Nitrogen fixation and growth response of Alnus rubra following fertilization with urea or biosolids

Abstract

Nitrogen fertilization of forests using biosolids offers a potentially environmentally friendly means to accelerate tree growth. This field study was designed to analyze the effects of nitrogen fertilization on the symbiotic, nitrogen (N)-fixing relationship between Alnus rubra Bong. (red alder) and Frankia. Anaerobically digested, class B biosolids and synthetic urea (46% N) were applied at rates of 140, 280 and 560 kg ha-1 available N to a well-drained, sandy, glacial outwash soil in the Indianola series (mixed, mesic Dystric Xeropsamments). Plots were planted with A. rubra seedlings. At the end of each of two growing seasons trees were harvested and analyzed for the rate of N fixation (as acetylene reduction activity), biomass and foliar N. At year 1, there was no N fixation for trees grown with urea amendments, but control (17 μmol C2H4 g-1 hr-1) and biosolids (26-45 μmol C2H4 g-1 hr-1) trees were fixing N. At the end of year 2, all trees in all treatments were fixing N (7 μmol C2H4 g-1 hr-1, 4-16 μmol C2H4 g-1 hr-1, and 20-29 μmol C2H4 g-1 hr-1 for control, urea and biosolids respectively). Trees grown with biosolids amendments were larger overall (year 1 shoot biomass 10 g, 5 g, and 23 g for control, urea, and biosolids respectively, year 2 shoot biomass 50 g, 51 g, and 190 g for control, urea, and biosolids respectively) with higher concentrations of foliar N for both years of the study (year 1 foliar N 26 g kg-1, 27 g kg -1, and 40 g kg-1 for control, urea, and biosolids respectively, year 2 foliar N 17 g kg-1, 19 g kg-1, and 23 g kg-1 for control, urea, and biosolids respectively). Trees grown with urea amendments appeared to use the urea N over Frankia supplied N, whereas the biosolids trees appeared to be able to use both N in biosolids and N from Frankia. The results from this study indicated that the greater growth of A. rubra may have been responsible for the observed higher N demand. Biosolids may have supplied other nutrients to the trees to support this accelerated growth.