autumn (September). Soil temperatures at 0−20 cm depth were better predictors of CO2 fluxes than deeper soil temperatures. N2O fluxes were varied in different treatments with the highest N2O fluxes in SLG and the lowest N2O fluxes in VBP. Meanwhile, no significant correlation was found between N2O fluxes and air or soil temperatures. Temporally, negative N2O fluxes were observed from June to October, indicating that soil N2O fluxes were reduced and emitted as N2, which was the terminal step of the microbial denitrification process. Most of the study sites were CO2 sources during the warm season and CO2 sinks in the cold season. Thus, soil temperature plays an important role in CO2 fluxes. We also found that the CO2 flux was positively related to pH in a 10 cm soil layer and positively related to moisture content (MC) in a 50 cm soil layer in VSBP and VSLG. However, the CO2 flux was negatively related to pH in a 30 cm soil layer in SBP and SLG. Our findings highlight the effects of vegetation removal on GHG fluxes, and aid in the scientific management of wetland plants.

Performance Assessment of Different Precipitation Databases (Gridded Analyses and Reanalyses) for the New Brazilian Agricultural Frontier: SEALBA

Since the early 2000s, Brazil has been one of the world’s leading grain producers, with agribusiness accounting for around 28% of the Brazilian GDP in 2021. Substantial investments in research, coupled with the expansion of arable areas, owed to the advent of new agriculture frontiers, led the country to become the world’s greatest producer of soybean. One of the newest agricultural frontiers to be emerging in Brazil is the one known as SEALBA, an acronym that refers to the three Brazilian states whose areas it is comprised of—Sergipe, Alagoas, and Bahia—all located in the Northeast region of the country. It is an extensive area with a favorable climate for the production of grains, including soybeans, with a rainy season that takes place in autumn/winter, unlike the Brazilian regions that are currently the main producers of these kinds of crops, in which the rainfall regime has the wet period concentrated in spring/summer. Considering that precipitation is the main determinant climatic factor for crops, the scarcity of weather stations in the SEALBA region poses an obstacle to an accurate evaluation of the actual feasibility of the region to a given crop. Therefore, the aim of this work was to carry out an assessment of the performance of four different precipitation databases of alternative sources to observations: two from gridded analyses, MERGE and CHIRPS, and the other two from ECMWF reanalyses, ERA5, and ERA5Land, and by comparing them to observational records from stations along the region. The analysis was based on a comparison with data from seven weather stations located in SEALBA, in the period 2001–2020, through three dexterity indices: the mean absolute error (MAE), the root mean squared errors (RMSE), and the coefficient of Pearson’s correlation (r), showing that the gridded analyzes performed better than the reanalyses, with MERGE showing the highest correlations and the lowest errors (global average r between stations of 0.96, followed by CHIRPS with 0.85, ERA5Land with 0.83, and ERA5 with 0.70; average MAE 14.3 mm, followed by CHIRPS with 21.3 mm, ERA5Land with 42.1 mm and ERA5 with 50.1 mm; average RMSE between stations of 24.6 mm, followed by CHIRPS with 50.8 mm, ERA5Land with 62.3 mm and ERA5 with 71.4 mm). Since all databases provide up-to-date data, our findings indicate that, for any research that needs a complete daily precipitation dataset for the SEALBA region, preference should be given to use the data in the following order of priority: MERGE, CHIRPS, ERA5Land, and ERA5.

Asymmetrical warming of growing/non-growing season increases soil respiration during growing season in an alpine meadow

Soil respiration (R_s) is an important carbon flux in the global carbon cycle, and understanding the influence of global warming on R_s is critical for precise prediction future climate change. Actually, global warming is expected to be seasonally asymmetric, however, it is still unclear on the response of R_s to asymmetrical warming of growing/non-growing season in alpine regions. In this study, an experiment with asymmetrical warming of growing/non-growing season (including three treatments, CK: control; GLNG: warming magnitude of growing season lower than non-growing season; GHNG: warming magnitude of growing season higher than non-growing season) was performed in an alpine meadow of the Northern Tibet since June 2015. The 'GLNG' and 'GHNG' treatments increased mean R_s by 71.22% (1.89 μmol CO₂ m^-2 s^-1) and 34.32% (0.91 μmol CO₂ m^-2 s^-1) during growing season in 2019, respectively. However, the 'GLNG' and 'GHNG' treatments did not significantly affect mean R_s during growing season in 2015, 2016, 2017 and 2018, respectively. The variation coefficient of growing season mean R_s was 32.95% under the CK treatment in 2015-2019. Therefore, warming may have a lagging effect on R_s. The warming scene with a greater warming during non-growing season may have a stronger effect on R_s than the warming scene with a greater warming during growing season. Inter-annual variation of R_s may be greater than the warming effect on R_s in alpine meadows.

Effects of Warming and Precipitation on Soil CO2 Flux and Its Stable Carbon Isotope Composition in the Temperate Desert Steppe

The stable carbon (C) isotope of soil CO2 efflux (δ13CO2e) is closely associated with soil C dynamics, which have a complex feedback relationship with climate. Three levels of warming (T0: ambient temperature (15.7 °C); T1: T0 + 2 °C; T2: T0 + 4 °C) were combined with three levels of increased precipitation (W0: ambient precipitation (245.2 mm); W1: W0 + 25%; W2: W0 + 50%) in order to quantify soil CO2 flux and its δ13CO2e values under nine treatment conditions (T0W0, T0W1, T0W2, T1W0, T1W1, T1W2, T2W0, T2W1, and T2W2) in desert steppe in an experimental beginning in 2015. A non-steady state chamber system relying on Keeling plots was used to estimate δ13CO2e. The temperature (ST) and moisture (SM) of soil as well as soil organic carbon content (SOC) and δ13C values (δ13Csoil) were tested in order to interpret variations in soil CO2 efflux and δ13CO2e. Sampling was carried out during the growing season in 2018 and 2019. During the experiment, the ST and SM correspondingly increased due to warming and increased precipitation. CO2 flux ranged from 37 to 1103 mg m−2·h−1, and emissions peaked in early August in the desert steppe. Warming of 2 °C to 4 °C stimulated a 14% to 30.9% increase in soil CO2 efflux and a 0.4‰ to 1.8‰ enrichment in δ13CO2e, respectively. Increased precipitation raised soil CO2 efflux by 14% to 19.3%, and decreased δ13CO2e by 0.5‰ to 0.9‰. There was a positive correlation between soil CO2 efflux and ST and SOC indicating that ST affected soil CO2 efflux by changing SOC content. Although the δ13CO2e was positively correlated with ST, it was negatively correlated to SM. The decline of δ13CO2e with soil moisture was predominantly due to intensified and increased diffusive fractionation. The mean δ13CO2e value (−20.2‰) was higher than that of the soil carbon isotope signature at 0–20 cm (δ13Csoil = −22.7‰). The difference between δ13CO2e and δ13Csoil (Δe-s) could be used to evaluate the likelihood of substrate utilization. 13C enriched stable C pools were more likely to be utilized below 20 cm under warming of 2 °C in the desert steppe. Moreover, the interaction of T × W neither altered the CO2 emitted by soil nor the δ13CO2e or Δe-s, indicating that warming combined with precipitation may alleviate the SOC oxidation of soil enriched in 13C in the desert steppe.

Projected soil organic carbon loss in response to climate warming and soil water content in a loess watershed

BACKGROUND

Soil organic carbon (SOC) plays a crucial role in the global carbon cycle and terrestrial ecosystem functions. It is widely known that climate change and soil water content (SWC) could influence the SOC dynamics; however, there are still debates about how climate change, especially climate warming, and SWC impact SOC. We investigated the spatiotemporal changes in SOC and its responses to climate warming and root-zone SWC change using the coupled hydro-biogeochemical model (SWAT-DayCent) and climate scenarios data derived under the three Representative Concentration Pathways (RCPs2.6, 4.5, and 8.5) from five downscaled Global Climate Models (GCMs) in a typical loess watershed--the Jinghe River Basin (JRB) on the Chinese Loess Plateau.

RESULTS

The air temperature would increase significantly during the future period (2017-2099), while the annual precipitation would increase by 2.0-13.1% relative to the baseline period (1976-2016), indicating a warmer and wetter future in the JRB. Driven by the precipitation variation, the root-zone SWC would also increase (by up to 27.9% relative to the baseline under RCP4.5); however, the SOC was projected to decrease significantly under the future warming climate. The combined effects of climate warming and SWC change could more reasonably explain the SOC loss, and this formed hump-shaped response surfaces between SOC loss and warming-SWC interactions under both RCP2.6 and 8.5, which can help explain diverse warming effects on SOC with changing SWC.

CONCLUSIONS

The study showed a significant potential carbon source under the future warmer and wetter climate in the JRB, and the SOC loss was largely controlled by future climate warming and the root-zone SWC as well. The hump-shaped responses of the SOC loss to climate warming and SWC change demonstrated that the SWC could mediate the warming effects on SOC loss, but this mediation largely depended on the SWC changing magnitude (drier or wetter soil conditions). This mediation mechanism about the effect of SWC on SOC would be valuable for enhancing soil carbon sequestration in a warming climate on the Loess Plateau.

