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 (N-15) of plants and the soil were investigated in a 7year warming experiment in a permafrost-affected alpine meadow on the Qinghai-Tibetan Plateau. The results revealed that warming significantly decreased the N-15 in the plant (aboveground and belowground parts) and different soil fractions (clay and silt fraction, aggregate, and bulk soil). The decreased soil N-15 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 N-15, suggesting a restrained rather than an open N cycle. Furthermore, enhanced cryoturbation under warming, identified by a downward redistribution of Cs-137 into deeper layers, promoted N protection from transformation. Overall, the decrease in soil N-15 indicated higher rates of N input through fixation relative to N loss through nitrification and denitrification in permafrost-affected ecosystems under warming conditions.