Quantifying biological nitrogen fixation of agroforestry shrub species using ¹⁵N dilution techniques under greenhouse conditions

Abstract

Some land-use systems in Saskatchewan, Canada include the nitrogen-fixing trees buffaloberry (Shepherdia argentea Nutt.), caragana (Caragana arborescens Lam.) and sea buckthorn (Hippophae rhamnoides L.). These species provide various ecological functions such as ameliorating soil moisture, light and temperature but little work has been done quantifying biological nitrogen fixation by these species. Greenhouse experiments were conducted to quantify N₂-fixation using the ¹⁵N natural abundance and the ¹⁵N dilution methods. Buffaloberry failed to form nodules in all but one of the four replicates in the natural abundance experiment. Using the ¹⁵N dilution method, the percentage of N derived from atmosphere (%Ndfa) in the shoot of buffaloberry averaged 64 %. For caragana, the mean  %Ndfa was 59 and 65 % and seabuckthorn was 70 and 73 % measured using the natural abundance and dilution methods, respectively. Because of large variability in biomass production between plants grown in the natural abundance experiment and the dilution experiment, the amounts of N₂ fixed also were very variable. Buffaloberry fixed an average of 0.89 g N m⁻²; the average for caragana ranged from 1.14 to 4.12 g N m⁻² and seabuckthorn ranged from 0.85 to 3.77 g N m⁻² in the natural abundance and dilution experiments, respectively. This corresponds to 16 kg N ha⁻¹ year⁻¹ for buffaloberry; an average of 15–73 kg N ha⁻¹ year⁻¹ in caragana and 11–67 kg N ha⁻¹ year⁻¹ in seabuckthorn. The substantial amounts of N₂ fixed by these species indicate that they have the potential to contribute to the overall N balance in land-use systems in which they are included.