An unusual soil layer and its impact on nitrogen movement in Loch Vale Watershed, Colorado

Background/Question/Methods

The discovery of an unusual black organic soil layer was found within a experimental fertilization plot located at Loch Vale Watershed in Rocky Mountain National Park.   The experimental plots are part of a long-term study of nitrogen saturation within the park. A preliminary chemical analysis of the layer and a survey of the experimental plots were conducted to determine the black soil layer’s composition, extent, and any effects the layer may have on anthropogenic nitrogen deposition.  I collected soil cores with and without the soil layer and performed KCL and diH2O extracts to look at characteristics of the black soil layer in comparison to the surrounding A and B horizons.  A survey of the experimental plots was also performed to determine the layer’s extent and if it is correlated with nitrogen saturation at the site.  Finally, a laboratory leaching experiment was conducted to see if the black organic layer had an effect on nitrogen loss and retention within the soils.  Results/Conclusions

Chemical analysis revealed that the black layer did not have a significantly higher concentration of inorganic nitrogen (8.11mg NH₄/g-soil and 384ug NO₃/g-soil) or total N (0.39% N), which were similar to amounts measured in the surrounding A and B horizons.  More significant differences were found between the organic and mineral soil horizons, with the organic horizon having higher %C (23.87%), %N (1.29%), and NH₄⁺ content (24.68mg NH4/g-soil). Surveys revealed the black layer occurred discontinuously in forest soils throughout the watershed and it occurrence had no correlation with experimental fertilization at the site.  During the leaching experiment, only cores from fertilized soils had higher concentrations of NH₄⁺ and NO₃^- in collected leachates, indicating an increase in nitrogen leaching from previously fertilized soils with continued treatment. The black layer does not have any significant effect on N concentrations in leachate.  My investigation indicated that the black layer is most likely a well-developed organic-rich Bh horizon in the forested Lithic Cryoboralf soils of Loch Vale.