The rates and controls of asymbiotic N2 fixation associated with organic detritus such as decomposing woody roots have rarely been evaluated in forests. Little is known about how abiotic and biotic factors influence asymbiotic N2 fixation in decomposing woody roots. The primary objective of this study was to examine how asymbiotic N2 fixation in woody roots after six to nine years of decomposition in three old-growth coniferous forests in Oregon responded to abiotic factors (e.g., temperature, oxygen concentration, and moisture content) and biotic factors (e.g., species, root size, and incubation site) by measuring acetylene reduction (AR) rates. The secondary objective was to estimate the annual N2 fixation by dead roots over six to nine years after a stand-replacing disturbance of an old-growth Douglas-fir (Pseudotsuga menziesii) forest. Decomposing woody roots of three root sizes of Douglas-fir, western hemlock (Tsuga heterophylla), and Ponderosa pine (Pinus ponderosa) were used in this study. We measured AR rates at 5, 13, 20, 30, and 40°C and at oxygen concentrations of 0, 1, 2, 4, 8, and 21% in dead root samples that had been previously wetted. Unaltered root samples with different moisture contents were used to measure the effect of moisture content on AR rates.
Results/Conclusions
The optimal temperature for AR in decomposing woody roots was 20°C with an AR rate of 9.6 nmol/g/d. Asymbiotic nitrogen fixation was greatly reduced above and below this temperature. The highest AR rates (13.8 nmol/g/d) occurred at an oxygen concentration of 2%. Optimal AR activity occurred above 60% root moisture content. Decomposing woody roots at the H.J. Andrews site had the highest AR rates, followed by the coastal Cascade Head site and the significantly lower, dry Pringle Falls site. The median AR rates of western hemlock, Douglas-fir, and ponderosa pine roots were 2.82, 4.32, and 5.12 nmol/g/day, respectively, with western hemlock roots having significantly lower rates than ponderosa pine. The mean AR rates of woody roots were significantly lower in fine roots than small and medium roots. However, they did not differ significantly after six to nine years of decomposition. The potential annual N2 fixation by dead roots over six to nine years after a stand-replacing disturbance of an old-growth Douglas-fir forest at H.J. Andrews site was around 6 kg N/ha/year with a range of 2 to 10 kg/ha/year, suggesting that dead roots have potentials to provide a significant N source to this N-limited forests.