Diurnal evolution of the temperature sensitivity of CO2 efflux in permafrost soils under control and warm conditions

Experimental warming increases ecosystem respiration by increasing above-ground respiration in alpine meadows of Western Himalaya

Alpine ecosystems in the Himalaya, despite low primary productivity, store considerable amount of organic carbon. However, these ecosystems are highly vulnerable to climate warming which may stimulate ecosystem carbon efflux leading to carbon-loss and positive feedback. We used open-top chambers to understand warming responses of ecosystem respiration (ER) and soil respiration (SR) in two types of alpine meadows viz., herbaceous meadow (HM) and sedge meadow (SM), in the Western Himalaya. Experimental warming increased ER by 33% and 28% at HM and SM, respectively. No significant effect on SR was observed under warming, suggesting that the increase in ER was primarily due to an increase in above-ground respiration. This was supported by the warming-induced increase in above-ground biomass and decrease in SR/ER ratio. Soil temperature was the dominant controlling factor of respiration rates and temperature sensitivity of both ER and SR increased under warming, indicating an increase in contribution from plant respiration. The findings of the study suggest that climate warming by 1.5-2 °C would promote ER via increase in above-ground respiration during the growing season. Moreover, net C uptake in the alpine meadows may increase due to enhanced plant growth and relatively resistant SR under warming.

Much stronger tundra methane emissions during autumn freeze than spring thaw

Warming in the Arctic has been more apparent in the non-growing season than in the typical growing season. In this context, methane (CH₄ ) emissions in the non-growing season, particularly in the shoulder seasons, account for a substantial proportion of the annual budget. However, CH₄ emissions in spring and autumn shoulders are often underestimated by land models and measurements due to limited data availability and unknown mechanisms. This study investigates CH₄ emissions during spring thaw and autumn freeze using eddy covariance CH₄ measurements from three Arctic sites with multi-year observations. We find that the shoulder seasons contribute to about a quarter (25.6 ± 2.3%, mean ± SD) of annual total CH₄ emissions. Our study highlights the three to four times higher contribution of autumn freeze CH₄ emission to total annual emission than that of spring thaw. Autumn freeze exhibits significantly higher CH₄ flux (0.88 ± 0.03 mg m^-2  hr^-1 ) than spring thaw (0.48 ± 0.04 mg m^-2  hr^-1 ). The mean duration of autumn freeze (58.94 ± 26.39 days) is significantly longer than that of spring thaw (20.94 ± 7.79 days), which predominates the much higher cumulative CH₄ emission during autumn freeze (1,212.31 ± 280.39 mg m^-2  year^-1 ) than that during spring thaw (307.39 ± 46.11 mg m^-2  year^-1 ). Near-surface soil temperatures cannot completely reflect the freeze-thaw processes in deeper soil layers and appears to have a hysteresis effect on CH₄ emissions from early spring thaw to late autumn freeze. Therefore, it is necessary to consider commonalities and differences in CH₄ emissions during spring thaw versus autumn freeze to accurately estimate CH₄ source from tundra ecosystems for evaluating carbon-climate feedback in Arctic.

Effects of multiple environmental variables on tundra ecosystem respiration in maritime Antarctica

Summertime ecosystem respiration (ER) rates through seven sites were measured at an upland tundra on Fildes Peninsula in maritime Antarctica to investigate effects of topographic gradient, vegetation types and climatic factors on tundra ER rates. Overall the highest ER rates occurred at the tundra top, followed at the middle slope, and the lowest rates at the lower slope. The daily highest ER rates occurred at noon whereas the lowest at 6 am. There was a significant positive correlation (P < 0.05) between tundra ER and 0-10 cm soil temperature, but a significant negative correlation (P < 0.01) between ER and soil moisture. A high Q10 value of 2.69 was obtained when all the data were combined, indicating soil temperature sensitivity of tundra ER. The mean ER at the tundra sites with moss coverage (72.2 ± 4.4 mg CO2 m-2 h-1) was significantly higher (P < 0.01) than that at the sites with lichen coverage (46.8 ± 8.7 mg CO2 m-2 h-1). The tundra sites without snow coverage experienced significant CO2 release, whereas the emission through ER was very low at the tundra sites with snow coverage. Our results indicated that topographic gradient, soil temperature, soil moisture, vegetation types and snow coverage might affect tundra ER in maritime Antarctica.

Soil organic carbon and nitrogen pools drive soil C-CO2 emissions from selected soils in Maritime Antarctica

The ongoing trend of increasing air temperatures will potentially affect soil organic matter (SOM) turnover and soil C-CO₂ emissions in terrestrial ecosystems of Maritime Antarctica. The effects of SOM quality on this process remain little explored. We evaluated (i) the quantity and quality of soil organic matter and (ii) the potential of C release through CO₂ emissions in lab conditions in different soil types from Maritime Antarctica. Soil samples (0-10 and 10-20cm) were collected in Keller Peninsula and the vicinity of Arctowski station, to determine the quantity and quality of organic matter and the potential to emit CO₂ under different temperature scenarios (2, 5, 8 and 11°C) in lab. Soil organic matter mineralization is low, especially in soils with low organic C and N contents. Recalcitrant C form is predominant, especially in the passive pool, which is correlated with humic substances. Ornithogenic soils had greater C and N contents (reaching to 43.15gkg^-1 and 5.22gkg^-1 for total organic carbon and nitrogen, respectively). C and N were more present in the humic acid fraction. Lowest C mineralization was recorded from shallow soils on basaltic/andesites. C mineralization rates at 2°C were significant lower than at higher temperatures. Ornithogenic soils presented the lowest values of C-CO₂ mineralized by g of C. On the other hand, shallow soils on basaltic/andesites were the most sensitive sites to emit C-CO₂ by g of C. With permafrost degradation, soils on basaltic/andesites and sulfates are expected to release more C-CO₂ than ornithogenic soils. With greater clay contents, more protection was afforded to soil organic matter, with lower microbial activity and mineralization. The trend of soil temperature increases will favor C-CO₂ emissions, especially in the reduced pool of C stored and protected on permafrost, or in occasional Histosols.