Effect Of Rhizobia Inoculant On Soybean Nodulation And Arbuscular Mycorrhizal Fungi Colonization Under Greenhouse And Field Conditions

ABSTRACT

In the highland of South-Kivu, DR.Congo, soybean farming is increasingly practiced by smallholder farmers but it productivity remains low. Both productivity and N2-fixing abilities of legumes can be enhanced not only by Rhizobium spp but also by colonization of their roots by Arbuscular Mycorrhizal Fungi (AMF). The objectives of this study were:- to evaluate symbiotic effectiveness of bradyrhizobia isolates for soybean inoculant production, their effect on indigenous AMF root colonization, and the suitability of Walungu peat/DR. Congo for inoculants production. The study was carried out in the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria. Nine strains of B. Japonicum from IITA collection and two reference strains (USDA110 and USDA532c) used in commercial inoculant, two controls (Urea and un-inoculated, non fertilized plots), Walungu peat from D.R Congo and the commercial standards America Peat Technology (APT) were investigated. Strains effectiveness for soybean nodulation was evaluated in both screenhouse and field conditions. In the field experiment the study also evaluated the effect of each individual strain on indigenous AMF colonization with soybean roots. The experimental design used was a randomized complete block design (RCBD) replicated 4 times giving a total of 52 plots in the experiment. Data were recorded on leave chlorophyll content, nodulation parameters (number and dry weight of nodules) above ground biomass parameters (dry weight, nitrogen and phosphorus content), and AMF colonization (presence or absence of hyphae, arbuscular and vesicular).In the laboratory experiment, the suitability of Walungu peat as carrier material for inoculant production was assessed under sterile and unsterile conditions and at three different temperatures (4° C, 15° C and laboratory temperature), as compared to the standards APT, using plate count method. The results showed that RANI22, RACA6 and IRJ2180A were effective in improving most of the parameters under study (chlorophyll, above ground dry weight, above ground nitrogen and phosphorus content, nodule number and dry weight) in both screenhouse and field experiment. At 8WAP leaf chlorophyll content increased by 27, 25, and 23% for RANI22, RACA6, and IRJ2180A respectively in the screenhouse experiment and 40-47% in the field experiment as compared to the control (uninoculated- no fertilized plots). On another hand, nitrogen fertilizer also showed increase in some of the parameters especially chlorophyll and nitrogen content but not significantly different from one or another of the three strains. In screenhouse conditions for instance, nitrogen fertilizer, RANI22 and RACA6 increased the above ground nitrogen content by 43-51% (p˂.001) as compared to the control, and in the field experiment the same parameter was increased by 69-66% (p˂.001) by RANI22, RACA6, IRJ2180A, and nitrogen fertilizer relative to the control. None of B. japonicum strains used in the current study or nitrogen addition affected significantly the infection rate of indigenous AMF as indicated by 100% (p˃0.05) hyphae in all the treatments. Walungu peat limed and APT were equally effective in increasing the growth and survival of the initial B. japonicum (USDA110) population (5x 108 ) over a period of 2 months. Higher population density (3.9 x109 and 3.5x 109 ) (p˂0.05) was recorded in sterile condition at 4° C and 15° C respectively. RANI22, RACA6, IRJ2180A and Walungu peat amended are alternative inputs for high quality inoculant production and soybean performance