The effects of warming and nitrogen addition on ecosystem respiration in a Tibetan alpine meadow: The significance of winter warming

Altitudinal patterns of alpine soil ammonia-oxidizing community structure and potential nitrification rate

Nitrogen availability limits the net primary productivity in alpine meadows on the Qinghai-Tibetan Plateau, which is regulated by ammonia-oxidizing microorganisms. However, little is known about the elevational patterns of soil ammonia oxidizers in alpine meadows. Here, we investigated the potential nitrification rate (PNR), abundance, and community diversity of soil ammonia-oxidizing microorganisms along the altitudinal gradient between 3,200 and 4,200 m in Qinghai-Tibetan alpine meadows. We found that both PNR and amoA gene abundance declined from 3,400 to 4,200 m but lowered at 3,200 m, possibly due to intense substrate competition and biological nitrification inhibition from grasses. The primary contributors to soil nitrification were ammonia-oxidizing archaea (AOA), and their proportionate share of soil nitrification increased with altitude in comparison to ammonia-oxidizing bacteria (AOB). The alpha diversity of AOA increased by higher temperature and plant richness at low elevations, while decreased by higher moisture and low legume biomass at middle elevations. In contrast, the alpha diversity of AOB increased along elevation. The elevational patterns of AOA and AOB communities were primarily driven by temperature, soil moisture, and vegetation. These findings suggest that elevation-induced climate changes, such as shifts in temperature and water conditions, could potentially alter the soil nitrification process in alpine meadows through changes in vegetation and soil properties, which provide new insights into how soil ammonia oxidizers respond to climate change in alpine meadows.IMPORTANCEThe importance of this study is revealing that elevational patterns and nitrification contributions of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) communities were primarily driven by temperature, soil moisture, and vegetation. Compared to AOB, the relative contribution of AOA to soil nitrification increased at higher elevations. The research highlights the potential impact of elevation-induced climate change on nitrification processes in alpine meadows, mediated by alterations in vegetation and soil properties. By providing new insights into how ammonia oxidizers respond to climate change, this study contributes valuable knowledge to the field of microbial ecology and helps predict ecological responses to environmental changes in alpine meadows.

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.

Winter warming alleviates the severely negative effects of nitrogen addition on ecosystem stability in a Tibetan alpine grassland

Many recent studies have explored how global warming and increased nitrogen (N) deposition affect the structure and function of natural ecosystems. However, how ecosystems respond to the combination of warming and N enrichment remains unexplored, especially under asymmetric seasonal warming scenarios. We conducted a decade-long field experiment in an alpine grassland to investigate the effects of warming (ambient condition (NW), winter-only (WW), and year-round (YW) warming) and N addition on the temporal stability of communities. Although N addition significantly reduced community temporal stability in NW, WW, and YW, WW relieved the severely negative effects of N addition compared to NW and YW (from 47.7 % in NW and 76.1 % in YW to 18.6 % in WW under 80 kg N hm^-2 year^-1). The most remarkable finding is that the main factors driving community stability shifted with warming patterns. The increase in community dominance under NW was a significant driver of the decreased temporal stability in the community. However, the decrease in community stability caused by N addition was ascribed to the decreased stability of both dominant and common species under WW. In contrast, N addition decreased community temporal stability mainly via a decrease in species asynchrony under YW. Our results suggested that warming patterns can modulate the effects of N enhancement on community stability. To predict the effects of climate change on alpine grasslands accurately, the idiosyncratic effects of asymmetric seasonal warming under future climate change scenarios should be considered.

Aridity modifies the responses of plant stoichiometry to global warming and nitrogen deposition in semi-arid steppes

Global warming and nitrogen (N) deposition are known to unbalance the stoichiometry of carbon (C), N, and phosphorus (P) in terrestrial plants, but it is unclear how water availability regulates their effects along a natural aridity gradient. Here, we conducted manipulative experiments to determine the effects of experimental warming (WT) and N addition (NT) on plant stoichiometry in desert, typical, and meadow steppes with decreasing aridity. WT elevated air temperatures by 1.2-2.9 °C using open-top chambers. WT increased forb C:N ratio and thus its N use efficiency and competitiveness in desert steppes, whereas WT reduced forb C:N and C:P ratios in typical and meadow steppes. Plant N:P ratio, which reflects nutrient limitation, was reduced by WT in desert steppes but not for typical or meadow steppes. NT reduced plant C:N ratios and increased N:P ratios in all three steppes. NT reduced forb C:P ratios in desert and typical steppes, but it enhanced grass C:P ratio in meadow steppes, indicating an enhancement of P use efficiency and competitiveness of grasses in wet steppes. WT and NT had synergetic effects on grass C:N and C:P ratios in all three steppes, which helps to increase grasses' productivity. Under WT or NT, the changes in community C:N ratio were positively correlated with increasing aridity, indicating that aridity increases plants' N use efficiency. However, aridity negatively affected the changes in N:P ratios under NT but not WT, which suggests that aridity mitigates P limitation induced by N deposition. Our results imply that warming could shift the dominant functional group into forbs in dry steppes due to altered stoichiometry, whereas grasses become dominated plants in wet steppes under increasing N deposition. We suggest that global changes might break the stoichiometric balance of plants and water availability could strongly modify such processes in semi-arid steppes.

