We tested the effects of seven vitamins [biotin, Ca-pantothenate, cyanocobalamin (B12), niacin, riboflavine (B2), and pyridoxine (B6), thiamin (B1)] at two concentrations of 0.01 and 1 µmole/g soil on the Nitrification of NH4+ (NH4Cl at 26 µmol/g soil) to NO3- during a 165-day period. Two controls were also tested: control A, soil to which no vitamin or NH4Cl was added, and control B, soil to which only NH4Cl was added. All soils were incubated at 25°C and kept moist at ca.65% of field capacity. The amount of NH4+ extractablefrom soils treated with NH4Cl decreased and approached that of the control A about 20 days after NH4Cl was added to the soils. The increase in the concentration of NO3- in soil showed a biaphasic pattern in all treatments; the first plateau was reached on day 20 (when the concentration of NH4+ in the soil had reached its minimum) and did not change much for the next 20 days. Subsequently, a surge in NO3- production was noted which leveled off after another ca. 100 days. None of the vitamins at the rate of 0.010 µmol/g soil had any effect onthe rate of NH4+ decrease or that of NO3 - increase in the soil. At 1 µmol/g, the addition of vitamins B12 niacin, pyridoxine, and biotin significantly increased the amount of NO3-produced in the soil. On day 165, riboflavin increased the NO3- in soil by 2.1 µmol/g (8.47 vs. 6.37 µmol/g soil), thiamin had increased it by 1.52 µmol/g (7.89 vs. 6.37 µmol/g soil) and Ca-pantothenate by 0.5 µmol/g (6.87 vs. 6.37), when compared with control B soil to which only NH4Cl had been added. Since the addition of 1 µmol/g of riboflavin, thiamin and Capantothenate had added 4, 4 and 2 µmol/g of nitrogen (in the vitamin molecule) to soils,respectively, the extra NO3 - produced when these vitamins were added to the soil may have been the product of microbial decomposition of these vitamins. It is concluded that the vitamins tested did not play a role in the phenomenon of added nitrogen interaction (ANI) or the priming effect.

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).