Controls on soil solution nitrogen along an altitudinal gradient in the Scottish uplands

Area of Science:

• Ecology
• Environmental Science
• Soil Science

Background:

• Nitrogen (N) deposition poses a significant threat to upland ecosystems, causing acidification, eutrophication, and biodiversity loss.
• Understanding the fate of deposited nitrogen is crucial for managing these sensitive environments.

Purpose of the Study:

• To investigate the spatial variability of soil solution nitrogen (N) and identify the factors driving these patterns in a pristine Scottish catchment.
• To assess the influence of soil properties, hydrology, and habitat type on nitrogen retention and leaching.

Main Methods:

• A 5-year monitoring study was conducted across six sites along an elevation gradient in the Cairngorm Mountains.
• Data collected included bulk deposition chemistry, soil carbon content, soil solution chemistry, soil temperature, and soil moisture.
• Statistical analyses were used to determine the best predictors of soil solution inorganic and organic nitrogen concentrations.

Main Results:

• Highest soil solution inorganic nitrogen concentrations were observed in alpine soils at higher elevations.
• Soil carbon stock, dissolved organic carbon (DOC), and site hydrology were key predictors of nitrate (NO3-) concentration.
• Ammonium (NH4+) concentrations were influenced by net mineralization, particularly in heath and peaty sites.
• Soil solution dissolved organic nitrogen was strongly related to DOC and temperature.

Conclusions:

• Spatial variations in soil solution nitrate are controlled by habitat nitrogen retention capacity, influenced by biological uptake and soil/water contact time.
• Soil percent carbon was a more effective predictor of inorganic nitrogen than total soil carbon mass.
• A fine-scale approach to assessing surface water vulnerability to nitrogen leaching is recommended over broad-scale critical loads, especially for sensitive regions.