High asymbiotic N2 fixation rates in woody roots after six years of decomposition:: controls and implications

The rates and controls of asymbiotic N-2 fixation associated with organic detritus such as decomposing woody roots have rarely been evaluated in forests. We measured acetylene reduction (AR) rates in woody roots after six years of decomposition in three sites of coniferous forest in Oregon, U.S.A., calibrated with measurements of N-15(2) fixation. Incubation site and root size significantly affected the AR rates of decomposing roots. Decomposing roots at the H.J. Andrews site had the highest AR rates, followed by a wetter site, Cascade Head, and a drier site, Pringle Falls. The mean AR rates of dead roots increased with increasing root size, while mass loss decreased with increasing root size. However, root species did not significantly affect AR with Douglas-fir, western hemlock, and ponderosa pine having mean rates of 13.7, 12.0, and 16.0 nmol/g/day, respectively. Mean N-2 fixation rates in decomposing roots are at least four times higher than rates reported for other asymbiotic sources of N2 fixation such as dead wood and litter. The average conversion ratio for AR data measured by N-15(2) fixation in dead roots was 4.5:1. A preliminarily estimate of potential annual N-2 fixation by dead roots after a stand-replacing disturbance of an old-growth Douglas-fir forest at the H.J. Andrews site was 6.3 kg N/ha/year with a range of 2.1 to 10.4 kg N/ha/year, suggesting that dead roots have potentials to provide a significant N source to these N-limited forests.