Soil gas fluxes

Pollutants: 

Ecosystems: 

Description: 

The gases NO and N2O are produced in soils by nitrifying and denitrifying bacteria, and the magnitude of the emissions is controlled by the availability of N as ammonium (NH4) or nitrate (NO3) and also by certain climatic and soil properties which promote nitrification or denitrification, e.g. temperature, rainfall, organic matter content (Skiba & Smith 2000).

Studies of a range of semi-natural ecosystems, which have received various forms of N deposition, suggest that measurements of soil NO and N2O emissions may be useful indicators of soils where N supply exceeds demand of vegetation (Skiba et al. 1998).

Previous experience:

Skiba et al. (1998) showed that nitrogen additions by fertilisation, manure and atmospheric deposition increased emission of N2O. For example, fluxes of N2O and NO were closely related to atmospheric N deposition in the vicinity of livestock farms in southern Scotland (Pitcairn et al.2002; Skiba et al. 1998). Emissions of N2O and NO from woodland soil were large close to the poultry houses and decreased with increasing distance from the poultry houses. Emissions represented around 0.8% of the atmospheric N deposited. Larger losses of N2O were measured in a sheep grazed pasture (1.7% of N input), and in an experimental sitka spruce plantation which had received acid mist containing N (3.7 % of N input).


Suitability to indicate atmospheric concentrations:

not suitable.

Suitability to indicate atmospheric depositions:

There is a robust relationship between soil gas emissions and N deposition.

Suitability to indicate environmental impacts:

Emissions of N2O and NO represent a biological response of soils to added nitrogen, so that the method by definition assesses environmental impact.

Sensitivity to other factors:

The method is sensitive to climatic and soil properties which promote nitrification or denitrification, e.g. temperature, rainfall, organic matter content (Skiba & Smith 2000).Freeze thaw cycles can influence the availability of soil mineral N and thus can potentially increase emissions. The nitrogen history of a site will strongly modify its capacity for N2O and NO emissions.


Timescale:

More suited for indication of long-term changes (several years to decades).

Limitations:

Emissions depend on the presence of an active nitrifying and denitrifying population and optimal soil aeration, which is controlled by soil moisture and its relationship to the proportion of water filled pore spaces, and soil texture. The method can be assessed on most terrestrial habitats.


Expertise in field:

Specialist staff and equipment are needed to obtain gas samples in the field.

Expertise in laboratory:

Specialist staff and equipment are needed carry out gas analysis in the laboratory.


Cost (per unit sample):  £500-1000

Cost Comment:  To set up equipment at 3 positions in the field and collect samples, plus gas analysis and data interpretation in lab and would probably take 3 days work at the cost £300 per day + equipment running costs. Total costs per site ~£920


Robustness: