Interactions of tissue and fertilizer nitrogen on decomposition dynamics of lignin‐rich conifer litter

High tissue nitrogen (N) accelerates decomposition of high‐quality leaf litter in the early phases of mass loss, but the influence of initial tissue N variation on the decomposition of lignin‐rich litter is less resolved. Because environmental changes such as atmospheric N deposition and elevated CO 2 can alter tissue N levels within species more rapidly than they alter the species composition of ecosystems, it is important to consider how within‐species variation in tissue N may shape litter decomposition and associated N dynamics. Douglas‐fir ( Pseudotsuga menziesii ) is a widespread lignin‐rich conifer that dominates forests of high carbon (C) storage across western North America, and displays wide variation in tissue and litter N that reflects landscape variation in soil N. We collected eight unique Douglas‐fir litter sources that spanned a two‐fold range in initial N concentrations (0.67–1.31%) with a narrow range of lignin (29–35%), and examined relationships between initial litter chemistry, decomposition, and N dynamics in both ambient and N fertilized plots at four sites over 3 yr. High initial litter N slowed decomposition rates in both early (0.67 yr) and late (3 yr) stages in unfertilized plots. Applications of N fertilizer to litters accelerated early‐stage decomposition, but slowed late‐stage decomposition, and most strongly affected low‐N litters, which equalized decomposition rates across litters regardless of initial N concentrations. Decomposition of N‐fertilized litters correlated positively with initial litter manganese (Mn) concentrations, with litter Mn variation reflecting faster turnover of canopy foliage in high N sites, producing younger litterfall with high N and low Mn. Although both internal and external N inhibited decomposition at 3 yr, most litters exhibited net N immobilization, with strongest immobilization in low‐N litter and in N‐fertilized plots. Our observation for lignin‐rich litter that high initial N can slow decomposition yet accelerate N release differs from findings where litter quality variation across species promotes coupled C and N release during decomposition. We suggest reevaluation of ecosystem models and projected global change effects to account for a potential decoupling of ecosystem C and N feedbacks through litter decomposition in lignin‐rich conifer forests.