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.

Warming has a minor effect on surface soil organic carbon in alpine meadow ecosystems on the Qinghai-Tibetan Plateau

The alpine meadow ecosystem on the Qinghai-Tibetan Plateau (QTP) is very sensitive to warming and plays a key role in regulating global carbon (C) cycling. However, how warming affects the soil organic carbon (SOC) pool and related C inputs and outputs in alpine meadow ecosystems on the QTP remains unclear. Here, we combined two field experiments and a meta-analysis on field experiments to synthesize the responses of the SOC pool and related C cycling processes to warming in alpine meadow ecosystems on the QTP. We found that the SOC content of surface soil (0-10 cm) showed a minor response to warming, but plant respiration was accelerated by warming. In addition, the warming effect on SOC was not correlated with experimental and environmental variables, such as the method, magnitude and duration of warming, initial SOC content, mean annual temperature, and mean annual precipitation. We conclude that the surface SOC content is resistant to climate warming in alpine meadow ecosystems on the QTP.

Degradation shifts plant communities from S- to R-strategy in an alpine meadow, Tibetan Plateau

The replacement of dominant sedges/grasses with secondary forbs is common in alpine rangelands, but the underlying plant ecological strategies and their relevance to leaf traits and their variabilities of different plant functional groups remain largely unknown. Here, we measured key leaf traits and analyzed the competitor, stress-tolerator and ruderal (CSR) strategies of major species with different functional groups (sedges, grasses and forbs) in an alpine meadow along a degradation gradient on the Tibetan Plateau. Our results indicated that S-selected species were dominant in both non-degraded (C:S:R = 1:95:4%) and severely degraded (C:S:R = 2:87:11%) meadows. However, there was a shift from S- to R-strategy in the communities after rangeland degradation. More specifically, sedges and grasses with a "conservative" strategy maintained stronger S-strategy to tolerate degraded and stressful conditions. In contrast, forbs with an "opportunistic" strategy (increase 9.5% in R-score) tended to adapt to degraded stages. Moreover, 51.1% and 23.9% of the increased R-scores in forbs were accounted by leaf mass per area and specific leaf area, respectively. Generally, higher leaf water and nitrogen contents coupled with larger variations in leaf traits and flexible SR strategies in forbs enabled them to capitalize on lower soil water and nutrient availability. Our findings highlighted that the contrasting strategies of plant species in response to the decrease in available resources might lead to niche expansion of secondary forbs and loss of diversity in the degraded alpine meadow. The emerging alternative stable states in the degraded rangelands might bring about a predicament for rangeland restoration.

Permafrost response to temperature rise in carbon and nutrient cycling: Effects from habitat-specific conditions and factors of warming

Permafrost is experiencing climate warming at a rate that is two times faster than the rest of the Earth's surface. However, it is still lack of a quantitative basis for predicting the functional stability of permafrost ecosystems in carbon (C) and nutrient cycling. We compiled the data of 708 observations from 89 air-warming experiments in the Northern Hemisphere and characterized the general effects of temperature increase on permafrost C exchange and balance, biomass production, microbial biomass, soil nutrients, and vegetation N dynamics through a meta-analysis. Also, an investigation was made on how responses might change with habitat-specific (e.g., plant functional groups and soil moisture status) conditions and warming variables (e.g., warming phases, levels, and timing). The net ecosystem C exchange (NEE) was found to be downregulated by warming as a result of a stronger sensitivity to warming in respiration (15.6%) than in photosynthesis (6.2%). Vegetation usually responded to warming by investing more C to the belowground, as belowground biomass increased much more (30.1%) than aboveground biomass (2.9%). Warming had a minor effect on microbial biomass. Warming increased soil ammonium and nitrate concentrations. What's more, a synthesis of 70 observations from 11 herbs and 9 shrubs revealed a 2.5% decline of N in green leaves. Compared with herbs, shrubs had a stronger response to respiration and had a decline in green leaf N to a greater extent. Not only in dry condition did green leaf N decline with warming but also in wet conditions. Warming in nongrowing seasons would negatively affect soil water, C uptake, and biomass production during growing seasons. Permafrost C loss and vegetation N decline may increase with warming levels and timing. Overall, these findings suggest that besides a positive C cycling-climate feedback, there will be a negative feedback between permafrost nutrient cycling and climate warming.