Effect of precipitation on respiration of different reconstructed soils

Respiration and hydrothermal characteristics of four reconstructed soils in barren gravel land at a site in Shaanxi Province were monitored before, during, and after two precipitation events. Both precipitation events significantly reduced soil temperature but there were great fluctuations in temperature after the second precipitation event. Moreover, precipitation increased the moisture content of the reconstructed soils. Before the first precipitation event, the soil volumetric water content was relatively stable, while it gradually decreased before the second precipitation event. The first precipitation event significantly stimulated the respiration rate for all reconstructed soils, while the second precipitation event generally inhibited it, especially during the precipitation event. The key factors influencing respiration for different reconstructed soils were different between the precipitation events. When soil volumetric water content showed persistent variation before precipitation, soil moisture was the most influential factor. In contrast, if water content was stable, soil temperature was more influential. Soil moisture and temperature jointly influenced soil respiration before, during, and after the precipitation event, while soil moisture was always the most influential factor after precipitation.

Warming and increased precipitation indirectly affect the composition and turnover of labile-fraction soil organic matter by directly affecting vegetation and microorganisms

Global warming accompanied by precipitation changes impacts soil carbon sequestration. A three-year field manipulation experiment with warming (+2 °C above ambient temperature) and increased precipitation (+15% and +30% above ambient precipitation) was conducted in an alpine grassland to investigate the response of soil organic matter (SOM) to future climate change on the Qinghai-Tibet Plateau (QTP). Labile-fraction SOM (LF-SOM) fingerprints were characterized by pyrolysis-gas chromatography/tandem-mass spectrometry (Py-GC-MS/MS), and organic compounds in LF-SOM were used as indicators to quantify the contributions of vegetation input and microbial degradation to LF-SOM transformation. Increased precipitation promoted LF-SOM accumulation, which were mainly due to the positive effect of increased precipitation on vegetation productivity. Plant-derived compounds in LF-SOM (including lignin, long-chain alkyl compounds, polysaccharides and phenols) were more sensitive to increasing soil moisture than microbial-derived (including short-chain alkyl compounds, N compounds and chitin) and aromatic-derived compounds (including aromatics and polyaromatics). In contrast, warming alone intensified the effect of drought on the alpine grassland, which had negative effects on both vegetation and microorganisms and reduced LF-SOM. Warming plus increased precipitation not only alleviated the water loss caused by warming but also increased soil temperature, which was more favorable for the growth of microorganisms. This was reflected in the increase in microbial-derived compounds in LF-SOM with increasing soil temperature, which contributed to LF-SOM degradation. Aromatic-derived compounds, as refractory compounds in soil, showed no significant response to either warming or increased precipitation treatments. Acidobacteria (approximately 25%) and Actinobacteria (approximately 20%), as the dominant soil bacterial communities in the alpine grassland, were significantly correlated with plant-derived compounds. At the same time, there were significant correlations between Proteobacteria and microbial-derived compounds, as well as between Firmicutes and aromatic-derived compounds (relative abundance). Under future climate change, microbial activity will increase as temperature increases, which will promote LF-SOM degradation only if precipitation also increases.

Hydro-Meteorological Trends in the Yarlung Zangbo River Basin and Possible Associations with Large-Scale Circulation

Climate change poses potential challenges to sensitive areas, such as high-elevation regions. The Yarlung Zangbo River (YLZR) basin is located in the southeast of the Qinghai-Tibetan Plateau. It contains large amounts of snow and numerous glaciers that are vulnerable to climate change. Based on daily observational data at 17 meteorological stations in and around the YLZR basin during 1957–2015, the variability of precipitation, air temperature, and streamflow were analyzed. The nonparametric Mann–Kendall test, Sen’s slope estimate method, cross wavelet transform (XWT), and wavelet coherence (WTC) were used to identify the annual seasonal trends. the abrupt changes of precipitation and air temperature, and their associations with large-scale circulation. The results showed that the YLZR basin experienced an overall rapid warming and wetting during the study period, with an average warming rate of 0.33 °C/10 a and wetting rate of 4.25 mm/10a, respectively. Abrupt change points in precipitation and air temperature occurred around the 1970s and 1990s, respectively. The abrupt change points of three hydrological stations occurred around the late 1960s and the late 1990s, respectively. The precipitation, annual average temperature, and the streamflow of the three hydrological stations were negatively correlated with the Pacific decadal oscillation (PDO) and the multivariate El Niño-Southern Oscillation (ENSO) index (MEI), reaching a significant level of 0.05.

