Organic or Inorganic Nitrogen and Rhizobia Inoculation Provide Synergistic Growth Response of a Leguminous Forb and Tree

Abstract

Our objective was to better understand how organic and inorganic nitrogen (N) forms supplied to a tree, Robinia pseudoacacia, and a perennial forb, Lupinus latifolius, affected plant growth and performance of their symbiotic, N-fixing rhizobia. In one experiment, we tested five sources of N [none; three inorganic forms (ammonium, nitrate, ammonium-nitrate); and an organic form (arginine)] in combination with or without rhizobia inoculation. We measured seedling morphology, allometry, nodule biomass, and N status. A second experiment explored combinations of supplied ¹⁵N and inoculation to examine if inorganic or organic N was deleterious to nodule N-fixation. Plant growth was similar among N forms. A positive response of nodule biomass to N was greater in Robinia than Lupinus. For Robinia, inorganic ammonium promoted more nodule biomass than organic arginine. N-fixation was concurrent with robust supply of either inorganic or organic N, and N supply and inoculation significantly interacted to enhance growth of Robinia. For Lupinus, the main effects of inoculation and N supply increased growth but no interaction was observed. Our results indicate that these important restoration species for forest ecosystems respond well to organic or inorganic N forms (or various forms of inorganic N), suggest that the nodulation response may depend on plant species, and show that, in terms of plant growth, N supply and nodulation can be synergistic.

Keywords: Lupinus latifolius; Robinia pseudoacacia; amino acid; arginine; inorganic nitrogen; isotopic nitrogen; nitrogen fixation; organic nitrogen.

Figure 1

Growth responses of black locust and lupine seedlings (98 and 56 days after planting, respectively) supplied inorganic or organic nitrogen with or without rhizobia inoculation and grown in a greenhouse. Noninoculated (A) and inoculated (B) black locust seedlings. Lupine seedlings fertilized with arginine (left plants noninoculated, right plants inoculated) (C) or ammonium-nitrate (left plants noninoculated, right plants inoculated) (D).

Figure 2

Nitrogen isotope ratio (δ¹⁵N) and N content in shoots and roots of lupine seedlings 56 days after planting in the greenhouse. Seedlings were supplied with either arginine (A and B) or ammonium-nitrate (C and D) after either being inoculated (white boxes) or not inoculated (gray boxes) with rhizobia. Vertical boxes represent approximately 50% of the observations and lines extending from each box are the upper and lower 25% of the distribution. The solid horizontal line in the center of each box is the median value and the dotted line is the mean.

Figure 3

Nitrogen isotope ratio (δ¹⁵N) and N content in leaves, stems, and roots of black locust seedlings 70 days after planting in the greenhouse. Seedlings were supplied with either arginine (A and B) or ammonium-nitrate (C and D) after either being inoculated (white boxes) or not inoculated (gray boxes) with rhizobia. Vertical boxes represent approximately 50% of the observations and lines extending from each box are the upper and lower 25% of the distribution. The solid horizontal line in the center of each box is the median value and the dotted line is the mean.