Alpine Swamp Meadow Research Articles

Patchy degradation‐induced changes in soil aggregates and organic carbon in an alpine swamp meadow on the Qinghai‐Tibetan Plateau

AbstractSoil cracking induced by grassland degradation has become increasingly common in the Qinghai‐Tibetan Plateau in recent decades, and the intensification of soil cracking has implications for soil carbon (C) cycling. However, how patchy degradation affects soil aggregates and organic C and the underlying mechanisms remain unclear. In the present study, we investigated the changes in soil aggregates and organic C across various degradation stages in patchy alpine swamp meadows. The results showed a hump‐shaped trend in the proportion of macroaggregates, mean weight diameter (MWD), macroaggregate‐associated organic C (MAOC), and their contribution to soil organic carbon (SOC) throughout the degradation sequence. Conversely, the proportion of microaggregates initially decreased and then increased, with the contribution of microaggregate‐associated organic C (MIOC) to SOC increasing as degradation progressed. Regression and piecewise structural equation modeling (SEM) showed that soil texture played a decisive role in the change in MAOC and MIOC, and enzyme activity was the key factor leading to the difference between MAOC and MIOC. Enhanced enzyme activity accelerated the decomposition of MAOC while favoring the accumulation of MIOC. These findings are crucial for understanding the mechanisms influencing the physical protection of SOC by aggregates under patchy degradation. This emphasized soil cracking as a turning point in aggregate and organic C changes and a trigger for severe degradation in alpine swamp meadows, warranting proactive measures to mitigate its onset and progression.

Dynamics of carbon and water vapor fluxes in three typical ecosystems of Heihe River Basin, Northwestern China

Understanding the dynamics of carbon and water vapor fluxes in arid inland river basin ecosystems is essential for predicting and assessing the regional carbon-water budget amid climate change. However, studies aiming to unravel the mechanisms driving the variations and coupling process of regional carbon-water budget in a changing environment in arid regions are limited. Here, we used the eddy covariance technique to analyze the relationship between CO2 and H2O fluxes in three typical ecosystems across the upper, middle, and lower reaches of an arid inland river basin in Northwestern China. Our results showed that all ecosystems acted as carbon sinks, with the alpine swamp meadow, cropland, and desert shrubland sequestrating −300.2 ± 0.01, −644.8 ± 2.9, and − 203.7 ± 22.5 g C m−2 yr−1, respectively. Air temperature (Ta) primarily controlled daily gross primary productivity (GPP) and net ecosystem CO2 exchange (NEE) in the irrigated cropland during the growing season, while soil temperature (Ts) and vapor pressure deficit (VPD) regulated these parameters in the alpine swamp meadow and desert shrubland. Additionally, Ta and net radiation (Rn) controlled daily evapotranspiration (ET) in cropland, while Ts and Rn regulated ET at other sites. Consequently, carbon and water vapor fluxes of all three ecosystems tended to be energy-limited during the growing season. The differential responses of carbon and water vapor fluxes in the upper, middle, and lower reaches of these ecosystems to biophysical factors determined their distinct coupling and variations in water use efficiency. Notably, the desert shrub ecosystem in the lower reach of the basin maintained a stable balance between carbon gain and water loss, indicating adaptation to aridity. This study provides valuable insights into the underlying mechanisms behind the changes in carbon and water vapor fluxes and water-use efficiency in arid river basin ecosystems.

Nitrogen dynamics of alpine swamp meadows are less responsive to climate warming than that of alpine meadows

The freeze-thaw cycle mediates permafrost soil hydrothermal status, nitrogen (N) mineralization, and loss. Furthermore, it affects root development and competition among nitrophilic and other species, shaping the pattern of N distribution in alpine ecosystems. However, the specific N dynamics during the growing season and N loss during the non-growing season in response to climate warming under low- and high-moisture conditions are not well documented. Therefore, we added 15N tracers to trace the fate of N in warmed and ambient alpine meadows and alpine swamp meadows in the permafrost region of the Qinghai-Tibet Plateau. During the growing season, warming increased 15N recovery (15Nrec) in shoots of K. humilis, litters, 0–5 and 5–20 cm roots in the alpine meadow by 149.94 % ± 52.87 %, 114.58 % ± 24.43 %, 61.11 % ± 32.27 %, and 97.12 % ± 42.92 %, respectively, while increased 15Nrec of litters by 151.55 % ± 27.06 % in the alpine swamp meadow. During the non-growing season, warming reduced 15N stored in roots by 486.77 % ± 57.90 %, though increased the 15N recovery in 5–20 cm soil depth by 76.68 % ± 39.42 % in the alpine meadow, whereas it did not affect N loss during the non-growing season in the alpine swamp meadow. Overall, warming promoted N utilization by increasing the plant N pool during the growing season, and enhanced root N loss and downward migration during the non-growing season due to the freeze-thaw process, which may result in fine root turnover and cell destruction releasing N in the alpine meadow. Conversely, the N dynamics of alpine swamp meadows were less responsive to climate warming.

