Freshwater Umbrella Research 2004-7

WP 4: Influence of climate change on the impacts of atmospheric deposition on upland freshwaters

Task 4.2 — Recovery and climate manipulation experiments at the Afon Gwy

The key research question to be addressed in this Task is: how will reduced N deposition and climate change affect key N processes and transfer of nitrogen from terrestrial to freshwater systems?

A unique experiment has been established in the Plynlimon catchment under the NERC Thematic programme GANE to identify the effects of reduced N and S deposition and warming on the vegetation, soils and water quality of a typical upland grassland. The experiment utilises a novel retractable roof system which allows transparent roofs to be drawn across the vegetation during rain events thus excluding N inputs in rain whilst 'clean' rain is applied to the experimental plots beneath the covers using an irrigation system. Warming is carried out using an additional set of reflective covers which are drawn over the vegetation at night thus preventing heat loss due to infrared radiation.

Initial results from the recovery treatments indicate a fast response in nitrate leaching once nitrogen deposition is reduced (within months) indicating 'new' deposited nitrate is directly contributing to nitrate transferred to freshwater systems. No change in sulphate or ammonium leaching was observed indicating recovery may be delayed and a more complex relationship exists between inputs and leaching losses. In the warming treatment, an initial pulse of nitrate leaching was observed but this has since declined indicating a possible increase in plant uptake which compensates for increased N immobilisation in the soil. These data in combination with another warming experiment in a heathland in north Wales partly funded by the EU (VULCAN and now Euro-limpacs projects) can provide crucial information concerning the likely effects of warming at the ecosystem level which combines soil and plant responses and their interaction and acclimation to warming over time. This type of information is required by the UK NFC if the effect of climate change on key N process such as immobilization and uptake are to be included in future critical load calculations.

Warming and reduction treatments at the roof experiment site in the Afon Gwy will continue for a further 3 years. Responses in soil water quality will be monitored through fortnightly collections at two soil horizons. Waters will be analysed for all major anions and cations. Combining these results from soil and vegetation measurements funded within the Terrestrial Umbrella will enable the impact on key soil processes to be identified and the implications for freshwater quality to be assessed. Of particular interest is the effects of warming on the retention capacity or possibly mobilisation of stored N and S from the soil organic pool, and the rate of recovery of soil water chemistry in response to reduced inputs of N and S.