Interactive Effects of N Deposition, Land Management and Weather Patterns on Soil Solution Chemistry in a Scottish Alpine Heath

Nitrogen (N) deposition and land management practices can have profound impacts on the structure and functioning of alpine ecosystems occupying headwaters of major river systems. Such impacts have the potential to result in loss of N to surface waters and acidification, both of which could have serious consequences for water quality and downstream habitats. We present results from a 6-year study of Calluna-dominated alpine heathland in Scotland, designed to assess the interactive effects of N addition (0 and 50 kg N ha⁻¹ y⁻l), simulated accidental fire and grazing on soil solution chemistry. Both N addition and to a lesser extent burning had significant effects on soil solution chemistry, whereas grazing had no significant impact. Soil solution nitrate (NO₃ ⁻) and ammonium (NH₄ ⁺) concentration showed a rapid response to N addition and N addition also resulted in acidification of soil solution. A ‘flush' of base cations, which normally buffer soil from the acid effects of deposited N, accompanied the excess N released from the soil to soil solution. The N treatment had no effect on dissolved organic carbon (DOC), however, concentrations were significantly (14%) lower at the burned plots. In addition to treatment effects, temporal analysis of data from control plots demonstrated that soil solution chemistry was influenced by extremes in weather conditions. Peak NO₃ ⁻ concentrations were observed in soil solution following the cold winter of 2000-2001 when there were frequent freeze/thaw events. A large pulse of base cations was lost from the soil following the dry year of 2003. These weather-induced responses potentially exacerbate the treatment effects observed in this study.