Soil Type, Topography, and Land Use Interact to Control the Response of Soil Respiration to Climate Variation

The effects of soil and topography on the responses of soil respiration (Rs) to climatic variables must be investigated in the southeastern mountainous areas of China due to the rapid land-use change from forest to agriculture. In this study, we investigated the response of Rs to soil temperature (ST), precipitation over the previous seven days (AP7), and soil water content (SWC) across two hillslopes that had different land uses: a tea garden (TG) and a bamboo forest (BF). Meanwhile, the roles of soil properties including soil clay content and total nitrogen (TN), and topography including elevation, profile curvature (PRC), and slope on the different responses of Rs to these climatic variables were investigated. Results showed that mean Rs on the BF hillslope (2.21 umol C m−2 s−1) was 1.71 times of that on the TG hillslope (1.29 umol C m−2 s−1). Soil clay content, elevation, and PRC had negative correlations (p < 0.05) with spatial variation of Rs, and ST was positively correlated (p < 0.01) with temporal variation of Rs on both hillslopes. Across both hillslopes ST explained 33%–73% and AP7 explained 24%–38% of the temporal variations in Rs. The mean temperature sensitivities (Q10s) of Rs were 2.02 and 3.22, respectively, on the TG and BF hillslopes. The Q10 was positively correlated (p < 0.05) with the temporal mean of SWC and TN, and negatively correlated (p < 0.05) with clay and slope. The mean AP7 sensitivities (a concept similar to Q10) were greatly affected by clay and PRC. When Rs was normalized to that at 10 °C, power or quadratic relationships between Rs and SWC were observed in different sites, and the SWC explained 12%–32% of the temporal variation in Rs. When ST and SWC were integrated and considered, improved explanations (45%–81%) were achieved for the Rs temporal variation. In addition, clay and elevation had vital influences on the responses of Rs to SWC. These results highlight the influences of soil, topographic features, and land use on the spatial variations of the Rs, as well as on the responses of Rs to different climatic variables, which will supplement the understanding of controlling mechanisms of Rs on tea and bamboo land-use types in Southeastern China.

Impact of diurnal unsymmetrical warming on soil respiration in an agroecological system of the Lhasa region

Purpose

The impact of diurnal unsymmetrical rise in temperature on soil respiration (R_s) is not fully understood; thus, we explored such a warming influence on R_s in an agroecological system of the Lhasa.

Materials and methods

A field warming experiment (C: control; DW: daytime warming; NW: nighttime warming; DW+NW: daytime plus nighttime warming) was carried out in a naked barley ecological system.

Results and discussion

The DW, NW and DW+NW treatments dramatically increased soil temperature and decreased soil moisture but did not markedly modify R_s. The effects of DW and NW on soil respiration sensitivity (Q₁₀) during the daytime and nighttime were different; they had no effects on daytime Q₁₀ of R_s, but a significant inhibitory effect on nighttime Q₁₀ of R_s.

Conclusions

A diurnal unsymmetrical rise in temperature brought about different results for the Q₁₀ of R_s but did not cause changes in R_s under different experimental treatments in agroecological systems of the Lhasa.

Dynamics of Soil Respiration in Alpine Wetland Meadows Exposed to Different Levels of Degradation in the Qinghai-Tibet Plateau, China

The effects of degradation of alpine wetland meadow on soil respiration (Rs) and the sensitivity of Rs to temperature (Q10) were measured in the Napa Lake region of Shangri-La on the southeastern edge of the Qinghai-Tibet Plateau. Rs was measured for 24 h during each of three different stages of the growing season on four different degraded levels. The results showed: (1) peak Rs occurred at around 5:00 p.m., regardless of the degree of degradation and growing season stage, with the maximum Rs reaching 10.05 μmol·m-2·s-1 in non-degraded meadows rather than other meadows; (2) the daily mean Rs value was 7.14-7.86 μmol·m-2·s-1 during the mid growing season in non-degraded meadows, and declined by 48.4-62.6% when degradation increased to the severely degraded level; (3) Q10 ranged from 7.1-11.3 in non-degraded meadows during the mid growing season, 5.5-8.0 and 6.2-8.2 during the early and late growing seasons, respectively, and show a decline of about 50% from the non-degraded meadows to severely degraded meadows; (4) Rs was correlated significantly with soil temperature at a depth of 0-5 cm (p < 0.05) on the diurnal scale, but not at the seasonal scale; (5) significant correlations were found between Rs and soil organic carbon (SOC), between biomass and SOC, and between Q10 and Rs (p < 0.05), which indicates that biomass and SOC potentially impact Q10. The results suggest that vegetation degradation impact both Rs and Q10 significantly. Also, we speculated that Q10 of alpine wetland meadow is probable greater at the boundary region than inner region of the Qinghai-Tibet Plateau, and shoule be a more sensitive indicator in the studying of climate change in this zone.