Consequences of increasing levels of atmospheric nitrogen deposition on ombrotrophic peatlands: a plant-based perspective.

Nitrogen (N) is a fundamental nutrient for forms of life on Earth. Most of the N (about 98%) is lithospheric and, being firmly linked to inorganic mineral phases, plays little part in the biogeochemical cycle of the element. The second big N reservoir is the atmosphere, containing about 2% of the total N of the Earth. The biosphere and the hydrosphere contain the lowest portion of total N (< 0.01%), although N in the biosphere is highly reactive and rapidly recycled. In spite of the fact that the total amount of N in the lithosphere, hydrosphere, and atmosphere is greater than all other macronutrients combined, i.e., carbon (C), phosphorus (P), oxygen (O), and sulphur (S), N is the element least readily available for living organisms. In other words, there is a seeming paradox in that all terrestrial organisms live in an atmosphere containing 78% by volume of N, yet N is commonly the limiting nutrient in an ecosystem. The reason of course is that only a very small fraction of N in the biosphere is in a form available to plants. The largest amount of N in the biosphere is atmospheric N2, a molecular species, chemically un-reactive under ambient conditions. The reactive N includes all those N forms biologically and chemically active such as ammonia (NH3), ammonium (NH4+), nitrogen oxide (NOx), nitric acid (HNO3), nitrous oxide (N2O), nitrate (NO3-), and organic N forms (proteins, enzymes, urea, nucleic acids). Under natural conditions, reactive N is created by biological fixation and lightning, with the former processes assumed to fix about 130-330 Tg yr-1, two orders of magnitude greater than the latter. In addition, another characteristic of the N cycle in a pre-human world is the balance between biological fixation and denitrification without significant N accumulation within ecosystems or redistribution among ecosystems. The N cycle has been gradually, but dramatically altered as human population has increased, especially since the Industrial Revolution. In particular, on land the anthropogenic fixation of reactive N is equal to the amount of naturally fixed N, the driving force being the need to feed a population that has been growing exponentially. Specifically, the main causes for increasing supplies of N in natural and semi-natural ecosystems (in decreasing order of magnitude) are: the widespread use of synthetic fertilizers obtained through the Haber-Bosch process, cultivation of leguminous crops, and fossil fuel combustion. The massive supply of reactive N in the environment is causing deleterious effects from deep in the ground to high in the stratosphere. Although the synthetic fixation of N has provided a mean to support the development of human society, the unwise use of fertilizers cause many concerns not only in a pure ecological context, but also in relation to the negative socio-economical effects for the human life. On the whole, the major concerns related to the altered N cycle can be briefly summarized as follows: - Production of tropospheric ozone with connected ecological human-health problems; - Alteration of nutrient balance of natural ecosystems with additional negative effects on biodiversity; - Acidification and eutrophication of aquatic ecosystems; - Contribution to global warming and stratospheric ozone depletion. Alteration of the N cycle has particularly dramatic effects on those ecosystems that are both nutrient-poor and dependent on atmospheric inputs for their nutrient and mineral supplies, a typical example being the ombrotrophic peatland (bog). Peatlands are formed where there is an imbalance between plant biomass production and plant biomass decomposition causing a net accumulation of partially decomposed plant remnants (peat). Such processes take place where there is a surplus of precipitation over evapotranspiration, associated with suitable geomorphologic conditions favouring the creation of anoxic conditions. Ombrotrophic peatlands are unaffected by local groundwater and maintain their own suspended water table. Bogs receive nutrients exclusively from atmospheric sources, so they become mineral and nutrient poor. From a biological point of view, the most peculiar feature of ombrotrophic peatlands is the presence of a dense mat of Sphagnum plants forming the bulk of living and dead biomass. Sphagnum is a genus of bryophytes adapted to thrive under low nutrient availability. Like all bryophytes Sphagnum plants absorb nutrients by direct influx through the single cell-layered leaves. Few vascular plants are able to live in ombrotrophic peatlands due to the low nutrient availability, the acidity of the substrate, the anoxic conditions, and the low temperature of the rooting environment. Peatlands are C sinks. To what extent increased N deposition can alter the biogeochemistry and biodiversity of bogs is becoming a matter of concerns, particularly because no decrease of N inputs is foreseeable. The objective of this chapter is therefore to analyze the effect of increasing N availability in ombrotrophic peatlands, with particular attention to the effects on plant ecology at both species and community level.