Combined influence of external nitrogen and soil contact on plant residue decomposition and indications from stable isotope signatures

External nitrogen (N) supply has been testified to exert important impacts on plant residue decomposition. The influence of N may be interactive with soil contact in terrestrial ecosystems. However, the joint mechanisms of decomposition of plant residues driven by soil contact and N addition remain incomplete. Using contrasting residues, including needles of Chinese fir (Cuninghamia lanceolata) (Cl) (relatively hard to degrade) vs. leaves of eucalyptus (Eucalyptus urophylla) (Eu) (relatively easy to degrade), a full factorial experiment was conducted by 360-day experiment to investigate the combined effect of N addition and soil contact on residue decay. As the microbe-manipulated decomposition could leave an imprint on the residue carbon (C) and N stable isotope, variations of the two signatures (δ¹³C and δ¹⁵N) were synchronously monitored. Our results firstly showed that added N sped up initial decomposition, while it played an opposite role in subsequent stage, and soil contact always stimulated decay. Under soil contact condition, we found a markedly more accelerating effect of N addition on decay of Cl than without soil contact. Linking with residue N dynamics, we thought that although N immobilized from soil could not completely meet microbial needs for decay of Cl, this N limitation was just relieved by added N, leading to this synergistic effect. At late decay stage, the N inhibiting influence was partly offset under soil contact condition, and this phenomenon was more dramatic for Eu. Our results lastly revealed that the ¹³C and ¹⁵N signatures mirrored and explained the underlying mechanisms of the above interactions. Overall, we concluded that external N and soil contact could interactively affect decay, depending on plant residue decomposability. These results would be used to accurately predict C sequestration for terrestrial ecosystems under heightened N scenario in the future.