Emissions of reactive oxidized nitrogen (NO and NO2),
collectively known as NOx, from human activities are c.
21 Tg N
annually, or 70% of global total emissions. They occur predominantly
in industrialized regions, largely
from fossil fuel combustion, but also from increased use of N
fertilizers. Soil emissions of NO not only make an
important contribution to global totals, but also play a part in
regulating the dry deposition of NO and NO2 (NOx)
to plant canopies. Soil microbial production of NO leads to a soil
‘compensation point’ for NO deposition or
emission, which depends on soil temperature, N and water status.
In warm conditions, the net emission of NOx
from plant canopies contributes to the photochemical formation of ozone.
Moreover, the effect of NOx emissions
from soil is to reduce net rates of NO2 deposition to
terrestrial surfaces over large areas.
Increasing anthropogenic emissions of NOx have led to an
approximate doubling in surface O3 concentrations
since the last century. NOx acts as a catalyst for the
production of O3 from volatile organic compounds (VOCs).
Paradoxically, emission controls on motor vehicles might lead to increases
in O3 concentrations in urban areas.
Removal of NO and NO2 by dry deposition is regulated to some
extent by soil production of NO; the major
sink for NO2 is stomatal uptake. Long-term flux measurements
moorland in Scotland show very small
deposition rates for NO2 at night and before mid-day of
1–4 ng NO2-N m−2 s−1,
and similar emission rates during
afternoon. The bi-directional flux gives 24-h average deposition velocities
1–2 mm s−1, and implies a long
life-time for NOx due to removal by dry deposition.
Rates of removal of O3 at the ground are also influenced
by stomatal uptake, but significant non-stomatal uptake
occurs at night and in winter. Measurements above moorland showed 40%
of total annual flux was stomatal, with
60% non-stomatal, giving nocturnal and winter deposition velocities of
2–3 mm s−1 and daytime summer values
of 10 mm s−1. The stomatal uptake is responsible for adverse
effects on vegetation. The critical level for O3
exposure (AOT40) is used to derive a threshold O3
flux for wheat of 0·5 μg m−2 s−1.
Use of modelled
stomatal fluxes rather than exposure might give more reliable estimates
loss; preliminary calculations
suggest that the relative grain yield reduction (%) can be estimated as
38 times the stomatal ozone flux (g m−2)
above the threshold, summed over the growing season.