Long term effects of enhanced nitrogen deposition on a lowland dry heath in southern Britain
✍ Scribed by S. A. Power; M. R. Ashmore; D. A. Cousins; N. Ainsworth
- Publisher
- Springer Netherlands
- Year
- 1995
- Tongue
- English
- Weight
- 389 KB
- Volume
- 85
- Category
- Article
- ISSN
- 0049-6979
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✦ Synopsis
Experi_mental additions of ammonium sulphate to a nitrogen-poor dry heathland have been carried out since 1989. There are four nitrogen treatments: a control (receiving artificial rain only), a low treatment which receives an additional 7.7 kg N hff I yr "l, a high treatment receiving 15.4 kg N hff 1 yr 1 and an alternating treatment which receives either the control or tile high nitrogen additions, in alternate years. The estimated background deposition at the study site is 13-18 kg N ha 1 yr "1, a value similar to the critical load that has been suggested for the conversion of lowland heath to grassland. Over the past 5 years there have been significant stimulations in shoot growth, flowmug, canopy density and litter produaion. Flowering, in particular, strongly reflects nitrogen additions in the altemating treatment.
Current models of the response of dry Calluna heathland to enhanced nitrogen deposition suggest that higher tissue nitrogen levels ~ occur and will be accompanied by heightened sensitivity to secondary stresses. This may in turn lead to canopy breakdown and replacement by grassland. The application of nitrogen at deposition rates only shghtly in excess of the critical load over five years has produced small, non-si/~gaificant increases in shoot nitrogen content. Flowever, there is clear evidence of a large positive effect on shoot growth, flowering, litter production and canopy density of Calluna. The observation of these responses at the application rates used in this study supports the current proposals for critical loads of nitrogen for lowl~ad heaths.