High rates of asymbiotic nitrogen fixation have been observed in dead roots. However, little is known about how biotic and abiotic factors influence asymbiotic nitrogen fixation in dead roots. The objective of this study was to examine how nitrogen fixation in dead roots responded to three abiotic factors: temperature, oxygen concentration, and moisture content by measuring acetylene reduction rates in dead roots. For measuring the response to temperature and oxygen, we collected dead roots of Douglas-fir (Pseudotsuga menziesii) from one site in western Oregon, while the effect of moisture content was tested with samples from three species including Douglas-fir, western hemlock (Tsuga heterophylla), and Ponderosa pine (Pinus ponderosa) from three sites in western Oregon. We measured acetylene reduction 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 acetylene reduction.
Results/Conclusions
The optimal temperature for acetylene reduction in roots was 20°C with an acetylene reduction rate of 9.6 nmol/g/d and a standard error of 3.2 nmol/g/d. Asymbiotic nitrogen fixation was greatly reduced above and below this temperature with acetylene reduction rates of 1.2 ± 0.6 nmol/g/d at 5°C and 0.2 ± 0.2 nmol/g/d at 40°C. The highest acetylene reduction rates (13.8 ± 4.2 nmol/g/d) occurred at an oxygen concentration of 2%. Rates dropped sharply under no oxygen condition (3.0 ± 2.8 nmol/g/d) and at an oxygen concentration of 21% (3.0 ± 0.8 nmol/g/d). Optimal acetylene reduction activity occurred above 60% root moisture content and activity was near zero below 60%. These abiotic factors have a similar influence on asymbiotic nitrogen fixation activity in other substrates like dead wood. In the Pacific Northwest, annual nitrogen fixation activity in dead roots tends to be primarily controlled by temperature while above-ground substrates like dead wood can have reduced activity during the dry summer months when lower moisture contents and higher oxygen concentrations can reduce activity.
