Atmospheric water vapor and soil moisture jointly determine the spatiotemporal variations of CO(2) fluxes and evapotranspiration across the Qinghai-Tibetan Plateau grasslands.

Alpine grasslands play important functions in mitigating climate change and regulating water resources. However, the spatiotemporal variability of their carbon and water budgets remains unquantified. Here, 47 site-year observations of CO2 and water vapor fluxes (ET) are analyzed at sites situated along a hydrothermal gradient across the Qinghai-Tibetan Plateau, including an alpine wetland (wettest), an alpine shrub (coldest), an alpine meadow, an alpine meadow-steppe, and an alpine steppe (driest and warmest). The results show that the benchmarks for annual net ecosystem exchange (NEE) are -79.3, -77.8, -66.7, 20.2, and 100.9 g C m-2 year-1 at the meadow, shrub, meadow-steppe, steppe, and wetland, respectively. The peak daily NEE normalized by peak leaf area index converges to 0.93 g C m-2 d-1 at the 5 sites. Except in the wetland (722.8 mm), the benchmarks of annual ET fluctuate from 511.0 mm in the steppe to 589.2 mm in the meadow. Boosted regression trees-based analysis suggests that the enhanced vegetation index (EVI) and net radiation (Rn) determine the variations of growing season monthly CO2 fluxes and ET, respectively, although the effect is to some extent site-specific. Inter-annual variability in NEE, ecosystem respiration (RES), and ET are tightly (R2 > 0.60) related to the inter-growing season NEE, RES, and ET, respectively. Both annual RES and annual NEE are significantly constrained by annual gross primary productivity (GPP), with 85% of the per-unit GPP contributing to RES (R2 = 0.84) and 15% to NEE (R2 = 0.12). Annual GPP significantly correlates with annual ET alone at the drier sites of the meadow-steppe and the steppe, suggesting the coupling of carbon and water is moisture-dependent in alpine grasslands. Over half of the inter-annual spatial variability in GPP, RES, NEE, and ET is explained by EVI, atmospheric water vapor, topsoil water content, and bulk surface resistance (rs), respectively. Because the spatial variations of EVI and rs are strongly regulated by atmospheric water vapor (R2 = 0.48) and topsoil water content (R2 = 0.54), respectively, we conclude that atmospheric water vapor and topsoil water content, rather than the expected air/soil temperatures, drive the spatiotemporal variations in CO2 fluxes and ET across temperature-limited grasslands. These findings are critical for improving predictions of the carbon sequestration and water holding capacity of alpine grasslands.

Li H ，Wang C ，Zhang F ，He Y ，Shi P ，Guo X ，Wang J ，Zhang L ，Li Y ，Cao G ，Zhou H ... -  《-》

Carbon and water fluxes in an alpine steppe ecosystem in the Nam Co area of the Tibetan Plateau during two years with contrasting amounts of precipitation.

Wang Y ，Zhu Z ，Ma Y ，Yuan L ... -  《-》

Different responses of ecosystem carbon exchange to warming in three types of alpine grassland on the central Qinghai-Tibetan Plateau.

Ganjurjav H ，Hu G ，Wan Y ，Li Y ，Danjiu L ，Gao Q ... -  《Ecology and Evolution》

Seasonal and Inter-Annual Variations of Carbon Dioxide Fluxes and Their Determinants in an Alpine Meadow.

The alpine meadow is one of the most important ecosystems on the Qinghai-Tibet Plateau (QTP) due to its huge carbon storage and wide distribution. Evaluating the carbon fluxes in alpine meadow ecosystems is crucial to understand the dynamics of carbon storage in high-altitude areas. Here, we investigated the carbon fluxes at seasonal and inter-annual timescales based on 5 years of observations of eddy covariance fluxes in the Zoige alpine meadow on the eastern Tibetan Plateau. We found that the Zoige alpine meadow acted as a faint carbon source of 94.69 ± 86.44 g C m-2 y-1 during the observation periods with large seasonal and inter-annual variations (IAVs). At the seasonal scale, gross primary productivity (GPP) and ecosystem respiration (Re) were positively correlated with photosynthetic photon flux density (PPFD), average daily temperature (Ta), and vapor pressure (VPD) and had negative relationships with volumetric water content (VWC). Seasonal variations of net ecosystem carbon dioxide (CO2) exchange (NEE) were mostly explained by Ta, followed by PPFD, VPD, and VWC. The IAVs of GPP and Re were mainly attributable to the IAV of the maximum GPP rate (GPPmax) and maximum Re rate (Remax), respectively, both of which increased with the percentage of Cyperaceae and decreased with the percentage of Polygonaceae changes across years. The IAV of NEE was well explained by the anomalies of the maximum CO2 release rate (MCR). These results indicated that the annual net CO2 exchange in the alpine meadow ecosystem was controlled mainly by the maximum C release rates. Therefore, a better understanding of physiological response to various environmental factors at peak C uptake and release seasons will largely improve the predictions of GPP, Re, and NEE in the context of global change.

Wang S ，Chen W ，Fu Z ，Li Z ，Wang J ，Liao J ，Niu S ... -  《Frontiers in Plant Science》

Divergence in ecosystem carbon fluxes and soil nitrogen characteristics across alpine steppe, alpine meadow and alpine swamp ecosystems in a biome transition zone.

Alpine ecosystem carbon cycling is sensitive to climate change, particularly in the transition zones between biomes. Soil nitrogen conditions, including the ammonium to nitrate (NH4+/NO3-) ratio, regulate ecosystem carbon uptake by coupling carbon‑nitrogen cycle. The largest alpine pasture on Earth is distributed on the Tibetan Plateau, where alpine biome transition zones are also widely distributed. However, it is largely unknown how the soil NH4+/NO3- ratio and net ecosystem CO2 exchange vary among vegetation types in the alpine biome transition zones due to a lack of in situ field observations. Here, we investigated soil NH4+/NO3- ratio and ecosystem carbon fluxes across alpine steppe, alpine meadow and alpine swamp ecosystems in a biome transition zone on the central Tibetan Plateau. The results showed that soil NH4+/NO3- ratio was lowest in the alpine steppe (driest environment), which had the highest soil pH, and highest in the alpine swamp (wettest environment), which had the lowest soil pH. We proposed a theoretical framework describing how soil moisture regulates soil NH4+/NO3- ratio by altering both the denitrification process and soil pH. We further found that the growing season average net ecosystem CO2 exchange for the alpine steppe, alpine meadow and alpine swamp was -1.46, -1.90 and -5.43 μmol m-2 s-1, respectively. This divergence in net ecosystem CO2 exchange across the three grasslands is primarily explained by divergence in gross ecosystem photosynthesis, rather than ecosystem respiration. The air temperature sensitivity of ecosystem respiration (Q10) for the alpine steppe, alpine meadow and alpine swamp was 1.73 ± 0.05, 1.44 ± 0.03 and 2.43 ± 0.45, respectively. Our study highlights large differences in both soil nutrient and ecosystem carbon uptake across different vegetation types in an alpine biome transition zone. More in situ investigations in various biome transition zones are urgently needed to quantitatively understand the spatial pattern of alpine ecosystem carbon‑nitrogen cycling processes.

Liu Y ，Geng X ，Tenzintarchen ，Wei D ，Dai D ，Xu-Ri ... -  《-》