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Global climate warming could affect the methane (CH4) and nitrous oxide (N2O) fluxes between soils and the atmosphere, but how CH4 and N2O fluxes respond to whole‐soil warming is unclear. Here, we for the first time investigated the effects of whole‐soil warming on CH4 and N2O fluxes in an alpine grassland ecosystem on the Tibetan Plateau, and also studied the effects of experimental warming on CH4 and N2O fluxes across terrestrial ecosystems through a global‐scale meta‐analysis. The whole‐soil warming (0–100 cm, +4°C) significantly elevated soil N2O emission by 101%, but had a minor effect on soil CH4 uptake. However, the meta‐analysis revealed that experimental warming did not significantly alter CH4 and N2O fluxes, and it may be that most field warming experiments could only heat the surface soils. Moreover, the warming‐induced higher plant litter and available N in soils may be the main reason for the higher N2O emission under whole‐soil warming in the alpine grassland. We need to pay more attention to the long‐term response of greenhouse gases (including CH4 and N2O fluxes) from different soil depths to whole‐soil warming over year‐round, which could help us more accurately assess and predict the ecosystem‐climate feedback under realistic warming scenarios in the future. [ABSTRACT FROM AUTHOR]

Background and aims: Microbial communities play an important regulatory role in soil carbon and nutrient cycling in terrestrial ecosystems. Most studies on microbial communities and biogeochemical cycling focus on surface soils (0–20 cm). However, relatively little is known about how structure and functioning of microbial communities shift with depth in a soil profile, which is crucial to understand biogeochemical cycling in deep soils. Methods: We combined a number of complementary techniques to investigate the microbial biomass, community composition and diversity, and potential functioning along soil profile (0–70 cm) in two alpine ecosystems (meadow and shrubland) on the Tibetan Plateau. Results: The microbial biomass and fungi:bacteria ratio declined significantly with depth, while the ratio of Gram-positive to Gram-negative bacteria increased with depth in both ecosystems. Microbial community composition showed significant differences among soil depths and between ecosystems. The relative abundance of some phylum of archaea, bacteria or fungi (e.g. Basidiomycota, Bacteroidetes) changed significantly with soil depth and ecosystem type. Bacteria diversity declined with depth, while archaea richness (OTU number) increased with depth and fungi diversity and richness did not show clear trend with depth. The co-occurrence network analysis further showed that surface soil microbes were more connected and interacted among each other compared to deep soil microbes. Moreover, total enzymatic activities and soil C mineralization rate declined with depth in both ecosystems. We also detected shifts with depth in some functional guilds of bacteria (based on faprotax database) in both ecosystems and fungi (based on FUNGuild database) only in shrubland. Conclusions: The biomass, community composition and diversity, and potential functioning of microbial communities all shifted significantly along soil profile in both ecosystems, and the vertical patterns of diversity varied among different microbial groups. This may have important implications for carbon and nutrient cycling along the soil profile in alpine ecosystems. [ABSTRACT FROM AUTHOR]

Background and aims: Changes of soil organic matter (SOM) stability in alpine ecosystems can alter carbon release from the soil and consequently feed back to climate change. We sought to explore the altitudinal patterns of SOM stability and their relationships with climatic, vegetational and edaphic factors. Methods: We combined a number of biological, thermal, chemical, molecular and isotopic indices to characterize SOM stability in two soil layers (0–10 and 10–30 cm) of alpine grassland along two altitudinal gradients of 3200–4200 m (Haibei) and 4400–5200 m (Damxung) on the Tibetan Plateau, respectively. Results: There were significant altitudinal variations in SOM stability between the two transects. Greater biological, chemical and isotopic indices of SOM stability were observed in the middle altitude in Haibei, while opposite results were observed in Damxung. Except for a decreasing trend of SOM stability along altitudinal gradient in subsurface soils (10–30 cm) in Damxung, there was no significant altitudinal trend of thermal and molecular indices of SOM stability. The multiple indices of SOM stability were mutually associated and all showed greater SOM stability in subsurface soils than in surface soils across regions and altitudes. Conclusions: Along the altitudinal gradient, SOM stability showed opposite unimodal patterns in Haibei and Damxung, and had higher values in subsurface soils than in surface soils of the alpine grassland. Climatic, vegetational and edaphic factors collectively control SOM stability rather than just temperature or precipitation. [ABSTRACT FROM AUTHOR]

