Nitrogen transfer between decomposing leaves of different N status

Nitrogen movement among microsites is thought to be an important control on patterns of ecosystem-level N cycling. In particular, N transfer between decomposing leaves may explain why litter mixtures sometimes decompose differently than would be predicted from the decomposition dynamics of each species separately. We evaluated how N moves between leaves of differing N status in leaf-pair microcosms. We collected litter from six species of trees from French Guiana (three with high N concentration, three with low) and ¹⁵N-labeled the microbial communities growing on each species. We then established microcosms with one labeled and one unlabeled leaf in a fully factorial design (each species with every species, ¹⁵N on each species) and measured ¹⁵N transfer over 28 days. There was substantial transfer of the ¹⁵N label in all cases, averaging between 15% and 30% of the ¹⁵N originally on the labeled leaf. Net N transfer from high-N to low-N leaves resulted from greater gross ¹⁵N transfer from high-N to low-N leaves than in reverse. Gross ¹⁵N transfer was controlled entirely by the N status of the source leaf, rather than by the difference in N-status of the leaves or by the characteristics of the sink leaf. For example, as much ¹⁵N was transferred from a high-N leaf to another high-N leaf as to a low-N leaf. These results support the assumption from N mineralization theories that microbes at a specific site have first access to that N and therefore control how much N is available to move to other microsites in the soil system. The strength of the gradient between microsites may then control the rate at which available N moves, but not how much N is available to move. If N transfer among different litter species is important for synergistic effects on decomposition of litter mixtures it would not be driven by the N gradient as is often hypothesized, but by the characteristics of the source leaf.