Changes in soil oxidase activity induced by microbial life history strategies mediate the soil heterotrophic respiration response to drought and nitrogen enrichment.

Drought and nitrogen deposition are two major climate challenges, which can change the soil microbial community composition and ecological strategy and affect soil heterotrophic respiration (Rh). However, the combined effects of microbial community composition, microbial life strategies, and extracellular enzymes on the dynamics of Rh under drought and nitrogen deposition conditions remain unclear. Here, we experimented with an alpine swamp meadow to simulate drought (50% reduction in precipitation) and multilevel addition of nitrogen to determine the interactive effects of microbial community composition, microbial life strategy, and extracellular enzymes on Rh. The results showed that drought significantly reduced the seasonal mean Rh by 40.07%, and increased the Rh to soil respiration ratio by 22.04%. Drought significantly altered microbial community composition. The ratio of K- to r-selected bacteria (BK:r) and fungi (FK:r) increased by 20 and 91.43%, respectively. Drought increased hydrolase activities but decreased oxidase activities. However, adding N had no significant effect on microbial community composition, BK:r, FK:r, extracellular enzymes, or Rh. A structural equation model showed that the effects of drought and adding nitrogen via microbial community composition, microbial life strategy, and extracellular enzymes explained 84% of the variation in Rh. Oxidase activities decreased with BK:r, but increased with FK:r. Our findings show that drought decreased Rh primarily by inhibiting oxidase activities, which is induced by bacterial shifts from the r-strategy to the K-strategy. Our results highlight that the indirect regulation of drought on the carbon cycle through the dynamic of bacterial and fungal life history strategy should be considered for a better understanding of how terrestrial ecosystems respond to future climate change.

Spatio-temporal variation in soil thermal conductivity during the freeze-thaw period in the permafrost of the Qinghai–Tibet Plateau in 1980–2020

The Qinghai–Tibet Plateau (QTP) has the largest amount of permafrost in the low and middle latitudes, making it highly susceptible to the effects of global warming. In particular, the degradation of permafrost can be intensified by anomalous amplified warming. To accurately model the hydrothermal dynamics of permafrost and its future trends, the accumulation of high-precision, long-term data for the soil thermal conductivity (STC) in the active layer is crucial. However, no previous research has systematically investigated the spatio-temporal variation in the STC on the QTP over an extended period. Therefore, this study aims to fill this gap using the XGBoost model to analyze the STC in the permafrost on the QTP from 1980 to 2020. The findings of this study provide some preliminary insights. First, areas with high variation in the STC between the freeze–thaw periods over the 40 years gradually migrated from the western region to the central region. Second, since 2015, STC in more than 90 % of the permafrost region in the thawing period has shown positive growth. While, during the freezing period, the STC also exhibited an increase over most regions of the QTP, though the western region and parts of the northeastern region exhibited a decrease. Third, the spatial center of gravity for the STC during the freezing and thawing periods from 1980 to 2020 shifted. The mean STC was larger in the eastern and northeastern regions during the freezing period and larger in the western region during the thawing period. Fourth, both alpine swamp meadow and alpine meadow exhibited a gradual increase in the STC during the freeze–thaw period from 1980 to 2020. The conclusions and data products from this study are expected to support spatiotemporal modeling of the permafrost on the QTP and assist in the prognosis for its future.

