eventually saturated and start to release it (Nriagu et al. 1998). And climate change
may have a synergistic effect (Klaminder et al. 2010). It has been argued that
climate change-related processes, such as intensification of soil carbon mineralisation, melting of permafrost and increased erosion, may accelerate the shift of soils
from sink to source.
14.4 Effects of Airborne Nutrients on Ecosystems
Pollutants are not the only substances transported atmospherically. Some airborne
substances can provide important elements for life; in this case they are considered
nutrients. Nitrogen and phosphorus are of particular interest. Both are key limiting
nutrients in ecosystems, and their relative availability determines ecosystem productivity and species composition. Human activities have altered the global cycles
of N and P, leading to an increased flux of these elements through the atmosphere.
There is plenty of N in the earth’s atmosphere, but it is the form of inert gas
generally unavailable for organisms. Organisms need the so called reactive nitrogen, that is, nitrogen-bearing molecules that can be assimilated by primary producers. These molecules are principally nitrate and ammonium. The natural
production of reactive nitrogen is low, causing the productivity of many ecosystems
to be limited by nitrogen. But since the start of industrialisation, human production
of reactive nitrogen has increased dramatically by artificial nitrogen fixation and the
use of internal combustion engines (Elser 2011). Artificial nitrogen fixation is an
industrial process (Haber–Bosch process) that converts gaseous nitrogen into
ammonia, which can be oxidised afterwards to produce nitrite and nitrate. The
process has been massively used to produce fertilisers for agriculture, and the
increased use of artificial fertilisers has lead in turn to higher emissions of ammonia
to the atmosphere from soils and livestock, as well as dust rich in nitrogen from the
field crops. In parallel, the use of internal combustion engines in automobiles
mainly, but also other industrial combustion processes, have generalised the
burning of fossil fuel causing huge emissions of nitrogen oxides to the atmosphere.
Nitrogen oxides are precursors of nitrate, which has an acidifying effect but is also a
nutrient. As a consequence of all these increased emissions, atmospheric transport
and subsequent deposition has become the dominant distribution process of reactive
nitrogen on a global basis (Galloway et al. 2008), and nitrogen deposition to
ecosystems has increased from *0.5 kilograms per hectare and year (kg N ha
−1
year
−1 ) or less in pristine conditions to rates that are nowadays greater than one
order of magnitude, exceeding 10 kg N ha
−1 year
−1 on average in large areas of the
world. Similarly to the case of sulphur oxides, efforts are underway to abate the
nitrogen oxides emissions in Europe and North America, but emissions from
emerging economies demanding more energy may counteract these reductions on a
global balance. Furthermore, the production of reactive nitrogen related with
agriculture is about 75% of the total, and it is challenging to reduce it in a world
where hundreds of millions of people still suffer from a “fertilizer deficit”. And the
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may have a synergistic effect (Klaminder et al. 2010). It has been argued that
climate change-related processes, such as intensification of soil carbon mineralisation, melting of permafrost and increased erosion, may accelerate the shift of soils
from sink to source.
14.4 Effects of Airborne Nutrients on Ecosystems
Pollutants are not the only substances transported atmospherically. Some airborne
substances can provide important elements for life; in this case they are considered
nutrients. Nitrogen and phosphorus are of particular interest. Both are key limiting
nutrients in ecosystems, and their relative availability determines ecosystem productivity and species composition. Human activities have altered the global cycles
of N and P, leading to an increased flux of these elements through the atmosphere.
There is plenty of N in the earth’s atmosphere, but it is the form of inert gas
generally unavailable for organisms. Organisms need the so called reactive nitrogen, that is, nitrogen-bearing molecules that can be assimilated by primary producers. These molecules are principally nitrate and ammonium. The natural
production of reactive nitrogen is low, causing the productivity of many ecosystems
to be limited by nitrogen. But since the start of industrialisation, human production
of reactive nitrogen has increased dramatically by artificial nitrogen fixation and the
use of internal combustion engines (Elser 2011). Artificial nitrogen fixation is an
industrial process (Haber–Bosch process) that converts gaseous nitrogen into
ammonia, which can be oxidised afterwards to produce nitrite and nitrate. The
process has been massively used to produce fertilisers for agriculture, and the
increased use of artificial fertilisers has lead in turn to higher emissions of ammonia
to the atmosphere from soils and livestock, as well as dust rich in nitrogen from the
field crops. In parallel, the use of internal combustion engines in automobiles
mainly, but also other industrial combustion processes, have generalised the
burning of fossil fuel causing huge emissions of nitrogen oxides to the atmosphere.
Nitrogen oxides are precursors of nitrate, which has an acidifying effect but is also a
nutrient. As a consequence of all these increased emissions, atmospheric transport
and subsequent deposition has become the dominant distribution process of reactive
nitrogen on a global basis (Galloway et al. 2008), and nitrogen deposition to
ecosystems has increased from *0.5 kilograms per hectare and year (kg N ha
−1
year
−1 ) or less in pristine conditions to rates that are nowadays greater than one
order of magnitude, exceeding 10 kg N ha
−1 year
−1 on average in large areas of the
world. Similarly to the case of sulphur oxides, efforts are underway to abate the
nitrogen oxides emissions in Europe and North America, but emissions from
emerging economies demanding more energy may counteract these reductions on a
global balance. Furthermore, the production of reactive nitrogen related with
agriculture is about 75% of the total, and it is challenging to reduce it in a world
where hundreds of millions of people still suffer from a “fertilizer deficit”. And the
334
L. Camarero