increased use of biofuels from crops that need to be fertilised with nitrogen adds a
new dimension to the problem. Predictions are that reactive nitrogen deposition
may double by 2050, with some regions reaching 50 kg N ha
−1 year
−1 (Galloway
et al. 2004).
Nitrogen deposition has been measured routinely in many places since it was
recognised as an acidifying agent. In contrast, phosphorus deposition measurements
are much less frequent and, as a matter of fact, the knowledge about the atmospheric
phosphorus cycling is limited. In the absence of measurements, most of our present
understanding derives from modelling, which is based on the estimation of emissions coupled with models of atmospheric circulation to infer the transport and
distribution of atmospheric phosphorus. Phosphorus does not form easily gases that
can be emitted, so it has been assumed that it is mainly transported as particles.
Under this assumption, the estimations indicate that, on a global basis, most (>80%)
of the atmospheric phosphorus is in mineral aerosols, with primary biogenic particles and combustion particles having a weight only in non-dusty regions
(Mahowald et al. 2008). They also indicate that atmospheric phosphorus of human
origin is only 5% of the total. However, recent work suggests that gaseous phosphorus emissions are important during combustion processes. If gases are also
considered, human atmospheric phosphorus should then account for about 50% of
the total (Wang et al. 2015). Estimates are therefore still controversial, and this fact
highlights the need of a better understanding of the processes of production and
speciation of atmospheric phosphorus. In any case, human action may have an
strong effect on the atmospheric phosphorus balance, either by enhancing mineral
dust production by land-use changes and (indirectly) climate change, or by
increasing combustion sources, or both. Although global trends are not clear
(mainly because of the lack of data), some research suggests that a significant
increase in phosphorus deposition has occurred since pre-industrial times, and that
this increase can be causing a change in the biogeochemistry and the trophic status
of naturally oligotrophic systems, such as alpine lakes (Brahney et al. 2015). The
same can be claimed for nitrogen. Yet phosphorus is present as a constituent of
living matter in a lower proportion than nitrogen. Hence, for stoichiometric reasons,
smaller changes in phosphorus than nitrogen availability may potentially have a
larger effect on primary production. It is therefore necessary to take both nutrients
into account in order to assess their true impact.
The nitrate concentrations in streams indicate that the Pyrenean catchments are
saturated with nitrogen. The concept of nitrogen saturation means that the catchment
is receiving more nitrogen than organisms (plants and microbiota) living in the
catchment are able to take up and immobilise, so a part of it is leached out of the
catchment, mostly as nitrate in running waters. Nitrate concentrations in the Central
Pyrenees streams fluctuate seasonally, with basal values typically between 7 and
10 µeq L
−1 during the plant growing season and peaks as high as 20–30 µeq L
−1
during the dormant period. On a scale from 0 to 3 (Stoddard and Traaen 1995),
Pyrenean catchments are at saturation stage 2, defined as “saturated, with high N loss”.
The atmospheric nitrogen load from bulk deposition (deposition with rain and the
fine fraction of dry deposition) in the central Pyrenees in 2010 was c. 10 kg N ha
−1
14 Atmospheric Chemical Loadings in the High Mountain: Current …
335
new dimension to the problem. Predictions are that reactive nitrogen deposition
may double by 2050, with some regions reaching 50 kg N ha
−1 year
−1 (Galloway
et al. 2004).
Nitrogen deposition has been measured routinely in many places since it was
recognised as an acidifying agent. In contrast, phosphorus deposition measurements
are much less frequent and, as a matter of fact, the knowledge about the atmospheric
phosphorus cycling is limited. In the absence of measurements, most of our present
understanding derives from modelling, which is based on the estimation of emissions coupled with models of atmospheric circulation to infer the transport and
distribution of atmospheric phosphorus. Phosphorus does not form easily gases that
can be emitted, so it has been assumed that it is mainly transported as particles.
Under this assumption, the estimations indicate that, on a global basis, most (>80%)
of the atmospheric phosphorus is in mineral aerosols, with primary biogenic particles and combustion particles having a weight only in non-dusty regions
(Mahowald et al. 2008). They also indicate that atmospheric phosphorus of human
origin is only 5% of the total. However, recent work suggests that gaseous phosphorus emissions are important during combustion processes. If gases are also
considered, human atmospheric phosphorus should then account for about 50% of
the total (Wang et al. 2015). Estimates are therefore still controversial, and this fact
highlights the need of a better understanding of the processes of production and
speciation of atmospheric phosphorus. In any case, human action may have an
strong effect on the atmospheric phosphorus balance, either by enhancing mineral
dust production by land-use changes and (indirectly) climate change, or by
increasing combustion sources, or both. Although global trends are not clear
(mainly because of the lack of data), some research suggests that a significant
increase in phosphorus deposition has occurred since pre-industrial times, and that
this increase can be causing a change in the biogeochemistry and the trophic status
of naturally oligotrophic systems, such as alpine lakes (Brahney et al. 2015). The
same can be claimed for nitrogen. Yet phosphorus is present as a constituent of
living matter in a lower proportion than nitrogen. Hence, for stoichiometric reasons,
smaller changes in phosphorus than nitrogen availability may potentially have a
larger effect on primary production. It is therefore necessary to take both nutrients
into account in order to assess their true impact.
The nitrate concentrations in streams indicate that the Pyrenean catchments are
saturated with nitrogen. The concept of nitrogen saturation means that the catchment
is receiving more nitrogen than organisms (plants and microbiota) living in the
catchment are able to take up and immobilise, so a part of it is leached out of the
catchment, mostly as nitrate in running waters. Nitrate concentrations in the Central
Pyrenees streams fluctuate seasonally, with basal values typically between 7 and
10 µeq L
−1 during the plant growing season and peaks as high as 20–30 µeq L
−1
during the dormant period. On a scale from 0 to 3 (Stoddard and Traaen 1995),
Pyrenean catchments are at saturation stage 2, defined as “saturated, with high N loss”.
The atmospheric nitrogen load from bulk deposition (deposition with rain and the
fine fraction of dry deposition) in the central Pyrenees in 2010 was c. 10 kg N ha
−1
14 Atmospheric Chemical Loadings in the High Mountain: Current …
335