Canopy Versus Forest-Floor Nitrogen Addition Alters Nutrient Dynamics but Weakly Affects Litter and Deadwood Decomposition in a Temperate Oak Forest

Abstract The decomposition of dead plant matter (leaf litter and deadwood) is a key process for carbon (C) and nutrient cycles in forests. Increased nitrogen (N) deposition due to human activity may affect this process, with important consequences on forest biogeochemistry. Although there is extensive work on leaf litter, the research about the effect of N deposition on deadwood decomposition is still limited. Moreover, most N manipulation studies rely on ground N fertilization, neglecting the canopy, which may influence the quantity and the form of N reaching the forest soil. This study assesses how litter and deadwood decomposition can be influenced by N deposition, using an innovative experimental design, in which N (20 kg N ha −1 year −1 ) is applied above the canopy (treatment “above”) or on the forest floor (treatment “below”) during a three-year (36-month) field experiment. Litter decomposition was studied with the litterbag method; deadwood decomposition was determined by incubating blocks of deadwood (sapwood and heartwood) at different decay stages. Neither N treatments influenced the decomposition rates or carbon (C) loss patterns of litter and deadwood. Remaining N in litter and deadwood was not affected by treatments, except for decay class IV of the sapwood, where N decreased (release) under the below-canopy treatment but showed a net increase in total N content under the above-canopy treatment and in the control. The dynamics of Na, P, and S in the litter differed between the treatment below and the other treatments. Together, despite unchanged mass loss and C loss these treatment-specific nutrient responses suggest that canopy-mediated N delivery can modulate early biogeochemical trajectories of decomposing litter and deadwood even when bulk decay rates remain unaffected over the short term. However, further research is needed to assess the effect of N deposition on deadwood decomposition in the long term, especially considering the slow decay of heartwood.