Browsing by red deer negatively impacts on soil nitrogen availability in regenerating native forest

Herbivores can have important indirect effects on belowground properties and processes that govern ecosystem form and productivity. A major way that herbivores affect ecosystem productivity is by modifying feedbacks that occur between dominant plants and belowground properties, and especially by changing the rates of nutrient mineralisation. In this study, we examine the effects of browsing by red deer, relative to landscape factors such as variations in microclimate and topography, on soil biological properties and N cycling in regenerating forest in an upland region of the Scottish Highlands. The site at Creag Meagaidh in the Scottish Highlands was sampled three times over the growing season of 2001 and soil cores were taken from four exclosures. Measures of soil biological properties and nitrogen availability were made. The results presented here suggest that although spatial and temporal variation were the dominant factors affecting soil biological properties and nitrogen dynamics in this ecosystem, herbivory also had consistent and significant effects on these measures. Browsing significantly reduced measures of DOC (27%), NO3- (48%), NH4+ (49%) and N-mineralisation (53%). The microbial C:N ratio was also significantly greater (89%) in browsed than un-browsed areas, suggesting that the removal of deer reduced the extent that the microbial community was N-limited. These significant negative effects of browsing on soil nitrogen cycling have the potential to reduce ecosystem productivity. We conclude that removal of browsing over 14 years has accelerated native woodland regeneration, leading to subsequent increases in soil C- and N- mineralisation, further increasing plant N supply and tree growth in this ecosystem. (C) 2003 Elsevier Ltd. All rights reserved.