Evaluation for nitrogen fixation and yield of soybean cultivars under conditions of zero and conventional tillage by 15N technique

This research project evaluated the choice of a non-fixing control to quantify N2 fixation by 15N isotope dilution, and the effect of tillage regime and soybean cultivars on yield and N2 fixation of soybean in northern, central and northeastern of Thailand. Japanese non-nodulating lines Tol-O and A62-2 were the most appropriate control plants for 15N isotope dilution for Thai soybeans in these soils which contained indigenous rhizobia. Cereals such as maize, sorghum and barley were also appropriate controls at some sites. The choice of the appropriate no-fixing control plant for the 15N isotope dilution technique remains a dilemma and no alternative exists other than to use several possible controls with ecah experiment. The recommended Thai soybean cultivars (SJ. 1,2,4,5) and an advanced line 16-4 differed little in their ability to support N2 fixation or yield, possibly due to similar ancestry. The ten AVRDC (ASET) lines showed considerable genotypic control in their ability to utilize their three available N sources (soil, fertilizer, atmosphere) and to assimilate and translate them into yields. None of these lines were consistently superior to Thai cultivars SJ.4 or SJ.5 although ASET lines 129, 208 and 217 showed considerable promise. Neither recommended Thai nor ASET cultivars were affected by tillage regime. Zero tillage resulted in superior N2 fixation and yield at Khon Kaen site but conventional tillage was superior at Chiang Mai Site. Soybean cultivars grown in Thailand were well adapted to zero tillage. Nitrogen fixation of Thai soybean cultivars were similar to world figures, averaging more than 16 kg N/rai or 100 kg N/ha and supplying over 50 percent of the plant's N yield. However, seed yield at Chiang Mai site seldom exceeded 320 kg/rai or 2 ton/ha, which is well below yield for temperately grown soybeans. It is not clear why Thai soybeans support N2 fixation, but do not utilize the nitrogen for higher seed yields.