Permafrost collapse shifts alpine tundra to a carbon source but reduces N2O and CH4 release on the northern Qinghai‐Tibetan Plateau

Important unknowns remain about how abrupt permafrost collapse (thermokarst) affects carbon balance and greenhouse gas flux, limiting our ability to predict the magnitude and timing of the permafrost carbon feedback. We measured monthly, growing‐season fluxes of CO2, CH4, and N2O at a large thermokarst feature in alpine tundra on the northern Qinghai‐Tibetan Plateau (QTP). Thermokarst formation disrupted plant growth and soil hydrology, shifting the ecosystem from a growing‐season carbon sink to a weak source but decreasing feature level CH4 and N2O flux. Temperature‐corrected ecosystem respiration from decomposing permafrost soil was 2.7 to 9.5‐fold higher than in similar features from Arctic and Boreal regions, suggesting that warmer and dryer conditions on the northern QTP could accelerate carbon decomposition following permafrost collapse. N2O flux was similar to the highest values reported for Arctic ecosystems and was 60% higher from exposed mineral soil on the feature floor, confirming Arctic observations of coupled nitrification and denitrification in collapsed soils. Q10 values for respiration were typically over 4, suggesting high‐temperature sensitivity of thawed carbon. Taken together, these results suggest that QTP permafrost carbon in alpine tundra is highly vulnerable to mineralization following thaw, and that N2O production could be an important noncarbon permafrost climate feedback. Permafrost collapse altered soil hydrology, shifting the ecosystem from a carbon sink to carbon source but decreasing CH4 and N2O flux. Little to no vegetation recovery after stabilization suggests potentially large net carbon losses. High N2O flux compared to Arctic and Boreal systems suggests noncarbon permafrost climate feedback. Key Points Permafrost collapse altered soil hydrology, shifting the ecosystem from a carbon sink to carbon source but decreasing CH4 and N2O flux Little to no vegetation recovery after stabilization suggests potentially large net carbon losses High N2O flux compared to Arctic and Boreal systems suggest non‐carbon permafrost climate feedback