Nitrification Inhibition Properties in Root Exudates of Brachiaria humidicola Plants

Nitrification inhibition (NI) by climax ecosystems has been suggested for decades, and this inhibitory effect seems to be a feature of wild genotypes rather than commercial cultivars. Many plants particularly grasses were suggested to have NI activity, and recently Brachiaria humidicola (BH) was shown to have promising control on nitrification rates through root exudates. In this study effects of different treatments such as N form (NH4+ vs NO3-) and N concentrations (1, 2 and 4 mM N), plant age, light intensity and different collecting mediums for root exudates on the NI activity of root washings were investigated. This was done using a series of nutrient solution experiments. The results showed that BH root exudates collected in distilled water, independent of light intensity, plant age, N-forms, N-concentrations and root exudates collection periods, had no significant inhibition on nitrification. However, root exudates collected in a 1 mM NH4Cl medium had significant inhibition on nitrification process in a soil bioassay. This inhibition was more highlighted when plants were grown in presence of ammonium rather than nitrate. In comparison to drying with rotary evaporator, freezed dried root exudates indicated significant NI in root exudates of plants which were grown in NH4+ under low light, while this effect was not seen under higher light intensity or nitrate nutrition. Measuring electric conductivity of solutions from root washing also showed higher conductivity when ammonium presented in root medium, particularly in root exudates collecting medium over extended time (24 instead 6 hours