Where plant growth may be limited by P availability or when enhanced N deposition may lead to reduced availability and/or uptake, measurements of the N:P ratios in selected species may provide a better indication of N saturation. Optimum N:P ratios for plant growth range from 10-14 (van den Driesshe, 1974; Ingestad, 1979). Low ratios (<10) indicate N limited growth and high ratios (>14) indicate P limitation (Koerselma & Meuleman, 1996).
- In areas of southern Sweden with high inputs of atmospheric N, Sphagnum growth was found to be P limited and N:P ratios of around 34 were measured in Sphagnum capitula (Aerts et al. 1992). Conversely in northern Sweden where N inputs are small, Sphagnum growth was limited by N availability and N:P rations were very small (6).
- Long term increases in atmospheric N deposition has led to increased foliar N levels and reduced concentrations of P, K and Mg, in beech stands in north eastern France Duquesnay et al. (2000), leading to increases in ratios of N:P, N:K and N:Mg of 42, 19 and 77, respectively.
- Biodiversity and species richness have also been related to N:P ratios (Ertsen 1998). In selected Dutch grassland and heathland sites a N:P ratio >16 favoured biodiversity (Roem & Berendse, 2000).
Suitability to indicate atmospheric concentrations:
There is insufficient evidence available.
Suitability to indicate atmospheric depositions:
The evidence available is not adequate to support a robust relationship between N:P and N deposition. (However when determining tissue N as a bioindicator, the added determination of P at relatively modest extra cost, provides valuable information in the form of N:P).
Suitability to indicate environmental impacts:
There is some evidence of an ability to indicate species richness but more research is needed.
Sensitivity to other factors:
The ratio is sensitive to changes in available P in the soil such as may occur close to urban areas.
This method can be used to assess long-term or short-term change in deposition. However, care must be taken in selecting species suitable for the type of change. Tree species can be used for long-term change whereas mosses may be more suitable for response to short-term changes. For some sites, in high rainfall areas, Calluna may be more suitable than mosses.
The method is not limited to particular habitat or species types, but is most suitable for woodland and moorland ecosystems. Ideally several common species should be sampled to obtain values that could be compared with published values. At least 3 replicate samples should be collected at several points in the site. Sampling also needs to take account of adjacent vegetation as this strongly influences the N composition through competition. Approximately 2 g fresh weight of material should be collected to provide a minimum of 0.5 g of dried, ground material for chemical analysis.
Expertise in field:
Sampling requires training in species identification particularly of bryophytes or lichens, which many Agency staff may have already received. When age or growth phase of the plant or foliage is important, these must be identifiable by staff.
Ideally, samples should be collected, stored in cold conditions, cleaned (removal of litter, other species etc) and oven dried before being transported to an analytical laboratory by post. When suitable ovens are not available, samples can be air-dried before transportation to the laboratory.
Expertise in laboratory:
Determination of total tissue N and P in plants requires sample digestion (with strong acids in a fume cupboard) and subsequent analysis using specialist, but well proven laboratory techniques.
Cost (per unit sample): £10-100
Cost Comment: For 3 species, 3 replicates per species and 3 locations per site.
Total per sample: £30 (27 samples - £810).