Experimental warming increased soil nitrogen sink in the Tibetan permafrost

In permafrost soil, warming regulates the nitrogen (N) cycle either by stimulating N transformation or by enhancing cryoturbation, the mixture of soil layers due to repeated freeze thaw. Here N isotopic values (δ 15 N) of plants and the soil were investigated in a 7 year warming experiment in a permafrost‐affected alpine meadow on the Qinghai‐Tibetan Plateau. The results revealed that warming significantly decreased the δ 15 N in the plant (aboveground and belowground parts) and different soil fractions (clay and silt fraction, aggregate, and bulk soil). The decreased soil δ 15 N was associated with an increase in soil N stock due to greater N fixation. The incremental N retention in plants and soil mineral‐associated fractions from warming resulted in a decrease in soil inorganic N, which constrains the role of nitrification/denitrification in soil δ 15 N, suggesting a restrained rather than an open N cycle. Furthermore, enhanced cryoturbation under warming, identified by a downward redistribution of 137 Cs into deeper layers, promoted N protection from transformation. Overall, the decrease in soil δ 15 N indicated higher rates of N input through fixation relative to N loss through nitrification and denitrification in permafrost‐affected ecosystems under warming conditions.