Litter type mediates N addition effect on soil organicmatter priming

While plant litter input can influence soil organic matter (SOM) decomposition via the priming effect, current understanding is mainly from studies with addition of 13-carbon (C) labeled fresh leaves from seedlings. Little is known about the priming effect induced by fallen leaves as well as fine roots from mature trees with distinct chemical characteristics due to long-term nitrogen (N) addition. Here, using a natural 13C labeling approach, we conducted an incubation study under controlled conditions to investigate residue decomposition and the priming effect caused by fresh and fallen needles and fine roots from a boreal forest under 12-yr N addition. Decomposition rate of fine roots and fresh needles was higher than that of fallen needles. The priming effect induced by fine roots (-63.86%) were highest followed by fresh (-66.79%) and fallen needles (-70.61%), which was negatively related to soil mineral N availability. Long-term N addition increased N and P contents, especially for fine roots, and increased the priming effect caused by fine roots (-60.65% versus -73.48%), but not needles. These results suggest that increased N content of fine roots may enhance microbial activity of decomposition relative to the treatment of no N addition. Overall, we for the first time provide empirical evidence for the higher priming effect caused by fine roots than needles from mature trees in a boreal forest under long-term N addition, and emphasize a critical future need of the priming effect caused by fallen issues, especially roots.