Temperature sensitivity of methanogenesis and anaerobic methane oxidation in thermokarst lakes modulated by surrounding vegetation on the Qinghai-Tibet Plateau

Understanding the balance between methane (CH4) production (methanogenesis) and its oxidation is important for predicting carbon emissions from thermokarst lakes under global warming. However, the response of thermokarst lake methanogenesis and the anaerobic oxidation of methane (AOM) to warming, especially from Qinghai-Tibetan Plateau (QTP), is still not quantified. In this study, sediments were collected from 11 thermokarst lakes on the QTP. These lakes are surrounded with different vegetation types, including alpine desert (AD), alpine steppe (AS), alpine meadow (AM) and alpine swamp meadow (ASM). The results showed that methanogenesis and AOM rates exponentially increased with temperature, while the temperature sensitivity (Q10, average Q10 values of methanogenesis and AOM were 0.69–30 and 0.54–16.9 respectively) of methanogenesis were larger than AOM, but not significant, showing a similar temperature dependence of methanogenesis and AOM in thermokarst lake sediments. Thermokarst lake sediments in the ASM had higher methanogenesis and anaerobic oxidation potential, matching its higher NDVI and relative abundances of methanogens and SBM (syntrophic bacteria with methanogens). Although the thermokarst lake sediments AOM depleted 15 %–27.8 % of the total CH4 production, the AOM rate was lower than methanogenesis in thermokarst lake sediments, it did not offset increased CH4 production under anaerobic conditions. The increase in CH4 production in thermokarst lake sediments will likely lead to higher emissions within a warming world. These findings indicate that methanogenesis and AOM in thermokarst lake sediments are sensitive to climate change. Models should consider the Q10 values of methanogenesis and AOM and vegetation types when predicting carbon cycle in thermokarst lakes under global warming.

Effects of experimental warming on soil enzyme activities in an alpine swamp meadow on the Qinghai-Tibetan Plateau

Information about the response of soil enzymes to field warming in permafrost regions is scarce, and the potential mechanisms by which warming combined with biotic and abiotic factors affect soil enzyme activities in alpine grasslands remain unclear. A 3-year in situ experiment with two warming levels (2.7 °C and 5.3 °C) was conducted in an alpine swamp meadow to investigate the effects of experimental warming on 5 soil enzyme activities involved in soil carbon and nitrogen cycling, and the relationships between soil enzyme activities and soil variables related to physicochemical properties and nutrient levels were examined. Results showed that warming had obvious positive effects on soil moisture (SM), soil organic carbon, total nitrogen, and NH4+-N contents, especially in the surface soil layer. Soil NO3--N tended to decrease under moderate warming, while it significantly increased under high warming. Meanwhile, during the entire growing season, warming strongly enhanced invertase and amylase activities by 39.2–49.0% and 109.9–191.0%, respectively. In contrast, urease activities were significantly decreased by 36.8–55.7% in warming plots and there were no significant differences in catalase or cellulase activities among treatments during the whole growing season, while catalase activities in warming plots were significantly decreased by 2.6–4.0% in June. These inconsistent responses of soil enzyme activities to experimental warming could be partially explained by warming-induced changes in soil variables. Redundancy analysis showed that soil NO3--N and SM were the most important factors, which explained 29.8% and 19.7% of the variations in soil enzyme activities, respectively, suggesting that the warming-induced increase in SM could weaken the soil aeration status and enhance enzyme and substrate diffusion, strongly affecting soil nitrification and microbial enzyme production.

The drought-induced succession decreased ecosystem multifunctionality of alpine swamp meadow

Rapid climate change and intensified anthropogenic disturbances have dropped groundwater level and altered hydrology conditions, provoked the drought-induced succession in alpine swamp meadows. However, we currently lack general cognition on how the drought-induced succession influence the ecosystem structure and functions in alpine swamp meadows. Here, we examined the changes of the main above- and below-ground ecological functions following the drought-induced succession, including swamp meadow to alpine meadow, shrub meadow, and steppe meadow. We found that both above- and below-ground ecosystem multifunctionality (EMF) indexes of the swamp meadows both decreased with the drought-induced succession. Specifically, ecosystem functions of alpine swamp meadow such as plant productivity, soil water content, soil fertility, and carbon and nitrogen all declined consistently following the drought-induced succession. Further, our analysis showed that the dynamics of the above- and below-ground EMF were positively related to plant productivity, soil hydrological properties, and nutrient availability. The decreases in soil water and nutrient contents by the drought-induced succession were the major factors that resulted in the decreased EMF. Taken together, our results demonstratedthat the drought-induced succession decreased EMF of alpine swamp meadow, and our finding revealed that soil water and nutrient contents changes were critical indicators of the EMF change during the drought-induced succession. Thus, soil hydrology and nutrient functions conservation are key to maintaining the EMF of alpine swamp meadows under ongoing climate changes.

