Altered Decomposition of Coarse Woody Debris Under Chronic Experimental Nitrogen Deposition.

Coarse woody debris, which encompasses both down dead wood (DDW) and standing dead trees (snags), can play an important role in ecosystem C storage.   It is known that enhanced nitrogen (N) availability can retard litter decomposition, but its effects on wood decay in forests are not well understood.  Observed reductions in decomposition of leaf litter and soil organic matter have been linked to substrate quality and microbial communities.  If N impacts on decomposition are ubiquitous among litter types, DDW density and C:N ratios are likely to be impacted by elevated N. This study assessed the influence of chronic experimental elevated N deposition on the decomposition of DDW in four northern hardwood forests.  A subset of the DDW produced since 1994 was assessed for decay class and sampled for analyses of wood density, C:N ratio, and chitin.  The age of all DDW produced since 1994 was known, due to the long-term nature of the study.  Decreases in DDW density with age were significantly less for the N deposition treatment than the control, consistent with hypothesized effects of reduced lignolytic activity under elevated N availability.   Since fungi are the primary agents of wood decay and cause import of N into the wood, reduced decay rates should result in wider C:N ratios, due to slower C loss and less N import.  Elemental analysis of the DDW indicates that this indeed was the case, with C:N ratios in older DDW significantly higher for the N deposition treatment.   Analysis of DDW chitin content, to confirm reduced fungal mass for the N deposition treatment, are ongoing.  Overall, our results indicate that elevated N deposition can slow rates of wood decomposition in northern hardwood forests. This suppression of wood decomposition could potentially influence overall ecosystem C storage under predicted future levels of N deposition.