The high-altitude peatland carbon cycle: A review of the impacts of climate change, human disturbance and management

High-altitude peatlands (HAPs; defined as > 1,500 m) provide important ecosystem services including soil carbon (C) storage. However, temperatures in high-altitude regions have been rising rapidly in recent decades, while HAPs are increasingly affected by human activities such as intensive drainage and grazing. Collectively, climate change and land management may strongly affect the HAP C cycle. Here, we synthesise current global progress on the HAP C cycle, focussing on the impacts of climate change and land management. Warming increased both ecosystem respiration (ER) and methane (CH4) emissions (26 %-86 %), while impacts on net ecosystem exchange (NEE) of CO2 were still unclear. However, short-term drought decreased ER and CH4 emissions (7 %-96 %), along with NEE (12 %-52 %). Snow, permafrost, and glacier decline may also impact the C cycle in HAPs, although a limited number of studies have been conducted. Grazing and vegetation degradation impacts on HAP C cycling were related to grazing and degradation intensity, while generally decreasing soil organic C stocks (3 %-51 %). Moving from shallower to deeper WTLs stimulated ER (9 %-812 %), while reducing CH4 emissions (13 %-100 %), with variable effects on NEE (-53 %-700 %). Restoration by rewetting began to reverse the trend of drainage. We highlight several knowledge gaps, including limited understanding of climate change and land-management effects on gross primary productivity and dissolved organic carbon, while there is still limited knowledge of regional differences in HAP C cycling. Future research should focus on the interaction of land-use and climate change in HAPs, including HAP restoration, which may help future conservation of these valuable ecosystems.