Nodulation and Nitrogen-fixing Capacity of Rhizobial Isolates from China in Symbiosis with Maackia amurensis

Abstract

Abstract Maackia amurensis Rupr. & Maxim (Amur maackia) is a leguminous Asian tree capable of forming N2-fixing symbioses with soil-borne Bradyrhizobium spp. This trait sets Amur maackia apart from many legumes now produced in North American nurseries. Two determinants of N2-fixing capacity in legumes are the compatibility of the host plant and its bacterial microsymbiont and the metabolic efficiency of compatible bacteria. Our objectives were to isolate numerous rhizobia from the root zones of indigenous Amur maackia in China and to select isolates that form superior N2-fixing relationships with inoculated seedlings. Soil samples collected in the Heilongjiang Province of China were used as inocula to establish nodules on seedlings. Putative rhizobia were isolated from these nodules and cultured. Inoculation of additional seedlings with 170 of these isolates evoked nodulation, confirming their identity as rhizobia. Isolates that induced the most nodules were evaluated further. All selected isolates increased growth and total N content of Amur maackia compared to uninoculated controls. Three of the isolates induced more root nodules, and four evoked a higher total N content in plants than did isolate USDA 4349, a previously characterized strain of Bradyrhizobium selected for Amur maackia. Our results demonstrate marked variation among rhizobia compatible with Amur maackia and illustrate the potential to inoculate plants in nurseries and landscapes with superior bacteria, a practice that could reduce fertilizer use and improve performance of trees in N-deficient soils.