Nitrogen dynamics of fine root decomposition in western Oregon

Background/Question/Methods

Nutrient dynamics in belowground detritus are a significant yet poorly understood component of forest ecosystems.  We used ¹⁵N to evaluate how site environment and initial substrate quality influence decomposition and nitrogen (N) dynamics of fine roots.  A decomposition study was installed at Cascade Head (CAH) and H. J. Andrews (HJA) experimental forests in Oregon using ¹⁵N labeled fine roots of bigleaf maple, Douglas-fir and Sitka spruce.  Litter bags were buried at a depth of 10 cm and collected after 3, 6, 9, 12, 18, 24, and 30 months. 

Results/Conclusions

Decomposition rate constants for the first year of the study ranged from 0.0906 to 0.1017 yr^-1, following the order of bigleaf maple > Sitka spruce > Douglas-fir.  Decomposition rates were not significantly different between sites for single species, nor were the rates different between the three species within a given site.  Soil nitrate measured with resin bags averaged 20% greater at CAH than HJA, though the difference was not significant.  CAH exhibited significant seasonal differences in nitrate availability, with the highest availability occurring in late fall.  CAH also exhibited greater ammonium availability, though not significant, and there were no seasonal differences in ammonium availability at either site.  Soil phosphorus availability was significantly greater at HJA, and both sites exhibit significant seasonal differences in phosphorus availability, with the greatest availability occurring in late fall.  While no significant differences in decomposition rate constants have been observed, we expect that differences in initial N concentrations between the fine roots of the three species will result in differing rates of ¹⁵N release in the early stages of decomposition.