INFLUENCE OF THE PRESENCE OF NITRITE AND NITRATE IN SOIL ON MAIZE BIOMASS PRODUCTION, NITROGEN IMMOBILIZATION AND NITROGEN RECOVERY

When comparing nitrite (NO2- and nitrate (NO3-) toxicity to maize (Zea mays L.) growth, it is important to know the fate of applied nitrogen (N). A pot experiment, using potassium nitrite ((KNO2)-N-15) and pota ssium nitrate ((KNO3)-N-15) was conducted to determine the fate of N ( 0, 75, 150, and 225 mg N kg(-1) soil) applied to a sandy loam soil col lected from Gistel (Belgium). The total dry weight of the plants treat ed with NO2- was lower than that of the plants treated with NO3- at 15 and 26 days after N application (harvest 1 and harvest 2, respectivel y). Shoot and root biomass reduction started at a relatively low NO2- application rate (75 mg NO2--N kg(-1)). Biomass reduction increased, a t both harvests with increasing amounts of NO2- to more than 55% at th e highest application rate (225 mg NO3--N kg(-1)). In the NO3- treatme nt, a reduction of 16% in total plant dry biomass was recorded only at the highest application rate (225 mg NO2--N kg(-1)), at both harvest times. The N-15 plant uptake (shoots plus roots) at harvest 1 decrease d with increasing N application rates of both N forms (KNO2 and KNO3). Twenty-six days after the N application, the total N-15 taken up by t he plant increased in all treatments in comparison with 15 days after the N application. However, only at higher rates of N application (150 and 225 mg N kg(-1)) was the N-15 uptake by the NO2- fed plants signi ficantly lower than by the NO3- fed plants. The percentage of immobili zed N from the applied N was low (0-17.7%) at both harvests, irrespect ive of the N source. However, with relatively low N application rates (75 mg N kg(-1)), the immobilized N in the soil decreased with time. T his may be due to the re-mineralization of the applied N. The percenta ge of inorganic N-15 in the soil in NO2- treatments was slightly lower than in equivalent doses of NO3-. This might be due to higher losses of N as N-oxides. Unaccounted for N from the applied N ranged from 21% to 52% for the NO2- treatments and from 3% to 38% for the NO3- treatm ents.