Spatial–Temporal Correlation Considering Environmental Factor Fusion for Estimating Gross Primary Productivity in Tibetan Grasslands

Gross primary productivity (GPP) is an important indicator in research on carbon cycling in terrestrial ecosystems. High-accuracy GPP prediction is crucial for ecosystem health and climate change assessments. We developed a site-level GPP prediction method based on the GeoMAN model, which was able to extract spatiotemporal features and fuse external environmental factors to predict GPP on the Tibetan Plateau. We evaluated four models’ behavior—Random Forest (RF), Support Vector Machine (SVM), Deep Belief Network (DBN), and GeoMAN—in predicting GPP at nine flux observation sites on the Tibetan Plateau. The GeoMAN model achieved the best results (R2 = 0.870, RMSE = 0.788 g Cm−2 d−1, MAE = 0.440 g Cm−2 d−1). Distance and vegetation type of the flux sites influenced GPP prediction, with the latter being more significant. The different grassland vegetation types exhibited different sensitivity to environmental factors (Ta, PAR, EVI, NDVI, and LSWI) for GPP prediction. Among them, the site located in the alpine swamp meadow was insensitive to changes in environmental factors; the GPP prediction accuracy of the site located in the alpine meadow steppe decreased significantly with the changes in environmental factors; and the GPP prediction accuracy of the site located in the alpine Kobresia meadow also varied with environmental factor changes, but to a lesser extent than the former. This study provides a good reference that deep learning model is able to achieve good accuracy in GPP simulation when considers spatial, temporal, and environmental factors, and the judgement made by deep learning model conforms to basic knowledge in the relevant field.

Response of methanogenic community and their activity to temperature rise in alpine swamp meadow at different water level of the permafrost wetland on Qinghai-Tibet Plateau.

Wetlands are an important source of atmospheric methane (CH4) and are sensitive to global climate change. Alpine swamp meadows, accounting for ~50% of the natural wetlands on the Qinghai-Tibet Plateau, were considered one of the most important ecosystems. Methanogens are important functional microbes that perform the methane producing process. However, the response of methanogenic community and the main pathways of CH4 production to temperature rise remains unknown in alpine swamp meadow at different water level in permafrost wetlands. In this study, we investigated the response of soil CH4 production and the shift of methanogenic community to temperature rise in the alpine swamp meadow soil samples with different water levels collected from the Qinghai-Tibet Plateau through anaerobic incubation at 5°C, 15°C and 25°C. The results showed that the CH4 contents increased with increasing incubation temperature, and were 5-10 times higher at the high water level sites (GHM1 and GHM2) than that at the low water level site (GHM3). For the high water level sites (GHM1 and GHM2), the change of incubation temperatures had little effect on the methanogenic community structure. Methanotrichaceae (32.44-65.46%), Methanobacteriaceae (19.30-58.86%) and Methanosarcinaceae (3.22-21.24%) were the dominant methanogen groups, with the abundance of Methanotrichaceae and Methanosarcinaceae having a significant positive correlation with CH4 production (p < 0.01). For the low water level site (GHM3), the methanogenic community structure changed greatly at 25°C. The Methanobacteriaceae (59.65-77.33%) was the dominant methanogen group at 5°C and 15°C; In contrast, the Methanosarcinaceae (69.29%) dominated at 25°C, and its abundance showed a significant positive correlation with CH4 production (p < 0.05). Collectively, these findings enhance the understanding of methanogenic community structures and CH4 production in permafrost wetlands with different water levels during the warming process.

Response of soil hydrothermal processes within the active layer to variable alpine vegetation conditions on the Qinghai‒Tibet Plateau

