Comparative response ofPisum sativum nodulated with indigenous soilRhizobium populations and/or co-inoculated with aRhizobium leguminosarum strain: II. Nodulation, competition for nodulation, plant biomass and nitrogen accumulation

Plants nodulated by Rtuzobium populations from two soils that differed markedly in their acidity (Ruzyn6 soil p i t = 7.6, Lukavec soil pH = 4.9) produced a significantly higher amount of the nodule dry mass per plant and a higher ratio of the nodule dry mass per unit of plant dry biomass in comparison with plants inoculated or co-inoculated (7 • 107 cells per seedling) with Hup + strain 128C30. When inoculated with Lukavec soil, the nodulation was delayed. Production of dry plant biomass and the amount of nitrogen yielded by this biomass were by 10 and 16 % (nodulated by Ruzyn6 soil population) and 30 and 34 % (Lukavec soil population), respectively, lower than if inoculated or co-inoculated with 128C30. Potential dinitrogen fixation resulting from seasonal changes in acetylene-reducing and dihydrogen-evolving activities of the nbdulated roots represented up to 40 % of the total nitrogen of the plant biomass if nodulated with soil rhizobia and about 64 % when inoculated or co-inoculated with 128C30. Co-inoculation with 128C30 together with the soil populations of Rhizobium resulted in an induction of more than 90 % of the nodules by this strain. The necessity and success of inoculation of legumes with introduced Rhizobium strains depends, to some extent, on the size (Brockwell et al. 1988) and efficiency of natural soil Rhizobium populations. They may be affected by various soil types (Mytton and Livesey 1983; Engvild 1989), crop rotation history (Kucey and Hynes 1989) and co-evolution of specific soil populations of rhizobia and the corresponding plant host (Lie et al. 1987). Another factor is soil acidity, one of the main limiting nodulation factors (Munns 1986). In this work, a potential dinitrogen-fixation activity of Rhizobhun populations from two soils differing especially in their acidity values and the efficiency and competitive power of one Hup + strain ofRhizobium leguminosarum bv. viceae were tested. In connection with the differences in some physiological activities already published (~krdleta et al. 1991b), the corresponding data concerning nodulation, competition of the co-inoculated strain for the nodulation of the host, biomass production, the potential dinitrogen f'Lxation and the actual accumulation of nitrogen by the plant biomass are also shown. MATERIALS AND METHODS Two-d-old seedlings of (cv.) Bohat~ pea were inoculated and cultivated under standard growth conditions (~krdleta et al. 1991b). The inoculation treatments were as follows: (a) seedlings inoculated with 7 g air-dried soil samples, (b) seedlings inoculated with the soil samples and co-inoculated with 7 x 107 cells of the Hup § 128C30 strain (Nelson and Salminen 1982) per seedling, (c) seedlings inoculated with ),-irradiated (35 kGy) soil samples and co-inoculated with the above-mentioned strain at the same inoculum rate (control treatment). The samples of the soils were removed from experimental fields of the Rbsearch b~stitute for Crop Production, Prague-Ruzyn6 (gray-brown podzolic soil, pH(H20) = 7.6, pH(KcI) = 6.5, Nt = 1.52 278 V. ~KRDLETA et al. Vol. 36 mg/g dry matter) and of the Experimental Station Lukavec in Bohemia (brown sandy-loam soil, pH(H20) = 4.9, pH(KCI) --4.4, Nt = 1.09 mg/g dry matter). The dry root nodules, roots and tops of five randomly sampled plants from each treatment and harvest were weighed and analyzed for the combined nitrogen by the automated Dumas method (CHN 600 analyzer, LECO, USA). The potential dinitrogen fixation (PN2F) was calculated as follows: / ' N z F = 1/3 x TARA x RE x 24 x 2.8016 • 10 -5 [g N/d per plant] (1) where TARA = C2H2 reduced per h per nodulated root [/zmol] (2) and RE = 1 [H2 evolved (air)]/[C2H2 reduced (air)] (3) (Schubert and Evans 1976). The PN2F values for a growth season were obtained by integration of the areas under the corresponding daily PN2F curves (Eq. 1) using Simpson's discrete approximation. The experimental resuits were subjected to analysis of variance. RESULTS AND DISCUSSION During almost the whole growth period followed, the inoculation treatments significantly affected the nodulation (nodule dry mass per root) and the specific nodulation (nodular*on index; production of nodule dry mass per unit of plant biomass; Table I). Except for the early stages of growth Table I. Nodulation (N; mg nodule dry mass per root) and specific nodulation (SN; mg nodule dry mass per g plant dm) a