Interactive effects of exogenous combined nitrogen and phosphorus on growth and nitrogen fixation by azolla

Effects of N source and media-N and P levels were examined on growth, N uptake, and N₂ fixation ofAzolla pinnata withAnabaena azollae association (azolla) at two inoculum-P concentrations. Each expeiment was conducted for 7 days in a growth chamber using azolla at a predetermined inoculum-P concentration and the growth media containing a combination of four levels of P (0, 15, 75, and 200 M) and three levels (0, 1, and 5 mM) of either¹⁵N-enriched NH₄⁺ as ammonium sulfate or¹⁵N-enriched NO₃^– as potassium nitrate. Nitrogen uptake and N₂ fixation were measured by¹⁵N isotopic dilution method. Tissue P and N, N uptake, and N₂-fixation increased with increasing P concentration in the media regardless of the inoculum-P level of azolla. Increasing P concentration in the media increased growth of azolla at low inoculum P, but the effect on high inoculum-P azolla was either small or absen. High inoculum-P concentration resulted in increased growth, tissue-N and P concentrations, N uptake, and N₂ fixation by azolla. Ammonium in the growth media caused larger increase in tissue-N and greater repression of N₂ fixation than equimolar concentration of NO₃^–. In the presence of NH₄⁺ or NO₃^–, in the growth media, N uptake by azolla exceeded the corresponding decrease in N₂ fixation, resulting in an overall increase in tissue-N concentration. Phosphorus in the media tended to negate the inhibitory effect of NH₄⁺ or NO₃^– on N₂ fixation. A multiple regression model showed that the effect of tissue-N on N₂ fixation was negative while that of tissue-P was positive. Therefore, a relative change in tissue-N and P appeared to regulate N₂ fixation. Tissue-N and P had similar effects on relative growth rate of azolla also. Inoculum-P level of azolla was important in determining the response to media-P.This research was supported by a grant from USAID under Indo-US Science and Technology Initiative.