Nitrogen Excess in North American Ecosystems: Predisposing Factors, Ecosystem Responses, and Management Strategies

Abstract

Most forests in North America remain nitrogen limited, although recent studies have identified forested areas that exhibit symptoms of N excess, analogous to overfertilization of arable land. Nitrogen excess in watersheds is detrimental because of disruptions in plant/soil nutrient relations, increased soil acidification and aluminum mobility, increased emissions of nitrogenous greenhouse gases from soil, reduced methane consumption in soil, decreased water quality, toxic effects on freshwater biota, and eutrophication of coastal marine waters. Elevated nitrate (NO3−) loss to groundwater or surface waters is the primary symptom of N excess. Additional symptoms include increasing N concentrations and higher N:nutrient ratios in foliage (i.e., N:Mg, N:P), foliar accumulation of amino acids or NO3−, and low soil C:N ratios. Recent nitrogen‐fertilization studies in New England and Europe provide preliminary evidence that some forests receiving chronic N inputs may decline in productivity and experience greater mortality. Long‐term fertilization at Mount Ascutney, Vermont, suggests that declining and slow N‐cycling coniferous stands may be replaced by fast‐growing and fast N‐cycling deciduous forests. Symptoms of N saturation are particularly severe in high‐elevation, nonaggrading spruce–fir ecosystems in the Appalachian Mountains and in eastern hardwood watersheds at the Fernow Experimental Forest near Parsons, West Virginia. In the Los Angeles Air Basin, mixed conifer forests and chaparral watersheds with high smog exposure are N saturated and exhibit the highest streamwater NO3− concentrations for wildlands in North America. High‐elevation alpine watersheds in the Colorado Front Range and a deciduous forest in Ontario, Canada, are N saturated, although N deposition is moderate (∼8 kg·ha−1·yr−1). In contrast, the Harvard Forest hardwood stand in Massachusetts has absorbed >900 kg N/ha during 8 yr of N amendment studies without significant NO3− leaching, illustrating that ecosystems vary widely in the capacity to retain N inputs. Overly mature forests with high N deposition, high soil N stores, and low soil C:N ratios are prone to N saturation and NO3− leaching. Additional characteristics favoring low N retention capacity include a short growing season (reduced plant N demand) and reduced contact time between drainage water and soil (i.e., porous coarse‐textured soils, exposed bedrock or talus). Temporal patterns of hydrologic fluxes interact with biotic uptake and internal cycling patterns in determining ecosystem N retention. Soils are the largest storage pool for N inputs, although vegetation uptake is also important. Recent studies indicate that nitrification may be widespread in undisturbed ecosystems, and that microbial assimilation of NO3− may be a significant N retention mechanism, contrary to previous assumptions. Further studies are needed to elucidate the sites, forms, and mechanisms of N retention and incorporation into soil organic matter, and to test potential management options for mitigating N losses f...

Nitrogen biogeochemistry of a mature Scots pine forest subjected to high nitrogen loads

“…Experiments with labelled N elsewhere confirmed that added ammonium was especially retained within the organic layer and upper mineral soil where microbial activity was the highest (Gundersen 1998;Nadelhoffer et al 1999Nadelhoffer et al , 2004Feng et al 2008). The soil organic N pool was also identified as the major sink of deposited DIN in several other studies (Magill et al 1997;Fenn et al 1998;Rueth and Baron 2002). It is also likely that chemical exchange of NH 4 ?…”

“…The low N uptake of the forest and the coarse textured soil (low hydrological residence times and low cation exchange capacity due to low clay content) might have predisposed the forest to low N retention and excessive NO 3 -losses (Fenn et al 1998). Also soil acidity could exacerbate N losses in different ways (Venterea et al 2004).…”

Simulated nitrogen deposition affects wood decomposition by cord-forming fungi

“…Anthropogenic nitrogen (N) deposition now rivals natural N inputs, and this has had widespread effects on terrestrial and aquatic ecosystems (Vitousek et al 1997;Fenn et al 1998;Langham 1999). One of the most intensively-studied aspects is the effect of atmospheric N deposition on litter decay in forests (Berg and Laskowski 2005), as alterations in decomposition rates will effect forest carbon sequestration.…”

Abiotic immobilization of nitrate in two soils of relic Abies pinsapo-fir forests under Mediterranean climate

“…We hypothesize that soils under Abies pinsapo Wr forests would show less N retention capacity overall than reported in more mesic temperate forest ecosystems. Mediterranean ecosystems have a limited capacity to retain N inputs (Riggan et al 1985;Fenn et al 1996;Fenn and Poth 1999), principally due to high nitriWcation rates, short growing seasons, and temporal asynchrony between hydrological loss Xuxes and plant N demand, mainly as a consequence of summerdry periods followed by intense autumn rains (Fenn et al 1998;Meixner et al 2001). A second hypothesis is that rates of abiotic immobilization of soil NO 3 ¡ would be higher in the N limited stand than in the N saturated site.…”