The Role of Dead Wood as an Ecosystem Nitrogen Sink

Abstract:

The mechanisms leading to high nitrogen (N) retention in many forest ecosystems that receive chronically elevated N inputs from atmospheric deposition are still largely unknown. In 1975, Vitousek and Reiners put forth a conceptual model which states that the net retention of a limiting nutrient should correspond to the net biomass accumulation in the system (rate of accumulation of live and dead organic material). Recent data from Hubbard Brook Experimental Forest in the northeastern US, however, have revealed long-term trends in declining N export that predate reduction in declining atmospheric N inputs, and that have continued for decades after the cessation of net biomass accumulation. This has led to several proposed modifications to the original Vitousek-Reiners hypothesis. Lovett et al. (2018) put forth the “N bank” hypothesis, suggesting that through small changes in mineral soil N stocks, soils can act as an internal bank, supplying N during early succession when N accumulation in biomass exceeds atmospheric N inputs, and reaccumulating N for decades after net biomass accumulation has slowed down. Lajtha (2020) suggested that the role of high carbon:nitrogen coarse woody debris in the immobilization of N is underappreciated, and could fully account for the high levels of N retention seen at Hubbard Brook. Here, we quantitatively explore the potential of coarse woody debris to serve as an N sink during forest succession at Hubbard Brook Experimental Forest through a newly developed coarse woody debris model. In creating this model, we explore the sensitivity to various model structures including: 1) mortality assumptions (static, random, age- and size-related mortality), 2) the impact of standing dead wood and snags and their fall rates (static, random, size-related fall rates), 3) the separate tracking of wood (high C:N) and bark (lower C:N), and 4) approaches to modeling N release from woody debris. We will present estimates of the potential role of coarse woody debris in serving as a long-term ecosystem N sink at Hubbard Brook Experimental Forest, using long-term data to parameterize model inputs and compare with model outputs.