Can coarse woody material account for the nitrogen imbalance at Hubbard Brook Experimental Forest, New Hampshire, USA?

Abstract The decay of coarse woody litter serves a potentially important role in forest nitrogen cycling. The carbon-rich, nitrogen-poor chemistry of wood allows it to immobilize, store, and in later stages of decomposition, supply nitrogen to forest ecosystems. The decay of woody litter happens over decadal time scales, making direct observations of its importance to nitrogen cycling challenging. Modeling woody litter decay can provide insights into its role in nitrogen cycling but is complex because it is influenced by microbial stoichiometric demands, wood chemistry, time spent as standing versus downed wood, input rates from mortality and disturbances, decay rates, and whether these processes are dynamic over time. One ecosystem where these uncertainties are particularly relevant is the Hubbard Brook Experimental Forest in New Hampshire, USA, where long-term monitoring of a reference watershed has revealed a persistent imbalance between nitrogen inputs and losses. Microbial immobilization of nitrogen in decaying wood has been proposed as an unaccounted-for nitrogen sink. To test whether coarse dead wood contributes to this imbalance, we modeled nitrogen and carbon dynamics during decay and the processes influencing their cycling. We found that 1) Nitrogen dynamics in dead wood likely do not account for a substantial fraction of the nitrogen imbalance observed at Hubbard Brook, and 2) Low microbial carbon-use efficiency for wood decay (< 0.10) was most consistent with observed data and had a large influence on the capacity of wood to immobilize nitrogen and the fate of wood-derived carbon.