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. The risk prompted the Scottish Environmental Protection Agency (SEPA) to express concerns over the application of phosphorus fertiliser in the North Highlands, particularly in sensitive water body catchments.
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 and nitrogen response to aerial and hand fertiliser treatments. The 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. Based on the results, routine monitoring of fertiliser applications is not required, but it may be prudent to monitor water quality in particularly sensitive areas. 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).
To monitor the effects of aerial and hand forest fertiliser applications on water quality in nutrient sensitive water catchments.
To inform Forestry and Land Scotland’s ongoing fertiliser application programme and test the efficacy of the UKFS water guidelines and General Binding Rules related to fertiliser applications (GBR 18).
To inform future revisions of good practice guidance.
Findings and recommendations
The results show that both orthophosphate (the reactive form) and total phosphorus concentrations in stream waters increased following all of the aerial fertiliser applications. However, 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. It was difficult to discern any response in phosphorus levels to the hand fertiliser applications in the Loch Shin sub-catchments, confirming that this method of treatment poses a minor risk of fertiliser runoff.
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.
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.