title: Decomposition nitrogen is better retained than simulated deposition from mineral amendments in a temperate forest.
abstract: Nitrogen (N) deposition (NDEP) 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. A15N 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 NDEPinputs. If this recycled litter nitrogen is retained in ecosystem pools differently to added mineral N, then estimates of the effects of NDEPon the relative change in C (∆C/∆N) based on short-term isotope-labelled mineral fertilizer additions should be questioned. We used15N 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 this15N to an equivalent amount in simulated NDEPtreatments. By the end of the experiment, three times as much15N 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 more15N tracer from litter decomposition than from simulated mineral NDEP(7.5% and 4.5%) and compared to soil recovery, expressed proportionally more15N in the A layer than the O layer, potentially indicating uptake of organic N from decomposition. These results suggest effects of NDEPon forest ∆C/∆N may not be apparent from mineral15N tracer experiments alone. Given the importance of N recycling, an important but underestimated effect of NDEPis its influence on the rate of N release from litter.
authors.affiliation: School of Geosciences, University of Edinburgh, Crew Building, Edinburgh, EH9 3FF, Midlothian, UK.
authors.forename: Richard K F
authors.initials: RK
authors.lastname: Nair
authors.affiliation: Forest Research, Northern Research Station, Roslin, EH25 9SY, Midlothian, UK.
authors.forename: Michael P
authors.initials: MP
authors.lastname: Perks
authors.affiliation: School of Geosciences, University of Edinburgh, Crew Building, Edinburgh, EH9 3FF, Midlothian, UK.|ICREA at CREAF, Campus de UAB, Cerdanyola del Vallés, Bellaterra, Barcelona, Spain.
authors.forename: Maurizio
authors.initials: M
authors.lastname: Mencuccini
mesh_terms: D017753:Ecosystem; D065928:Forests; D008903:Minerals; D009584:Nitrogen / Q000378:metabolism*; D012987:Soil; D014197:Trees
chemical_list: D008903:Minerals; D012987:Soil; D009584:Nitrogen
keywords: 15N-nitrogen; forest; isotope trace; litter decomposition; litter nitrogen; nitrogen fertilization