Shallow groundwater inhibits soil respiration and favors carbon uptake in a wet alpine meadow ecosystem

Wet alpine meadows generally act as a significant carbon sink, since their low rate of soil decomposition determines a much smaller ecosystem respiration (Re) than photosynthesis. However, it remains unclear whether the low soil decomposition rate is determined by low temperatures or by nearly-saturated soil moisture. We explored this issue by using five years of measurements from two eddy-covariance sites with low temperature and significantly different soil water conditions. The results showed that both sites were carbon sinks. However, despite a smaller annual gross primary productivity, the wet site with a shallow groundwater showed a much higher carbon use efficiency and larger carbon sink than the dry site (which had a deeper water table) due to its much lower Re. Our analyses showed that Re of the wet site was significantly decreased under the nearly-saturated soil condition during the unfrozen seasons. This effect of nearly-saturated soil water on Re increased with soil depths. In contrast, at the dry site the high soil water content favored Re. The corresponding soil temperature at both sites expectedly showed large and positive effects on Re. These results demonstrated that the high carbon sink of the wet alpine meadow was mainly caused by the inhibiting effects of the nearly-saturated soil condition on soil respiration rather than by the low temperatures. Therefore, we argue that a warming-induced shrinking cryosphere may affect the carbon dynamics of wet and cold ecosystems through changes in soil hydrology and its impact on soil respiration. In addition, our study highlights the different responses of soil respiration to warming across soil depths. The thawing of frozen soil may cause larger CO₂ emission in the top soil, while it may also partially contribute to slowing down soil carbon decomposition in the deep soil through decreasing metabolic activity of aerobic organisms.