The decomposition process is a key component of the biogeochemical nitrogen (N) cycle and represent an important feedback at both the plant and ecosystem levels. A large proportion of the work on nutrient dynamics associated with decomposition has examined senesced leaf litter. A general pattern of decomposing leaf litter tends to immobilize N early in the decomposition and mineralize N during later phase of decomposition. However, there is less knowledge available on N dynamics of other litter types (e.g., roots, twigs) in decomposition. The primary objective of this talk will focus on the N dynamics of three root decomposition studies that were conducted in temperate coniferous forests at the Pacific Northwest. These studies include a chronosequence study of decomposing woody roots (up to 46 years), a 2-year time series decomposition of fine roots, and a 2.5-year decomposition study using 15N –labeled fine roots. The secondary objective of this talk will examine key factors that influence the N dynamics in decomposing roots.
Results/Conclusions
The chronosequence study indicated that decomposing woody roots of Sitka spruce, Douglas-fir, west hemlock, and ponderosa pine started to release N after 20-30% mass loss. All 15 fine roots of tree species demonstrated N were released during the first 2 years of decomposition. The rate of N release was positively correlated with the initial N concentration of fine roots. The decomposition study of 15N –labeled fine roots further showed that the difference between gross and net N mineralization was significant. The mineralization results suggest that initial root chemistry such as initial N concentration may be an important driver of N dynamics in decomposing roots. Asymbiotic N2 fixation in decomposing roots also provide external N source. Our studies demonstrate that greater amounts of N are cycling through than may be quantified by only measuring net mineralization. Our studies also challenge current leaf-based biogeochemical theory about patterns of N immobilization and mineralization.