Community species diversity mediates the trade-off between aboveground and belowground biomass for grasses and forbs in degraded alpine meadow, Tibetan Plateau

Although many empirical experiments have shown that increasing degradation results in lower aboveground biomass (AGB), our knowledge of the magnitude of belowground biomass (BGB) for individual plants is a prerequisite for accurately revealing the biomass trade-off in degraded grasslands. Here, by linking the AGB and BGB of individual plants, species in the community, and soil properties, we explored the biomass partitioning patterns in different plant functional groups (grasses of Stipa capillacea and forbs of Anaphalis xylorhiza). Our results indicated that 81% and 60% of the biomass trade-off variations could be explained by environmental factors affecting grasses and forbs, respectively. The change in community species diversity dominated the biomass trade-off via either direct or indirect effects on soil properties and biomass. However, the community species diversity imparted divergent effects on the biomass trade-off for grasses (scored at -0.72) and forbs (scored at 0.59). Our findings suggest that plant communities have evolved two contrasting strategies of biomass allocation patterns in degraded grasslands. These are the "conservative" strategy in grasses, in which plants with larger BGB trade-off depends on gigantic roots for soil resources, and the "opportunistic" strategy in forbs, in which plants can adapt to degraded lands using high variation and optimal biomass allocation.

Microbial metabolic response to winter warming stabilizes soil carbon

Current consensus on global climate change predicts warming trends with more pronounced temperature changes in winter than summer in the Northern Hemisphere at high latitudes. Moderate increases in soil temperature are generally related to faster rates of soil organic carbon (SOC) decomposition in Northern ecosystems, but there is evidence that SOC stocks have remained remarkably stable or even increased on the Tibetan Plateau under these conditions. This intriguing observation points to altered soil microbial mediation of carbon-cycling feedbacks in this region that might be related to seasonal warming. This study investigated the unexplained SOC stabilization observed on the Tibetan Plateau by quantifying microbial responses to experimental seasonal warming in a typical alpine meadow. Ecosystem respiration was reduced by 17%-38% under winter warming compared with year-round warming or no warming and coincided with decreased abundances of fungi and functional genes that control labile and stable organic carbon decomposition. Compared with year-round warming, winter warming slowed macroaggregate turnover rates by 1.6 times, increased fine intra-aggregate particulate organic matter content by 75%, and increased carbon stabilized in microaggregates within stable macroaggregates by 56%. Larger bacterial "necromass" (amino sugars) concentrations in soil under winter warming coincided with a 12% increase in carboxyl-C. These results indicate the enhanced physical preservation of SOC under winter warming and emphasize the role of soil microorganisms in aggregate life cycles. In summary, the divergent responses of SOC persistence in soils exposed to winter warming compared to year-round warming are explained by the slowing of microbial decomposition but increasing physical protection of microbially derived organic compounds. Consequently, the soil microbial response to winter warming on the Tibetan Plateau may cause negative feedbacks to global climate change and should be considered in Earth system models.

Unaltered soil microbial community composition, but decreased metabolic activity in a semiarid grassland after two years of passive experimental warming

Soil microbial communities regulate soil carbon feedbacks to climate warming through microbial respiration (i.e., metabolic rate). A thorough understanding of the responses of composition, biomass, and metabolic rate of soil microbial community to warming is crucial to predict soil carbon stocks in a future warmer climate. Therefore, we conducted a field manipulative experiment in a semiarid grassland on the Loess Plateau of China to evaluate the responses of the soil microbial community to increased temperature from April 2015 to December 2017. Soil temperature was 2.0°C higher relative to the ambient when open-top chambers (OTCs) were used. Warming did not affect microbial biomass or the composition of microbial functional groups. However, warming significantly decreased microbial respiration, directly resulting from soil pH decrease driven by the comediation of aboveground biomass increase, inorganic nitrogen increase, and moisture decrease. These findings highlight that the soil microbial community structure of semiarid grasslands resisted the short-term warming by 2°C, although its metabolic rate declined.

