THE CONTROLS ON DISSOLVED NITROGEN LOSSES FOLLOWING 2 INTENSITIES OF HARVESTING IN A SITKA SPRUCE FOREST (N WALES)

The controls on dissolved nitrogen losses following conventional and whole-tree harvesting were investigated in a Sitka spruce plantation in N. Wales (UK). Lysimeters containing the soil organic horizons were established with the treatments brash, ground vegetation and Sitka spruce seedlings in a limited factorial design; other treatments designed to investigate the effect of the physical protection provided by the brash and the brash throughfall on underlying organic horizons were also included. The lysimeter leachates were collected and analysed weekly for 15 months. The brash present following conventional harvesting increased nitrate leachate losses two fold during the experimental period relative to control lysimeters containing only soil organic matter with no vegetation or brash. Increased inputs of soluble organic nitrogen, potassium and phosphorus in the brash throughfall were found relative to the incoming rain. However, the increased nitrate loss may be attributable to improved microclimatic conditions stimulating mineralisation and nitrification rates beneath the brash as similar effects on nitrate loss were observed with an inert 'artificial brash' cover on the organic material. The presence of brash also increased hydrogen and aluminium loss from the organic horizons. The presence of less than 50% cover of Agrostis capillaris L., which establishes quickly in the absence of brash, reduced nitrate losses to 19% of the control and 10% of that from the brash treatment. Soluble organic nitrogen losses were unaffected and were an important component of nitrogen leachate losses.