Low concentrations of nitrate and ammonium stimulate nodulation and N2 fixation while inhibiting specific nodulation (nodule DW g-1 root dry weight) and specific N2 fixation (N2 fixed g-1 root dry weight) in soybean

Nitrate N is a major inhibitor of the soybean/Bradyrhizobium symbiosis in legumes and although this inhibition has been studied for many years, as yet no consensus has been reached on the specific and quantitative interactions between nitrate and ammonium supply and N-2 fixation. The effect of nitrate and ammonium supply on plant growth, nodulation and N-2 fixation capacity during the full growth cycle was investigated in both greenhouse and growth chamber experiments with three soybean genotypes. The results show that a high concentration of mineral N (10 mM), either as nitrate or ammonium or ammonium nitrate significantly suppressed nodule number, nodule dry weight and total N-2 fixed per plant of nodulated soybeans. However, lower mineral N concentrations, either 1 mM or 3.75 mM significantly enhanced nodule number, nodule dry weight and total N-2 fixed per plant, while specific nodulation (nodule dry weight g(-1) root DW, SNOD) and specific N-2 fixation (total N-2 fixed g(-1) root DW, SNF) were significantly reduced, particularly at the early vegetative growth stage V4, compared to the treatment with N-2 fixation as the only N source, in both growth chamber and greenhouse experiments. Therefore, we suggest that SNOD or SNF might be better indicators to express the suppressing effect of mineral N addition on nodule performance and N-2 fixed. Our studies also showed that ammonium alone was the more efficient N source than either ammonium nitrate or nitrate for soybean, as it resulted in higher biomass accumulation, nodule dry weight, total N accumulation and total N-2 fixed by 23, 20, 18 and 44%, respectively, compared to NO3- as the N source