Global assessment of nitrogen deposition effects on terrestrial plant diversity: a synthesis

Abstract

Abstract. Atmospheric nitrogen (N) deposition is a recognized threat to plant diversity in temperate and northern parts of Europe and North America. This paper assesses evidence from field experiments for N deposition effects and thresholds for terrestrial plant diversity protection across a latitudinal range of main categories of ecosystems, from arctic and boreal systems to tropical forests. Current thinking on the mechanisms of N deposition effects on plant diversity, the global distribution of G200 ecoregions, and current and future (2030) estimates of atmospheric N-deposition rates are then used to identify the risks to plant diversity in all major ecosystem types now and in the future.This synthesis paper clearly shows that N accumulation is the main driver of changes to species composition across the whole range of different ecosystem types by driving the competitive interactions that lead to composition change and/or making conditions unfavorable for some species. Other effects such as direct toxicity of nitrogen gases and aerosols, long-term negative effects of increased ammonium and ammonia availability, soil-mediated effects of acidification, and secondary stress and disturbance are more ecosystem-and site-specific and often play a supporting role. N deposition effects in mediterranean ecosystems have now been identified, leading to a first estimate of an effect threshold. Importantly, ecosystems thought of as not N limited, such as tropical and subtropical systems, may be more vulnerable in the regeneration phase, in situations where heterogeneity in N availability is reduced by atmospheric N deposition, on sandy soils, or in montane areas.Critical loads are effect thresholds for N deposition, and the critical load concept has helped European governments make progress toward reducing N loads on sensitive ecosystems. More needs to be done in Europe and North America, especially for the more sensitive ecosystem types, including several ecosystems of high conservation importance.The results of this assessment show that the vulnerable regions outside Europe and North America which have not received enough attention are ecoregions in eastern and southern Asia (China, India), an important part of the mediterranean ecoregion (California, southern Europe), and in the coming decades several subtropical and tropical parts of Latin America and Africa. Reductions in plant diversity by increased atmospheric N deposition may be more widespread than first thought, and more targeted studies are required in low background areas, especially in the G200 ecoregions.

Effects of experimental nitrogen additions on plant diversity in tropical forests of contrasting disturbance regimes in southern China

“…Soils are old and classified as lateritic red earth (oxisol) . In addition, our study sites have been experienced high atmospheric N deposition of >30 kg N ha À1 yr À1 since 1990 (Huang et al, 1994;Zhou and Yan, 2001;Fang et al, 2008), which greatly exceeds most areas considered at risk for N saturation (Gilliam, 2006;Bobbink et al, 2010). Therefore, P limitation is likely for our forests, and indeed has been suggested by the further study (Lu et al, unpublished data).…”

“…The rehabilitated forest originated as a pine plantation in 1930's that was naturally invaded and colonized by regional broadleaf species from animal and wind dispersal. However, it has yet to recover fully to its pre-disturbance condition over the past several decades (Mo et al, 2003(Mo et al, , 2004(Mo et al, , 2006, creating and maintaining N-limiting conditions (Mo et al, 2006), i.e., N demand by recovering woody vegetation continues to exceed supply of available N. Under N limitation, increasing N commonly decreases biodiversity by enhancing competitive abilities of fast-growing nitrophilous plants with high maximum growth rates, at the expense of slower growing neighbors of smaller stature (Aerts and Chapin, 2000;Gilliam, 2006;Hautier et al, 2009;Bobbink et al, 2010). Because there were no significant increases in understory plant growth in the rehabilitated forest, it is unlikely that competitive exclusion among understory plants contributed to the observed decline of understory cover.…”

“…At the same time, anthropogenic N emissions and resultant N deposition have increased substantially in China, and are projected to increase further due to agricultural and industrial activities (Zheng et al, 2002;Galloway et al, 2004;Liu et al, 2010). Lü and Tian (2007) concluded that total N deposition rates are highest (65 kg N ha À1 yr À1 ) in south-central China, with a mean rate of 19 kg N ha À1 yr À1 , higher than most reported for North American and Europe (MacDonald et al, 2002;Bobbink et al, 2010), where threats to forest ecosystem health have been suggested (Percy and Ferretti, 2004).…”

“…It is well known that increased N deposition typically decreases plant biodiversity in the affected ecosystems (see a review of Bobbink et al, 2010), although a few studies have reported no significant change (e.g., Gilliam et al, 2006). In contrast, less is known regarding the impact of N deposition on plant diversity of tropical/subtropical areas (Bobbink et al, 2010), which are typically N rich and more likely to be phosphorus (P) limited (Vitousek, 1984;Matson et al, 1999). Extensive work in tropical forests of the Hawai'i archipelago (see Vitousek, 2004) has shown that much of this arises from loss of available P through the long-term pedogenic processes of tropical soil formation.…”

Current and future ozone risks to global terrestrial biodiversity and ecosystem processes

“…Bobbink et al. (2010) assessed rates of N deposition in 2000, and under three scenarios by 2030, in the G200 ERs, comparing them with a generic effects threshold of 15 kg N ha −1  year −1 . This analysis highlighted ERs in South‐East Asia as being under greatest threat.…”