Effects of nitrogen deposition and empirical nitrogen critical loads for ecoregions of the United States

Abstract

Abstract. Human activity in the last century has led to a significant increase in nitrogen (N) emissions and atmospheric deposition. This N deposition has reached a level that has caused or is likely to cause alterations to the structure and function of many ecosystems across the United States. One approach for quantifying the deposition of pollution that would be harmful to ecosystems is the determination of critical loads. A critical load is defined as the input of a pollutant below which no detrimental ecological effects occur over the long-term according to present knowledge.The objectives of this project were to synthesize current research relating atmospheric N deposition to effects on terrestrial and freshwater ecosystems in the United States, and to estimate associated empirical N critical loads. The receptors considered included freshwater diatoms, mycorrhizal fungi, lichens, bryophytes, herbaceous plants, shrubs, and trees. Ecosystem impacts included: (1) biogeochemical responses and (2) individual species, population, and community responses. Biogeochemical responses included increased N mineralization and nitrification (and N availability for plant and microbial uptake), increased gaseous N losses (ammonia volatilization, nitric and nitrous oxide from nitrification and denitrification), and increased N leaching. Individual species, population, and community responses included increased tissue N, physiological and nutrient imbalances, increased growth, altered root : shoot ratios, increased susceptibility to secondary stresses, altered fire regime, shifts in competitive interactions and community composition, changes in species richness and other measures of biodiversity, and increases in invasive species.The range of critical loads for nutrient N reported for U.S. ecoregions, inland surface waters, and freshwater wetlands is 1-39 kg NÁha , spanning the range of N deposition observed over most of the country. The empirical critical loads for N tend to increase in the following sequence for different life forms: diatoms, lichens and bryophytes, mycorrhizal fungi, herbaceous plants and shrubs, and trees.The critical load approach is an ecosystem assessment tool with great potential to simplify complex scientific information and communicate effectively with the policy community and the public. This synthesis represents the first comprehensive assessment of empirical critical loads of N for major ecoregions across the United States.

Nitrogen deposition contributes to soil acidification in tropical ecosystems

“…Currently, 11% of the world's natural vegetation receives N deposition in excess of 10 kg N ha -1 yr -1 (Dentener et al, 2006;Pardo et al, 2011). Continuous atmospheric N deposition to terrestrial ecosystems can lead to pronounced soil acidification (Van Breemen et al, 1984), resulting in a net decrease in soil pH and acid neutralization capacity (ANC) of the soils (Larssen & Carmichael, 2000;Hédl et al, 2011).…”

“…Although soils are buffered by bicarbonate in the pH range above 6. (Krug & Frink, 1983;Boxman et al, 2008;Gruba et al, 2013), where atmospheric N deposition commonly exceeds the critical loads of ecosystems, leading to N saturation (Aber et al, 1998;Fenn et al, 2006;Thimonier et al, 2010;Pardo et al, 2011).…”

Consequence of altered nitrogen cycles in the coupled human and ecological system under changing climate: The need for long-term and site-based research

“…Independently derived critical levels for lichens and moss diversity have been found to be similar for northern and southern Europe, thus emphasizing the universal applicability of these plant groups as ecological indicators of N deposition. Pardo et al (2011) showed that, in the USA, empirical critical loads for N tend to increase according to the following sequence: lichens and bryophytes, mycorrhizal fungi, herbaceous plants and shrubs, and trees. In several studies in the USA, lichens invariably showed the lowest NH 3 critical levels (1 lg m -3 ) and N critical loads (1 kg N ha -1 year -1 ) of all biological indicators (Jovan et al 2012).…”

Consequences of atmospheric nitrogen deposition in terrestrial ecosystems: old questions, new perspectives

“…They found that while denitrification in these forest sites is a significant proportion of the total N budget, the gaseous N losses through this pathway did not respond to increased N deposition rates. This conclusion is particularly important for modeling efforts designed to assess the critical N load (see Pardo et al 2011), and whether atmospheric N input can be markedly offset by denitrification. In the third paper featured here, Houle et al examined long-term flux patterns for bulk precipitation, throughfall and canopy N uptake between 1997 and 2012 at two boreal forest sites in Quebec, Canada.…”