Decomposition nitrogen is better retained than simulated deposition from mineral amendments in a temperate forest

Nitrogen (N) deposition (N_DEP ) drives forest carbon (C) sequestration but the size of this effect is still uncertain. In the field, an estimate of these effects can be obtained by applying mineral N fertilizers over the soil or forest canopy. A ¹⁵ N label in the fertilizer can be then used to trace the movement of the added N into ecosystem pools and deduce a C effect. However, N recycling via litter decomposition provides most of the nutrition for trees, even under heavy N_DEP inputs. If this recycled litter nitrogen is retained in ecosystem pools differently to added mineral N, then estimates of the effects of N_DEP on the relative change in C (∆C/∆N) based on short-term isotope-labelled mineral fertilizer additions should be questioned. We used ¹⁵ N labelled litter to track decomposed N in the soil system (litter, soils, microbes, and roots) over 18 months in a Sitka spruce plantation and directly compared the fate of this ¹⁵ N to an equivalent amount in simulated N_DEP treatments. By the end of the experiment, three times as much ¹⁵ N was retained in the O and A soil layers when N was derived from litter decomposition than from mineral N additions (60% and 20%, respectively), primarily because of increased recovery in the O layer. Roots expressed slightly more ¹⁵ N tracer from litter decomposition than from simulated mineral N_DEP (7.5% and 4.5%) and compared to soil recovery, expressed proportionally more ¹⁵ N in the A layer than the O layer, potentially indicating uptake of organic N from decomposition. These results suggest effects of N_DEP on forest ∆C/∆N may not be apparent from mineral ¹⁵ N tracer experiments alone. Given the importance of N recycling, an important but underestimated effect of N_DEP is its influence on the rate of N release from litter.