Responses of Forest Ecosystems to Nitrogen Deposition

Environmental legislation in countries around the world has led to notable recent declines in the atmospheric deposition of nitrogen (N), although most decreases relate to oxidized N, with reduced N increasing in many areas [1]. Still, deposition of N remains high in many regions globally. For areas where chronic atmospheric deposition of N has led to N saturation, excess N chronically threatens the structure and function of ecosystems. Indeed, critical loads for N remain widely exceeded for many forests, leading to a variety of deleterious effects, including loss of biodiversity and altered biogeochemical cycles, all of which threaten the sustainability of impacted forests [2]. It is likely that the recovery of N-impacted sites might require extended periods of time, especially in locations where base cations, such as Ca⁺⁺, have been depleted by accelerated NO₃⁻ leaching. Thus, understanding the potential responses of forest ecosystems to N deposition remains essential.

This Special Issue of Forests explores the multifaceted responses of forest ecosystems to both increases and decreases in N deposition on a global scale. This includes effects on plants and plant assemblages, as well as effects of N on forest biogeochemistry, and comprises studies from Asia, Europe, and North America.

Certainly, the fundamental paradigms of N cycling in terrestrial ecosystems have shifted greatly over the past several decades [3], and N has been the focus of extensive research as both basic and applied science. More recently and more specifically this has been carried out by biogeochemists, extending from the discovery of N as an element in 1772, to its central place in von Liebig’s Law of Minimum for plant growth articulated in 1827, to the discovery of symbiotic N fixation in 1888, to the development of the Haber-Bosch process in 1913 (initiating of its use as fertilizer in crop production), and finally to the present awareness that excess N in the environment can alter the structure and function of ecosystems [4].

Chronic N deposition has been an environmental threat throughout most of Asia. In Japan, research has demonstrated that effects of excess N can include trophic interactions involving tree foliar tissue and defoliating insects, which can be particularly notable in N₂-fixing trees [5]. Excess N also influences the biogeochemical cycles of tropical Asian forests. Experimental additions of N significantly inhibited mineralization of soil carbon (C) in tropical forest soils of southern China (the Dinghushan Biosphere Reserve—DHSBR), increasing concentrations of dissolved organic carbon [6]. Deposition of N can interact with that of other nutrients, e.g., phosphorus (P), to alter plant tissue nutrient stoichiometry, especially in humid tropical forests that are typically N-rich, but limited by P. Another factorial field experiment at DHSBR demonstrated that additions of P, rather than N, greatly altered foliar N and P concentrations in understory plants [7].

Europe has long experienced excess N deposition from increased emissions of gaseous N from industrial, domestic, and agricultural sources. Long-term (~30 yr) data from the Australian Alps has shown that, although N deposition approximated or exceeded critical loads, >80% of this was retained in the ecosystem, maintaining lower hydrologic N loss; tree growth was identified as the main sink for N [8]. Indeed, forest dynamics often mitigate effects of high N deposition, including the response of soil nutrient cations, i.e., Ca⁺⁺, Mg⁺⁺, and K⁺. Work in hardwood and conifer stands of central Germany found significant differences between these stand types regarding the effects of excess N on the biogeochemical cycling of these essential plant nutrients [9].

Most research on effects of excess N on forest ecosystems has been carried out via plot-based field designs which have the advantage of being fully factorial and involving replications, but the disadvantage of lacking realistic simulations of increased deposition of N from the atmosphere. Accordingly, whole-watershed manipulations, wherein N is applied from rotary or fixed-wing aircraft, are rare. Research at the Fernow Experimental Forest (FEF), Tucker County, West Virginia, comprises both plot-based and whole-watershed approaches, with aerial additions of (NH₄)₂SO₄ at 35 kg N ha⁻¹ yr⁻¹ to the treatment watershed beginning in 1989. Although this site is an eastern deciduous forest, results from over a quarter century have relevant implications for other forest types, as well [10,11].

Past and ongoing work at FEF is highly varied with respect to forest response variables. An unexpected outcome has been the discovery that excess N can make some hardwood species more susceptible to damage from storm-related winds, which has implications for climate change and the future of impacted forests [12]. Excess N can also greatly alter the species composition and diversity of forest herbaceous communities, which is where up to 90% of plant diversity is found in forest ecosystems [10,13,14]. Recent efforts have demonstrated that this response can vary greatly with stand age and dominant tree species [15]. Not surprisingly, the response of forest trees to excess N can be highly species specific, and work at FEF has shown that several hardwood species, such as sweet birch (Betula lenta L.), black cherry (Prunus serotina Ehrh.), and red maple (Acer rubrum L.) responded positively to N additions, whereas yellow poplar (Liriodendron tulipifera L.) responded negatively [16]. These species-specific responses have implications for ecosystem-level effects of N on forests, as some hardwood species, especially sugar maple (Acer saccharum L.), can facilitate greater loss of N, via leaching of NO₃⁻, by enhancing rates of net nitrification [17].

Recent studies have speculated about the future of N-impacted forest ecosystems under conditions of decreased N deposition [1,18,19,20]. In 2019, aerial applications of NS ceased after 30 yr of treatment at FEF. This new phase of research at FEF now allows researchers the opportunity to test these predictions empirically [21], further advancing our knowledge and understanding of the complexity of N biogeochemistry in forest ecosystems.