Alpine vegetation plays an important role in the thermal stability of the permafrost under a warming climate, as it affects ground hydrothermal dynamics. The response of soil hydrothermal dynamics in the active layer to permafrost degradation under different alpine grassland types is unclear on the Qinghai‒Tibet Plateau. In this study, long-term soil temperature and soil water content in the active layer were monitored in situ from October 2010 to December 2018 at five sites in the Kaixinling permafrost region on the interior Qinghai‒Tibet Plateau along the Qinghai–Tibet Railway. The sites included an alpine steppe (AS), three alpine meadows (AM) with different degrees of degraded vegetation, and an alpine swamp meadow (ASM). Based on field-monitored data, the variations in soil temperature, soil water content, and freeze–thaw processes were examined in the active layer. The response characteristics of the soil hydrothermal processes to climate change were analysed under the different alpine grasslands. The results showed that the duration of the thawing and freezing stages of the active layer of the AMs was shorter than that of the ASM and the AS. The average mean annual soil temperature (MAST) in the active layer of the AM ((−1.25 ± 0.50) °C) was lower than those in the AS ((−0.71 ± 0.39) °C) and ASM ((−0.45 ± 0.57) °C), while the AM had the highest rate of soil temperature increase ((0.2 ± 0.06) °C per year). The annual amplitude of ground temperature in the active layer increased with the transition direction of the alpine vegetation type from ASM to AM to AS. The small surface offset (SO) and thermal offset (TO) (absolute values) indicated that the ground thermal state of the AM was more unstable, as it was more sensitive to the increase in air temperature than the ASM or the AS. Soil properties controlled the distribution of soil water content within the active layer, but vegetation improved the shallow soil structure by producing more belowground phytomass, thus, enhancing soil water content in the 0–30 cm layer. The average soil water content at depths of 0–30 cm was directly proportional (p < 0.05) to the phytomass. Soil water contents at depths of 0–30 cm in the ASM ((37.7 ± 5.3)%) and the AM ((40.8 ± 5.9)%) were significantly higher than those in the AS ((22.7 ± 3.2)%). These results provide valuable insight into the hydrothermal interactions between the degradation of permafrost and alpine vegetation under a warming climate.

Spatiotemporal Patterns and Regional Differences in Soil Thermal Conductivity on the Qinghai–Tibet Plateau

The Qinghai–Tibet Plateau is an area known to be sensitive to global climate change, and the problems caused by permafrost degradation in the context of climate warming potentially have far-reaching effects on regional hydrogeological processes, ecosystem functions, and engineering safety. Soil thermal conductivity (STC) is a key input parameter for temperature and surface energy simulations of the permafrost active layer. Therefore, understanding the spatial distribution patterns and variation characteristics of STC is important for accurate simulation and future predictions of permafrost on the Qinghai–Tibet Plateau. However, no systematic research has been conducted on this topic. In this study, based on a dataset of 2972 STC measurements, we simulated the spatial distribution patterns and spatiotemporal variation of STC in the shallow layer (5 cm) of the Qinghai–Tibet Plateau and the permafrost area using a machine learning model. The monthly analysis results showed that the STC was high from May to August and low from January to April and from September to December. In addition, the mean STC in the permafrost region of the Qinghai–Tibet Plateau was higher during the thawing period than during the freezing period, while the STC in the eastern and southeastern regions is generally higher than that in the western and northwestern regions. From 2005 to 2018, the difference between the STC in the permafrost region during the thawing and freezing periods gradually decreased, with a slight difference in the western hinterland region and a large difference in the eastern region. In areas with specific landforms such as basins and mountainous areas, the changes in the STC during the thawing and freezing periods were different or even opposite. The STC of alpine meadow was found to be most sensitive to the changes during the thawing and freezing periods within the permafrost zone, while the STC for bare land, alpine desert, and alpine swamp meadow decreased overall between 2005 and 2018. The results of this study provide important baseline data for the subsequent analysis and simulation of the permafrost on the Qinghai–Tibet Plateau.

Retrieving Soil Moisture in the Permafrost Environment by Sentinel-1/2 Temporal Data on the Qinghai–Tibet Plateau

Soil moisture (SM) products presently available in permafrost regions, especially on the Qinghai–Tibet Plateau (QTP), hardly meet the demands of evaluating and modeling climatic, hydrological, and ecological processes, due to their significant bias and low spatial resolution. This study developed an algorithm to generate high-spatial-resolution SM during the thawing season using Sentinel-1 (S1) and Sentinel-2 (S2) temporal data in the permafrost environment. This algorithm utilizes the seasonal backscatter differences to reduce the effect of surface roughness and uses the normalized difference vegetation index (NDVI) and the normalized difference moisture index (NDMI) to characterize vegetation contribution. Then, the SM map with a grid spacing of 50 m × 50 m in the hinterland of the QTP with an area of 505 km × 246 km was generated. The results were independently validated based on in situ data from active layer monitoring sites. It shows that this algorithm can retrieve SM well in the study area. The coefficient of determination (R2) and root-mean-square error (RMSE) are 0.82 and 0.06 m3/m3, respectively. This study analyzed the SM distribution of different vegetation types: the alpine swamp meadow had the largest SM of 0.26 m3/m3, followed by the alpine meadow (0.23), alpine steppe (0.2), and alpine desert (0.16), taking the Tuotuo River basin as an example. We also found a significantly negative correlation between the coefficient of variation (CV) and SM in the permafrost area, and the variability of SM is higher in drier environments and lower in wetter environments. The comparison with ERA5-Land, GLDAS, and ESA CCI showed that the proposed method can provide more spatial details and achieve better performance in permafrost areas on QTP. The results also indicated that the developed algorithm has the potential to be applied in the entire permafrost regions on the QTP.

