Atmospheric nitrogen deposition leads to soil acidification and increased nutrient leaching from soils. In severe cases it mobilizes aluminum resulting in toxicity to plants or downstream aquatic life. Since ecosystem responses to N loading are characterized by biogeochemical feedbacks that produce dynamic changes over time, long-term research approaches are essential to capturing the full extent of adaptive system response. To better understand the extent and duration of N deposition impacts on Rocky Mountain ecosystems we carried out a twenty-year N fertilization experiment at Loch Vale, a subalpine watershed in Rocky Mountain National Park.
We applied 25 kg ha-1 ammonium nitrate annually to three 30 X 30 m forest plots, each paired with a control plot, from 1996 to 2017. Atmospheric deposition contributed an annual average of 3 kg ha-1 N to the watershed over that time. We used lysimeters and soil samples to measure changes in pH, soluble cations, nitrate, dissolved organic carbon, and rates of nitrification and mineralization over time. We analyzed the results to elucidate the long-term ecosystem response to N loading and impacts on nitrification, acidification, and leaching of C, N, and major cations.
Results/Conclusions
After two years of N addition rates of nitrification peaked (11 X controls), followed by a 0.35 unit-decline in pH. Leaching of major cations peaked between 1999 and 2005 (~ 3 X controls). Elevated rates of nitrification diminished until by 2010 cation leaching was not significant. By 2015 we detected no significant difference in pH or nitrification between treatments and controls, even though N application was ongoing. We consider possible drivers of stabilizing feedbacks that ultimately mitigated against acidification and Al leaching.
Nitrification is a microbially-mediated process that generates acidity during the oxidation of ammonium. Previous work at Loch Vale found the soil microbial biomass declined with treatment, possibly indicating a reduction in nitrifier populations. We also observed elevated ammonium leaching, a potential stabilizing feedback caused by acidity produced by soil nitrifiers. Since N addition has led to amplifying feedbacks between acidity and Al leaching in other forest systems, understanding the apparent source of resilience to N addition at Loch Vale may prove useful to mitigating impacts of N deposition in other systems. These results also highlight the importance of long-term research in understanding the responses of complex ecosystems to global change drivers.