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 enriches watercourses.
This risk prompted the Scottish Environmental Protection Agency (SEPA) to express concerns over Forest Enterprise Scotland’s (FES, now Forestry and Land Scotland) aerial and hand phosphorus fertiliser application programme in the North Highland District (now North Region), particularly in sensitive water body catchments. Whilst past evidence suggests that well designed and managed applications would not impair water quality, SEPA, FES and Forestry Commission Scotland (FCS) agreed that it would be prudent to undertake water quality monitoring in a few sensitive catchments to check that this was the case and hopefully demonstrate the effectiveness of current good practice measures.
A programme of water quality monitoring was established in selected sub-catchments around Loch Shin, the River Oykel and the River Peffery to determine the phosphorus response to aerial and hand fertiliser treatments.
To monitor the effects of aerial and hand forest fertiliser applications on water quality in nutrient sensitive water catchments.
Findings and recommendations
The overall results of the study indicate that a good standard of forestry practice, including buffer widths of up to 50 m for aerial applications and 10 m for hand applications, was sufficient to protect water quality from fertiliser treatments. With respect to phosphorus, concentrations in all of the streams remained below the 13 µg l-1 high ecological status WFD standard for annual mean reactive phosphorus, even when up to 63% of the catchment was aerially fertilised. In terms of nitrogen, no issues were identified for nitrate but the total ammonia concentration (99th percentile) in one stream (the Peffery Tributary) varied between good and moderate status, albeit for a short time. This was thought to be related to over-flying of a section of the stream, highlighting the need for care in identifying and avoiding the treatment of buffer areas, especially where the treated area drains to nutrient-sensitive waters.
It is recommended that the results of this monitoring are used to inform future updates to UKFS Water Guidelines and GBRs related to fertiliser use in forestry (GBR 18).
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 & 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.