Background and aims: Long-term nitrogen (N) addition can affect soil organic carbon (SOC) pool within different soil fractions with different turnover rates. However, the mechanisms of these effects, particularly in alpine grassland ecosystems, are not clear. Methods: We studied the responses of SOC content in different soil fractions to N addition based on a six-year N addition field experiment in an alpine meadow ecosystem on the Tibetan Plateau. We measured soil chemical and microbial properties, and SOC content in bulk soil, particular organic matter (POM) and mineral-associated organic matter (MAOM) fractions in response to N addition. Results: N addition increased soil N availability, decreased soil pH and microbial biomass, but had minimal effect on plant biomass, soil enzyme activity, and SOC content in bulk soil. With increasing levels of N addition, SOC in the POM fraction (POC) showed a significant negative trend, while SOC in the MAOM fraction (MAOC) did not change significantly. Conclusions: As plant biomass input and soil enzyme activity were not significantly altered with N addition, the decline in POC was likely caused by changes in microbial physiology (carbon use efficiency), while the insignificant change in MAOC may be determined by the balance between input (from microbial necromass) and output (from microbial decomposition). Taken together, our study showed that the less-protected POC fraction is more vulnerable to N addition than the more-protected MAOC fraction in the alpine grassland. This finding may improve the prediction of soil C dynamics in response to N deposition in alpine grassland ecosystems on the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Soil organic carbon (SOC) decomposition can potentially feedback to climate change. However, the biotic, abiotic and inherent factors controlling the stability of soil carbon, and changes in these factors with soil depth, remain poorly understood. In this study, we combined a number of complementary methods to quantify the biological, thermal, chemical, molecular and isotopic indices of soil organic matter (SOM) stability along the soil profile (0–70 cm) in two contrasting alpine ecosystems (meadow and shrubland) on the Tibetan Plateau. Firstly, we conducted an aerobic lab–incubation experiment on root–free, sieved soils. The number of days to respire 5% of initial SOC, a biological index of SOM stability, decreased with soil depth. Moreover, the temperature at which half of SOM mass loss (TG–T50), a thermal index of SOM stability, increased with soil depth. Additionally, hot–water extractable organic carbon (HWEOC) per gram SOC, a chemical index of SOM stability, showed weak (meadow) and little (shrubland) declining trend with depth. Further, we used Fourier–transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) spectroscopy to characterize the molecular composition of SOM. The index of recalcitrance of FTIR spectra and the combined index of aliphaticity and aromaticity of NMR spectra both increased with depth, suggesting that the molecular composition of SOM was more complex with increasing depth. Finally, the isotopic values of SOM (13C and 15N) and the 14C–based SOC turnover time both increased with depth, indicating that the isotopic indices of SOM stability also increased with depth. Overall, our results suggest that the thermal, chemical, molecular and isotopic indices of SOM stability were mutually correlated and all showed increasing trend with increasing soil depth in the two alpine ecosystems, although the biological index (as measured by aerobic incubation of root–free sieved soils) showed the opposite results. • We used a number of techniques to characterize SOM stability along the soil profile in two alpine ecosystems on the Tibetan Plateau. • The biological index (as measured by aerobic incubation of root–free sieved soils) of SOM stability decreased with depth. • The chemical, thermal, molecular and isotopic indices of SOM stability all increased with depth. • We should interpret the biological index calculated from C mineralization rate of sieved root–free soils from lab–incubation studies with caution. • Combining biogeochemical, thermal, and spectroscopic evidences are imperative to explore SOM stability mechanisms. [ABSTRACT FROM AUTHOR]

The response of soil carbon storage to warming is critical for the climate due to the large flux of carbon between soil and atmosphere. Despite evidence that warming enhances carbon fluxes to and from the soil, how soil carbon stock respond to warming remains uncertain. Here we combine three complementary methods to explore the responses of soil carbon stock to warming in the alpine grassland on the Tibetan Plateau. These methods include sampling sites along temperature (elevation) gradients, measuring soils from two OTC warming experiments, and conducting a meta-analysis of field warming experiments across the plateau. Firstly, we sampled soils along two elevational gradients with similar vegetation (Kobresia meadow): 3200-4200 m on Mt. Qilian and 4400-5200 m on Mt. Nianqingtanggula. In the first gradient, with decreasing elevation (or warming), belowground biomass (BGB) did not change, while aboveground biomass (AGB), microbial biomass carbon (MBC) and soil organic carbon (SOC) all increased. In the second gradient, with decreasing elevation (or warming), AGB did not change, while BGB, MBC and SOC all declined. Moreover, the turnover rate of bulk SOC estimated by its 14C content, did not show clear trend with elevation in both gradients. These results suggest the responses of SOC dynamics to warming were inconsistent along the two elevation gradients. Secondly, in a seven-year OTC warming experiment in an alpine meadow (Kobresia humilis), four different levels of warming (0.4~2.0 C) with or without clipping, did not have significant effect on AGB, BGB, MBC, SOC and its two fractions (particulate organic matter (POM) and mineral-associated organic matter (MAOM)). Moreover, in a 20-year OTC warming experiment in two alpine meadows (dominated by Kobresia humilis and Potentilla fruticosa respectively), warming by 1-2 C did not change AGB, BGB, MBC, SOC and its two fractions (POM and MAOM), except that it reduced microbial biomass (MBC) and necromass (amino sugar) and MAOM-C in the surface soil of Potentilla fruticosa meadow. Finally, the meta-analysis of all existing warming experiments in alpine grassland on the plateau showed that warming overall had a minor effect on soil carbon stock. Such minor effect was independent of ecosystem (meadow or steppe), and the method, magnitude and duration of warming. Although evidence from more long-term, coordinated and distributed experiments and data from subsoils and permafrost are needed, our integrative analysis suggests that warming may not lead to soil carbon loss in alpine grasslands on the Tibetan Plateau. [ABSTRACT FROM AUTHOR]