Seed collection and Soil preparation: The experiment was carried out during kharif season from March, 2017 to May, 2017 and March, 2018 to June, 2018 in the net house of Soil Science Division, BARI, Joydebpur, Gazipur. Seeds of mungbean (BARI Mung-6) were collected from Pulses Research Centre, BARI, Gazipur. The silted (sandy clay loam) soils were collected from the bank of Turag River at Kodda, Gazipur mixed with cowdung at 5:1 ratio and was used as the potting media. Each pot (28 cm in diameter and 23 cm in height) was filled with approximately 8-kg soil leaving the upper 3 inches of pot was vacant to facilitate watering. The pH of cowdung was 6.7 and the nutrient contents were: organic matter 14.1%, N 0.8%, P 1.26%, K 0.88%, Ca 1.55%, Mg 0.82%, S 0.62%, Fe 0.25% and Mn 0.112%. The physical and chemical properties of the soil are presented in Table 1. The soil contained 12 AM (100-1 g soil) spores of indigenous mixed AM fungal species and the experiment was conducted under unsterilized soil conditions.
Soil analysis: Soil pH was measured by a combined glass calomel electrode (Jackson, 1958). Organic carbon was determined by the Wet Oxidation Method (Walkley and Black, 1934). Total N was determined by the modified Kjeldahl method (Jackson, 1962). Calcium, K and Mg were determined by NH4OAc extraction method (Black, 1965). Copper, Fe, Mn and Zn were determined by DTPA extraction followed by AAS reading. Boron was determined by CaCl2 extraction method. Phosphorus was determined by the Modified Olsen method (Neutral + Calcareous soils) according to Olsen et al. (1954). Sulphur was determined by CaH4(PO4)2.H2O extraction followed by turbidimetric turbidity method with BaCl2.
Fertilizer application: Chemical fertilizers @ 22.56 kg N ha-1 from urea, 24.45 kg P ha-1 from TSP, 34.27 kg K ha-1 from MoP, 7.95 kg S ha-1 from gypsum, 2.23 kg Zn ha-1 from ZnSO4, 0.81 kg B ha-1 from Boric acid and 0.34 kg Mo ha-1 from Na2MoO4.2H2O were applied (BARC, 2012). Half of urea-N and all other fertilizers were applied as basal during final land preparation and the remaining half amount of urea-N were applied after 10 days after sowing.
Preparation of salinity solution and mycorrhizal inoculum: Different concentrations of salinity were prepared according to New South Walles (NSW) Department of Primary Industries 2005, Australia and applied three times during the experimentation period. Firstly applied before sowing of mungbean seed, next after 25 DAS and then after 35 DAS. The arbuscular mycorrhizal inoculum was prepared from the roots and rhizosphere soils of Sorghum. Mycorrhizal species was originally isolated from different AEZ region, using the wet sieving and decanting method. The spores were left to multiply for 6 months on sorghum plants using unsterilized soil, collected from the same site, in the net house of Soil Science Division, BARI. Plants were irrigated with tap water as needed. A mixture of infected sorghum root and soil which contained spores was used as mycorrhizal inoculum. The soil-based AM fungal inoculum containing 150 g of rhizosphere soil (approximate 275 ± 20 spores 100 g-1 soil) and infected sorghum root fragments with a minimum infection level was inoculated to each mycorrhizal pot. The mycorrhizal inoculum was first placed in each pot at 3-5 cm depth and was covered with a thin soil layer of 1 cm immediately prior to the seed sowing of mungbean to facilitate fungal colonization of plant roots.
Design of experiment and treatments: The experiment was designed in RCBD with 10 treatments and 4 replications. Ten seeds were sown in each pot at 1 cm soil depth. After collecting germination percentage data the treatment was sustained with 3 vigorous seedlings in mycorrhizal and non-mycorrhizal pot, and the other seedlings were removed from the pot. There were ten treatments viz. T1: Control, T2: Arbuscular mycorrhiza (AM) + 50% P, T3: AM + 75% P, T4: AM + 100% P, T5: Rhizobium + 50% P, T6: Rhizobium + 75% P, T7: Rhizobium + 100% P, T8: AM + Rhizobium + 50% P, T9: AM + Rhizobium + 75% P and T10: AM + Rhizobium + 100% P.
Determination of germination percentage: The germination test was carried out according to ISTA rules (ISTA, 1976). For each treatment, 25 seeds were put into Petri dishes. The Petri dishes were put on a laboratory table at room temperature (25 ± 2oC). After 3 days, normal, abnormal and diseased seeds were counted. Germination of mungbean seed in the laboratory table was 84%. Ten seeds were sown in each pot. After 4, 6, 8 and 10 days germinated seeds were observed and counted. Germination percentage was calculated by the following formula: Germination (%) = {( Number of germinated seeds in each pot) / (Total number of seeds sown in each pot)} x 100.
Crop harvest: Mungbean were harvested after 62 days of sowing. Different growth parameters like plant height, pods plant-1, seeds pod-1, 100-seed weight, seed yield and stover yield were measured.
Statistical analysis: Data were statistically analyzed using Analysis of Variance (ANOVA) following CropStat package while the all pair comparisons were done by Statistix 10.