Diversidade e eficiência de isolados rizobianos para calopogônio (Calopogonium mucunoides) originados de um argissolo sob diferentes coberturas vegetais

Abstract

Biological nitrogen fixation by the legume-rhizobia symbiosis is one of the most important mechanisms of nitrogen supply to plants, and depends on the symbiotic efficiency of the rhizobial strain. We now know that legumes associate with 98 species from 18 different genera of rhizobia and native rhizobia biodiversity is fundamental for the selection of new, more efficient strains for biological nitrogen fixation. To this end, the present work aims to evaluate the effect of different vegetation covers in a single soil on diversity and symbiotic efficiency of calopo rhizobial isolates. Soil samples were collected at the 0-0,2 m deep layer, in areas with Brachiaria decumbens pastures, “sabiá” (Mimosa caesalpiniifolia) woodlots and Atlantic forest, with three separate areas for each cover, all in the same soil. Calopo plantlets were pre-germinated in Petri dishes with paper towel for three days, transplanted to Leonard jars, inoculated with 10 g of soil and harvested after 50 days. Four to five nodules were sampled per plant, used to rhizobial isolation in YMA media and later morphologically characterized. 1575 isolates were obtained, which were distributed in 398 groups. Authentication was conducted for a single isolate from each group, in randomized blocks with two replicates, in bags with 0.5 kg of sand:vermiculte (1:1) autoclaved mixture, and received Hoagland solution without N daily. Inoculation was done with 1 mL per plant of a bacterial broth with population estimated in 108 cells.mL-1. Plants were harvested 45 days after inoculation and separated in shoot, root and nodules, dried in oven until constant mass and weighted. The symbiotic efficiency test was conducted in randomized blocks, with four replicates, in polyethylene bags with 1.5 kg of the sand:vermiculite 1:1 autoclaved mixture. Treatments were 122 isolates selected after authentication, based on shoot dry mass, five uninoculated controls with 0, 50, 100, 150 and 200 kg.ha-1 of N, and a control inoculated with SEMIA 6152, recommended for the culture. The experiment was conducted as the earlier phase. Shoot dry matter was ground and nitrogen content was measured. 25 isolates were considered efficient and selected for the next phase. An experiment was conducted in randomized blocks, with four replicates, in polyethylene bags with 2.5 kg of soil. Treatments were the 25 selected isolates, five uninoculated treatments with 0, 30, 60, 90 and 120 kg.ha-1 of N, and a treatment inoculated with the SEMIA 6152 strain. Inoculations was done as per the authentication phase. The first cut was 45 days after inoculation, with the remaining cuts 45 days after the previous one. Shoot dry matter was ground and N contents were determined for the first cut. Data was submitted to analysis of variance, and the Scott-Knott means comparison test, and variables were correlated. Over 98% of the isolates have fast growth in culture media, although the recommended strain is B radyrhizobium japonicum¸ considered to have slow growth. After grouping, 163, 257 and 77 groups were found for the pasture, sabiá woodlots and Atlantic Forest areas, respectively, showing high diversity amongst the isolates. Vegetation cover did not affect rhizobial diversity. Mean shoot dry matter for the plants inoculated with the most efficient strains did not differ from the treatment receiving 150 kg. ha-1 of N. Shoot dry matter correlated positively with all evaluated variables. The highest proportion of efficient isolates was from sabiá woodlots, indicating that symbiotic efficiency may be affected by vegetation cover. There was high variability in nodule dry mass, as well as in N content and accumulation, indicating large differences in symbiotic capability among the isolates. Shoot dry matter was affected by N supply in both cuts. There was an increase in shoot dry matter in the second cut for all treatments, when compared to the first cut. There was no significant effect of vegetation cover in the evaluated variables for both cuts. Higher responses to the isolates were found in the second cut, and some isolates allowed shoot dry matter similar to those of the highest N fertilization treatments.