The response of the Dajiuhu Peatland ecosystem to hydrological variations: Implications for carbon sequestration and peatlands conservation

Peatlands, one of the largest biosphere carbon reservoirs, are facing climate change induced water-table draw -down and carbon loss. To provide insights into peatland protection and carbon-water cycle under the back-ground of global change, the detailed relationship between peatland ecology and water table depth (WTD) should be considered. In this study, we focused on carbon flux and synthesized datasets on WTD, carbon dynamics, vegetation and microbiologic activities in the Dajiuhu Peatland to better understand the ecological response to the hydrological variations. Based on modern monitoring, the daily CH4 emission decreased and CO2 emission increased with the fall of the WTD. When the WTD was deeper than 30 cm, daily carbon sequestration reduced until it approached 0 according to nonlinear models. By comparisons with previous spatial soil TOC, vegetation and microbiologic surveys, we proposed that approximately 30 cm is a critical WTD level (turning point or zone) affecting the structure and function of peatland ecosystems. Prolonged and severe droughts might trigger an ecological shift of peatland because increase in oxygen availability and decrease in water availability are not able to meet the physiological needs of living organisms, which further decreases CH4 emission due to limited methanogenesis and enhanced methanotrophy and weakens carbon sequestration through enhanced litter decomposition. This was also confirmed by palaeo-ecological records. In particular, precipitation-induced declines of WTD (WTD > 30 cm) during 9,500-9,200 cal yr BP, 6,000-4,000 cal yr BP and 3,600-3,200 cal yr BP caused a series of ecological changes, including the decreases in wet-preferred Sphagnum and herbs, enhanced aerobic bacteria activities and ecosystem respiration, and changed soil carbon sequestration. Drought during 6,000-4,000 cal yr BP induced low carbon sequestration. However, rapid vegetation succession and peat accumulation caused by the abrupt hydrological variations during 9,500-9,200 cal yr BP and 3,600-3,200 cal yr BP might have resulted in temporary increases in carbon sequestration. Our results demonstrate that WTD is crucial to the regulation of peatland ecological functions. This finding offers references for peatland carbon- -water cycles and management.