Characteristics of Evapotranspiration and Crop Coefficient Correction at a Permafrost Swamp Meadow in Dongkemadi Watershed, the Source of Yangtze River in Interior Qinghai–Tibet Plateau

The Qinghai–Tibet Plateau (QTP), known as the Earth’s third pole, is highly sensitive to climate change. Various environmental degradation has occurred due to the effects of climate warming such as the degradation of permafrost and the thickening of active layers. Evapotranspiration, as a key element of hydrothermal coupling, has become a key factor of the plateau environment for deciphering deterioration, and the FAO P-M model has a good physical foundation and simple model data requirements as a primary tool to study the plateau evapotranspiration. There has been a large research base, but the estimation of evapotranspiration in alpine regions is still subject to many uncertainties. This is reflected in the fact that the classification of underlying surface types has not been sufficiently detailed and the evapotranspiration characteristics of some special underlying surface types are still unclear. Therefore, in this work, we modified the FAO P-M coefficients based on the characteristics of actual evapotranspiration measured by the Eddy covariance system and the key influencing factors to better simulate the actual evapotranspiration in alpine swamp meadow. The results were as follows: (1) Both ETa measured by the Eddy covariance system and ET0 calculated by FAO P-M showed the same trend at the daily and annual scales and hysteresis was confirmed to exist, so the error caused by hysteresis should be considered in further research. (2) The annual ETa was 566.97 mm and annual ETa/P was 0.76, and about 11.19% of ETa occurred during the night. The ETa was 2.15 during the non-growing seasons, implying that a large amount of soil water was released into the air by evapotranspiration. (3) The evapotranspiration characteristics of alpine swamp meadow are formed under the following conditions: control of net radiation (Rn) affected by VPD during the growing season and affected by soil temperature and humidity during the non-growing season. Precipitation and soil water content are no longer the main controlling factors of evapotranspiration during the growing season at the alpine swamp meadow as the volume soil water content tends to saturate. (4) The basic corrected Kc was 1.14 during the initial and mid-growing season, 1.05 during the subsequent growing season, and 0–0.25 during the non-growing season, and the correction factor process can also provide ideas for correcting the Kc of other vegetation.

Water Budget, Biological Water Use, and the Soil Hydrological Cycle across Typical Ecosystems of the Heihe River Basin

Quantification of the water budget of an arid inland river ecosystem is essential but still a challenge for the sustainable development of water resources. In situ observed data were used to analyze the monthly and annual water budgets and the soil hydrological cycle for six typical ecosystems in the Heihe River Basin (HRB). The two-source model was used to partition evapotranspiration (ET) into transpiration (T) and evaporation, after which the validated model was applied to quantitatively analyze the biological water use fraction [T/Ecosystem Water Supply (WS)] for different ecosystems. There were differences in the water budgets of the different ecosystems due to differences in climate, vegetation, soil, and external inputs. Precipitation in the HRB decreased from upstream to downstream, whereas there was a gradual increase in ET. External sources of water (e.g., natural runoff from upstream, irrigation in the middle reaches, and groundwater recharge in the lower reaches) to soil layers played an important role in regulating the water budgets of HRB ecosystems. Cropland obtained the maximum biological water use fraction (0.50), followed by Populus euphratica (0.49), alpine meadow (0.49), alpine swamp meadow (0.44), Tamarix ramosissima (0.42), and Kalidium foliatum (0.4). The soil water residence time (at a depth of 40 cm) varied from 14 d to 97 d (average of 60 d). The order of plant species in terms of soil water residence time was: K. foliatum (88 d) &gt; T. ramosissima (72 d) &gt; alpine meadow (68 d) &gt; alpine swamp meadow (63 d) &gt; cropland (53 d) &gt; P. euphratica forest (20 d). Differences in the biological water use fraction and soil water residence time could be attributed to the characteristics of the water budget for each ecosystem. This study quantified the water budget, biological water use, and soil hydrological cycle across typical ecosystems in HRB, and can act as a reference for ecosystem management of the arid inland river basin.