Warming Increases Nitrous Oxide Emission from the Littoral Zone of Lake Poyang, China

Littoral wetlands are globally important for sustainable development; however, they have recently been identified as critical hotspots of nitrous oxide (N2O) emissions. N2O flux from subtropical littoral wetlands remains unclear, especially under the current global warming environment. In the littoral zone of Lake Poyang, a simulated warming experiment was conducted to investigate N2O flux. Open-top chambers were used to raise temperature, and the static chamber-gas chromatograph method was used to measure N2O flux. Results showed that the littoral zone of Lake Poyang was an N2O source, with an average flux rate of 8.9 μg N2O m−2 h−1. Warming significantly increased N2O emission (13.8 μg N2O m−2 h−1 under warming treatment) by 54% compared to the control treatment. N2O flux in the spring growing season was also significantly higher than that of the autumn growing season. In addition, temperature was not significantly related to N2O flux, while soil moisture only explained about 7% of N2O variation. These results imply that N2O emission experiences positive feedback effect on the ongoing warming of the climate, and abiotic factors (e.g., soil temperature and soil moisture) were not main controls on N2O variation in this littoral wetland.

Effects of Warming and N Deposition on the Physiological Performances of Leymus secalinus in Alpine Meadow of Qinghai-Tibetan Plateau

Warming and Nitrogen (N) deposition are key global changes that may affect eco-physiological process of territorial plants. In this paper, we examined the effects of warming, N deposition, and their combination effect on the physiological performances of Leymus secalinus. Four treatments were established in an alpine meadow of Qinghai-Tibetan plateau: control (CK), warming (W), N deposition (N), and warming plus N deposition (NW). Warming significantly decreased the photosynthetic rate (A_net ), stomatal conductance (g_s ), intercellular CO₂ concentration (C_i ), and transpiration rate (T_r ), while N deposition and warming plus N deposition significantly increased those parameters of L. secalinus. Warming significantly increased the VPD and L_s , while N deposition and warming plus N deposition had a significant positive effect. Warming negatively reduced the leaf N content, Chla, Chlb, and total Chl content, while N deposition significantly promoted these traits. Warming, N deposition, and their combination significantly increased the activity of SOD, POD, and CAT. Besides, warming and warming plus N deposition significantly increased the MDA content, while N deposition significantly decreased the MDA content. N deposition and warming plus N deposition significantly increased the Rubisco activity, while warming showed no significant effect on Rubisco activity. N deposition and warming plus N deposition significantly increased the Fv/Fm, ΦPSII, qP, and decreased NPQ, while warming significantly decreased the Fv/Fm, ΦPSII, qP, and increased NPQ. N deposition strengthened the relations between g_s , Chl, Chla, Chlb, Rubisco activity, and A_net . Under warming, only g_s showed a significantly positive relation with A_net . Our findings suggested that warming could impair the photosynthetic potential of L. secalinus enhanced by N deposition. Additionally, the combination of warming and N deposition still tend to lead positive effects on L. secalinus.

Response of plant production to growing/non-growing season asymmetric warming in an alpine meadow of the Northern Tibetan Plateau

A field growing/non-growing season asymmetric warming experiment (C: control, i.e., no warming in the entire year; GLNG: growing season warming lower than non-growing season warming; GHNG: growing season warming higher than non-growing season warming) was conducted in an alpine meadow of the Northern Tibetan Plateau in early June 2015. The effects of growing/non-growing season asymmetric warming on the normalized difference vegetation index (NDVI), soil adjusted vegetation index (SAVI), aboveground biomass (AGB) and gross primary production (GPP) in 2015-2017 were examined. The 'GLNG' and 'GHNG' treatments significantly increased the annual mean air temperature (T_a) by 2.95 °C and 2.76 °C, and the vapor pressure deficit (VPD) by 0.23 kPa and 0.28 kPa but significantly reduced the annual mean soil moisture (SM) by 0.02 m³ m^-3 and 0.02 m³ m^-3 respectively; however, changes in the annual mean T_a, VPD and SM were the same between the 'GLNG' and 'GHNG' treatments over the three years in 2015-2017. There were no significant differences in the SAVI and GPP among the 'C', 'GLNG' and 'GHNG' treatments over the three growing seasons in 2015-2017. The 'GLNG' and 'GHNG' treatments did not significantly affect the NDVI and AGB compared to 'C', whereas the NDVI and AGB under the 'GLNG' treatment were significantly greater than those under the 'GHNG' treatment over the three growing seasons in 2015-2017. The significant differences in NDVI and AGB between the 'GLNG' and 'GHNG' treatments may be attributed to the different effects under the 'GLNG' and 'GHNG' treatments on the non-growing season T_a, growing season water availability and soil nitrogen availability. Therefore, the non-growing season with a higher warming magnitude may have stronger effects on the aboveground plant production than did the growing season with a higher warming magnitude in the alpine meadow of the Northern Tibetan Plateau.