Evaluation of summer legumes for yield and measurement of nitrogenfixation by using different approaches

A field experiment was conducted in the irrigated environment for theevaluation of yield and measurement of Nitrogen and fixation by soybean, cowpeaand mung bean by three methods i.e. N difference, ureide and naturalabundance-15 methods. The experiment was laid out in a Randomized CompleteBlock design with four-site characterization and the legumes were sown in row30 cm apart in a treatment plot size measuring 6.4 m the maize (as areference) was sown 60 cm apart in separate treatment plot size. A basal doseof P at 100 kg P2O5 ha-1 and of 50 kg K2O ha-1 was applied to all treatment,plots at sowing. The crops were irrigated with canal water when needed. Thelegumes and reference crops were sampled at maximum biomass stage formeasuring biological yield, total crop N and natural abundance 15N content inplants to measure N2 fixed in legumes. The xylem sap samples were collectedat pod-fill stag for measuring relative abundance of ureide N in legumes. Thecraps were harvested at physiological maturity and the grain yields wererecorded. The grain and plant samples were analyzed for total N contents.Each treatment plot was sampled at 0-3 cm at harvest stage to determine theeffect of legume on soil organic C total N and mineral N. The grain and shootdry matter yields varied greatly with the legume species. The grain yieldranged from 3.2 t ha-1 for the pigeon pea to 0.8 t ha-1 for the mung bean.The soybean-1, cow pea and mungbean had produced almost similar yields i.e.between 0.8 and 1.0 t ha-1. Similarly, the shoot dry matter ranged from 11.5 tha-1 in pigeon pea to 7.0 t ha-1 in soybean-1. The N concentration in plantvaried significantly with the legume species ranging from 2.32% in pigeon peato 1.69% in soybean. Moreover, the concentration in grains of different legumesranged from 4.9% in soybean- 2 to 2.44% in pigeon pea. Similarly, the amountof total crop N was different for different crop species ranging from 269 kg Nha-1 in pigeon pea to 120 kg N t ha-1 in soybean-1. The %pfix value, werealso different for legumes but the result of the natural abundance I5N andureide methods were almost similar (r2 0.97). The average Pfix values forvarious legumes ranged from 50 to 80%. Similarly the amount of total fixed inlegume varied greatly with the legume species. Again, the results of thenatural abundance 15N and the xylem ureide methods were very close (r2.0.97) but values for that of N differences methods were higher. The averagevalues for N2 fixed in different legumes as measured by the 15N and ureidemethods ranged from 101 kg ha-1 in soybean-I to 355 kg in mung bean. Theresults further showed that all the legume had positive balance for N rangingfrom +51 kg N ha-1 to +322 kg Na-1 by the soil analysis at harvest stageshowed that organic C ranged from 0.32% ill the pigeon pea treatment to 0.57%in the mung bean treatment. The total N ranged from 0.05% in the soybean-2treatment to 0.066% in the soybean- 1 treatment. The mineral N ranged form 10mg in the pigeon pea plot to 18 mg N g-1 soil in the mung bean plot. Theresults showed that the levels of organic C, total and total mineral N in soilwere generally greater for the legumes than for the non-legume treatments.Thus, it could be concluded that the results of the natural abundance N andthe ureide methods were very similar but that of values for the N differencemethod were higher than the other two methods.