Microtopographic heterogeneity mediates the soil respiration response to grazing in an alpine swamp meadow on the Tibetan Plateau

Soil respiration (Rs) is a major component of terrestrial carbon cycling, which varies greatly with spatial heterogeneity or under grazing. There is scant evidence for the interactive effects of microtopography and grazing on Rs, particularly in alpine swamp meadow ecosystems where hummocks and hollows are common microtopographic features. We conducted a grazing exclusion experiment in an alpine swamp meadow on the central Tibetan Plateau. Seasonal variation in Rs and relevant environmental factors were observed between hummocks and hollows in the free grazing plots and grazing exclusion plots. The growing season Rs was significantly higher in the hummocks than that in the hollows. The responses of Rs to grazing varied greatly with microtopographic heterogeneity, with grazing significantly decreasing Rs in the hummocks but greatly increasing Rs in the hollows. The grazing-induced relative changes in aboveground biomass, belowground biomass, soil organic carbon and microbial biomass carbon were positively correlated with the changes in Rs. Grazing and microtopography caused changes in soil temperature and microbial biomass were the major factors that driving the seasonal variation in Rs. This information stresses the importance of hummock-hollow microtopography in regulating carbon cycling in grazed alpine swamp meadows.

Radiation, soil water content, and temperature effects on carbon cycling in an alpine swamp meadow of the northeastern Qinghai–Tibetan Plateau

Abstract. Predicted intensified climate warming will likely alter the ecosystem net carbon (C) uptake of the Qinghai–Tibetan Plateau (QTP). Variations in C sink–source responses to climate warming have been linked to water availability; however, the mechanisms by which net C uptake responds to soil water content in saturated swamp meadow ecosystems remain unclear. To explore how soil moisture and other environmental drivers modulate net C uptake in the QTP, field measurements were conducted using the eddy covariance technique in 2014, 2015, 2017, and 2018. The alpine swamp meadow presented in this study was a persistent and strong C sink of CO2 (−168.0 ± 62.5 g C m−2 yr−1, average ± standard deviation) across the entire 4-year study period. A random forest machine-learning analysis suggested that the diurnal and seasonal variations of net ecosystem exchange (NEE) and gross primary productivity (GPP) were regulated by temperature and net radiation. Ecosystem respiration (Re), however, was found mainly regulated by the variability of soil water content (SWC) at different temporal aggregations, followed by temperature, the second contributing driver. We further explored how Re is controlled by nearly saturated soil moisture and temperature comparing two different periods featuring almost identical temperatures and significant differences on SWC and vice versa. Our data suggest that, despite the relatively abundant water supply, periods with a substantial decrease in SWC or increase in temperature produced higher Re and therefore weakened the C sink strength. Our results reveal that nearly saturated soil conditions during the growing seasons can help maintain lower ecosystem respiration rates and thus enhance the overall C sequestration capacity in this alpine swamp meadow. We argue that soil respiration and subsequent ecosystem C sink magnitude in alpine swamp meadows could likely be affected by future changes in soil hydrological conditions caused by permafrost degradation or accelerated thawing–freezing cycling due to climate warming.

Vegetation Mapping in the Permafrost Region: A Case Study on the Central Qinghai-Tibet Plateau

