Changing C-N Interactions in the Forest Floor under Chronic N Deposition: Implications for Forest C Sequestration

Atmospheric N deposition has far-reaching impacts on forest ecosystems, including on-site impactssuch as soil acidification, fertilization, and nutrient imbalances, and off-site environmental impacts such as nitrateleaching and nitrous oxide emission. Although chronic N deposition has been believed to lead to forest Nsaturation, recent evidence suggests that N retention capacity, particularly in the forest floor, can be surprisinglyhigh even under high N deposition. This review aims to provide an overview of N retention processes in theforest floor and the implications of changing C-N interactions for C sequestration. The fate of available N in forestsoils has been explained by the competitive balance between tree roots, soil heterotrophs, and nitrifiers. However,high rates of N retention have been observed in numerous N addition experiments without noticeableincreases in tree growth and soil respiration. Alternative hypotheses have been proposed to explain the gap betweenthe input and loss of N in N-enriched, C-limited systems, including abiotic immobilization and mycorrhizalassimilation, both of which do not require additional C sources to incorporate N in soil N pools. Different fatesof N in the forest floor have different implications for C sequestration. N-induced tree growth can enhance Caccumulation in tree biomass as observed across temperate regions. C loss from forests can amount to oroutweigh C gain in N-saturated, declining forests, while another type of ‘C-N decoupling’ can have positive orneutral effects on soil C sequestration through hampered organic matter decomposition or abiotic N immobilization,respectively.