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 NDEP inputs. If this recycled litter nitrogen is retained in ecosystem pools differently to added mineral N, then estimates of the effects of NDEP on the relative change in C (Delta C/Delta 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 N-15 to an equivalent amount in simulated NDEP treatments. By the end of the experiment, three times as much N-15 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 NDEP (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 NDEP on forest Delta C/Delta N may not be apparent from mineral N-15 tracer experiments alone. Given the importance of N recycling, an important but underestimated effect of NDEP is its influence on the rate of N release from litter.