An accurate and detailed vegetation map is of crucial significance for understanding the spatial heterogeneity of subsurfaces, which can help to characterize the thermal state of permafrost. The absence of an alpine swamp meadow (ASM) type, or an insufficient resolution (usually km-level) to capture the spatial distribution of the ASM, greatly limits the availability of existing vegetation maps in permafrost modeling of the Qinghai-Tibet Plateau (QTP). This study generated a map of the vegetation type at a spatial resolution of 30 m on the central QTP. The random forest (RF) classification approach was employed to map the vegetation based on 319 ground-truth samples, combined with a set of input variables derived from the visible, infrared, and thermal Landsat-8 images. Validation using a train-test split (i.e., 70% of the samples were randomly selected to train the RF model, while the remaining 30% were used for validation and a total of 1000 runs) showed that the average overall accuracy and Kappa coefficient of the RF approach were 0.78 (0.68–0.85) and 0.69 (0.64–0.74), respectively. The confusion matrix showed that the overall accuracy and Kappa coefficient of the predicted vegetation map reached 0.848 (0.844–0.852) and 0.790 (0.785–0.796), respectively. The user accuracies for the ASM, alpine meadow, alpine steppe, and alpine desert were 95.0%, 83.3%, 82.4%, and 86.7%, respectively. The most important variables for vegetation type prediction were two vegetation indices, i.e., NDVI and EVI. The surface reflectance of visible and shortwave infrared bands showed a secondary contribution, and the brightness temperature and the surface temperature of the thermal infrared bands showed little contribution. The dominant vegetation in the study area is alpine steppe and alpine desert. The results of this study can provide an accurate and detailed vegetation map, especially for the distribution of the ASM, which can help to improve further permafrost studies.

Plant and microbial regulations of soil carbon dynamics under warming in two alpine swamp meadow ecosystems on the Tibetan Plateau

Increasing temperature plays important roles in affecting plant and soil microbial communities as well as ecological processes and functions in terrestrial ecosystems. However, mechanisms of warming influencing soil carbon dynamics associated with plant-microbe interactions remain unclear. In this study, open-top chambers (OTCs) experiments were carried out to detect the responses of plants, soil microbes, and SOC contents, physical fractions (by particle-size fractionation) and chemical composition (by solid-state 13C NMR spectroscopy) to warming in two alpine swamp meadows (Kobresia humilis vs K. tibetica) on the Tibetan Plateau. Our results showed that four years of warming had significant influences on plant belowground biomass, microbial community and SOC contents in the K. humilis swamp meadow, but had much weaker or minor effects in the K. tibetica swamp meadow with water-logged status and lower level of warming. In the K. humilis swamp meadow, warming increased microbial biomass, C-hydrolysis gene abundance and N-acetylglucosaminidase enzyme activity. These positive effects of warming on microbial biomass and functions further increased soil dissolved inorganic nitrogen and alleviated the nitrogen limitation for plant growth, potentially leading to higher plant biomass. Therefore, increases in SOC and particulate organic carbon (POC) under warming were likely attributed to the higher C input with promoted plant biomass overweighting the simultaneous higher C degradation and release in the K. humilis swamp meadow. Conversely, warming marginally reduced soil alkyl C, which was likely associated with enhanced decomposition by fungi and gram-positive bacteria. Overall, the increases in unprotected POC and decreases in recalcitrant alkyl C demonstrate the sensitivity of SOC physical fractions as well as chemical composition to climate warming in the K. humilis alpine swamp meadow, and suggest that the overall stability of SOC might be lower despite the gain in the content of SOC after climate warming in this alpine swamp meadow.

Plateau pika offsets the positive effects of warming on soil organic carbon in an alpine swamp meadow on the Tibetan Plateau

Both climate warming and plateau pika (Ochotona curzoniae) exert considerable impacts on carbon cycling in alpine ecosystems on the Tibetan Plateau. However, the interaction of warming and pika disturbance on soil organic carbon (SOC) dynamics in alpine ecosystems remains largely unknown. Here, we measured plant, soil and microbial properties after four-year warming in a swamp meadow (with or without pika disturbance) on the Tibetan Plateau. Our results showed that in non-pika plots, warming not only increased plant belowground biomass (48%), but also enhanced microbial biomass, hydrolytic enzyme activities, and bacterial functional genes (41–46%). More plant inputs likely outweighed faster microbial decomposition, leading to the accumulation of fast-cycling particulate organic carbon (POC, 49%) and bulk SOC (22%), but not slow-cycling mineral-associated organic carbon (MAOC). In pika-disturbed plots, however, warming did not significantly change plant belowground biomass (which dominates total plant biomass), microbial biomass and hydrolytic enzyme activities. Although bacterial functional genes were suppressed (−40%) and plant aboveground biomass was increased (118%), oxidative enzymes were significantly stimulated (17%), which likely counteracted the higher aboveground plant inputs and led to minor changes in soil carbon pools (SOC, POC and MAOC) with warming. Moreover, bacterial and fungal community structure were significantly altered by pika disturbance, but not warming. Overall, these findings demonstrated that pika disturbance could offset the short-term positive effect of warming on soil organic carbon in the alpine swamp meadow ecosystem.