CONCENTRATION OF NITRATE IN SOLUTION AS INFLUENCED BY SAMPLE HOLDING TIME AND METHODS OF ANALYSIS

Quality Assurance and Quality Control (QAQC) criteria (by the U.S. Env ironmental Protection Agency) stipulate 24 to 48 h of sample holding t ime for nitrate-nitrogen (NO3-N) analysis. There is no information on the effects of prolonged sample holding time or methods of analysis on the concentration of NO3-N in solutions. Fertigation water samples co ntaining various concentrations of NO3-N (which were collected at two minute intervals immediately affer pumping the liquid fertilizer into the irrigation water for a period of 40 min) and soil leachate samples collected at different depths, under citrus tree canopy, using suctio n lysimeters were analyzed for concentrations of NO3-N using ion chrom atograph (IC) and rapid flow analyzer (RFA) methods. Significant corre lations were found between the IC and RFA methods for concentrations o f NO3-N (r(2) = 0.90). Therefore, the two methods are similar with reg ard to the precision in measuring concentrations of NO3-N in solution samples. In a parallel experiment, concentration of NO3-N was measured (using the IC method) in soil leachate samples (collected from suctio n lysimeters): (i) within 24 h of sample collection, and (ii) after st orage for 50 (Set-I) or 80 d (Set-2) at -20 degrees C. The prolonged s torage of the samples had little effects on the concentration of NO3-N as evident from high correlation (r(2) = 0.97; with slope of 0.87) be tween the concentrations measured within 24 h and those measured after 50 to 80 d of storage. Therefore, the storage of solution samples at -20 degrees C up to 50 to 80 d without acidification does not alter th e concentration of NO3-N.