Forest fertilisation and Water Quality

Summary

Tree sapplingThis project aims to monitor the effects of aerial and hand forest fertiliser applications on water quality in sensitive water catchments.

Many of Scotland’s forests are growing on upland soils with low nutrient content and require fertilisation to improve tree establishment and growth. Fertilisation often has the desired effect of increasing forest productivity but it can have an adverse impact on the water environment if nutrient runoff, mainly phosphorus, enriches watercourses.

To determine the phosphorus response to aerial and hand fertiliser treatments in sensitive catchments, a programme of water quality monitoring was established in 2014 in a few selected sub-catchments around Loch Shin and the River Oykel; both watercourses are designated as a Special Area of Conservation (SAC).

The monitoring was prompted by the Scottish Environmental Protection Agency’s (SEPA) concerns over Forestry Commission Scotland’s (FCS) aerial and hand phosphorus fertiliser application programme in the North Highland District, particularly in sensitive water body catchments. Whilst past evidence suggests that the applications would not impair water quality, both SEPA and FCS agreed that it would be prudent to undertake water quality monitoring in a few catchments to ensure that water quality is not adversely affected and to check the effectiveness of current good practice measures.

The results will inform FCS’ ongoing fertiliser application programme and test the efficacy of the Water Guidelines and General Binding Rules related to fertiliser applications (GBR 18); any lessons learned will be incorporated into future revisions of best practice guidance.

Research objectives

To monitor the effects of aerial and hand forest fertiliser applications on water quality in sensitive water catchments.

Results so far

The results thus far show that both orthophosphate (the reactive form) and total phosphorus concentrations in stream waters increased following the aerial fertiliser but were well below the 28 µg l-1 good ecological status standard for annual mean reactive phosphorus in upland, low alkalinity waters. In contrast, it was difficult to discern any response in phosphorus levels to the hand fertiliser application.

The results indicate a considerable background level of both orthophosphate and total phosphorus in the streams around Loch Shin, which is subject to marked variation in response to weather (particularly storm events) and seasonal factors. The source of this phosphorus could be natural (weathering), historic fertiliser applications or due to forest felling activity. Further investigation of the soils and past land use and management would be needed to help identify the main cause.

Water sampling and analysis continues with an additional aerial fertiliser site added to the monitoring network.

Status

Forest phosphate fertilisation has been an issue of concern in the UK since the late 1970’s. Improvements to fertiliser practice following the introduction of the Forests and Water Guidelines in 1988, including better helicopter targeting systems and the use of buffer areas, succeeded in reducing phosphate losses to water although some concerns remain, particularly involving fertiliser applications to deep peat.

Hand fertiliser applications present low risk, but in some areas aerial treatments are necessary due to issues of accessibility, scale and cost, particularly on second and third rotation restocking sites. Whilst past evidence suggests that the applications could be undertaken without impairing water quality, both SEPA and FCS agreed that it would be prudent to check that this was the case.

Contact

Nadeem Shah

Funders and partners

This is a partnership project primarily funded by Forestry Commission Scotland with considerable support from Forest Enterprise Scotland; SEPA provide a significant in-kind contribution by analysing water samples in their laboratory.

Forestry Commission policy

The protection of the water environment is a key element of sustainable forestry and this research directly supports the Forestry Commission policy of achieving sustainable forest management in the UK.