Nitrogen Deposition Effects on Production and Decomposition of Coarse Woody Debris

Atmospheric nitrogen deposition has the potential to impact forest productivity, microbial associations, nutrient cycling, decomposition and stand dynamics. However, among the least studied aspects of these processes are the production and decomposition of dead woody biomass, or coarse woody debris (CWD). Coarse woody debris is made up of dead woody material on the ground called down dead wood (DDW) and dead standing trees which are often referred to as snags. Observed reductions in decomposition of CWD have been linked to substrate quality and microbial communities. If the decomposition effect is ubiquitous among litter types, CWD density, C:N, and biomass are likely to be impacted by elevated N deposition. Previous research found a slight increase in mortality in the N-amended plots. By assessing CWD volume and biomass, we can conjecture the potential impacts of N-deposition on stand dynamics. This study analyzes the influence of chronic simulated N-deposition on the decomposition of CWD along a latitudinal gradient in Michigan. Methodology included assessing DDW volume and biomass from ambient and elevated N deposition treatments, classifying downed dead wood and snags by their respective visual qualitative decay classes, and calculating wood density and C:N ratios. Since N deposition treatments began in 1994, DDW biomass has accrued by 16% in N amended plots. Wood density and C:N ratios were significantly and negatively correlated with years since tree death in ambient (control) treatment plots (P = 0.003 and 0.005 respectively). N amended wood density and C:N ratios did not significantly decline over times since tree death. Analysis of covariance, with years since tree death as a covariate, indicated that N deposition treatment caused an increase in average DDW density and C:N ratio (P = 0.063 and 0.060 respectively). Our analysis indicates that N deposition has the potential to slow rates of wood decomposition and therefore the accumulation of dead woody biomass over time