Nitrogen deposition effects on plant species diversity; threshold loads from field data

- We assembled species richness and N deposition data for five UK semi-natural habitats.
- We determined threshold deposition rates above which species richness declines.
- Woodland, heath and acid grassland show species loss at all deposition rates.
- Bog and calcareous grassland thresholds exceed the lowest deposition rates.
- The thresholds differ from empirical critical loads for four of the five habitats.

National-scale plant species richness data for Great Britain in 1998 were related to modelled contemporary N deposition (Ndep) using a broken stick median regression, to estimate thresholds above which Ndep definitely has had an effect. The thresholds (kg N ha−1 a−1) are 7.9 for acid grassland 14.9 for bogs, 23.6 for calcareous grassland, 7.8 for deciduous woodland and 8.8 for heath. The woodland and heath thresholds are not significantly greater than the lowest Ndep, which implies that species loss may occur over the whole range of contemporary Ndep. This also applies to acid grassland if it is assumed that Ndep has substituted for previous N fixation. The thresholds for bog and calcareous grassland are both significantly above the lowest Ndep. The thresholds are lower than the mid-range empirical Critical Loads for acid grassland, deciduous woodland and heath, higher for bogs, and approximately equal for calcareous grassland.