LEACHING IN SMALL LYSIMETERS OF NITRATE DERIVED FROM NITROGEN-15-LABELED FIELD PEA RESIDUES

Field pea (Pisum sativum L.) crop residues labeled with N-15 were inco rporated during September in small monolith lysimeters of a sandy loam soil. The leaching of N-15-labeled and nonlabeled NO3 to below 45 cm was studied during the subsequent 2 to 3 yr comparing (i) planted vs. unplanted soil and (ii) with and without pea residue incorporation. Th e peak flux of labeled NO3-N from uncropped soil occurred in December. During the first leaching period (September-April), 15 and 7% of the residue N was recovered as NO3 in the percolate in the two experiments , corresponding to 18 and 13% of total NO3-N in the percolates, respec tively. The difference observed in leaching of labeled N in the two ex periments was mainly due to a much higher drainage volume in the first experiment. The different residue particle sizes, 10 mm or <3 mm, use d in the two experiments may have influenced the immoblization-mineral ization of N in the soil differently and consequently the potential fo r NO3 leaching. Pea residue N constituted 10 to 15% and 5% of total NO 3-N leached from unplanted soil in the second and third leaching perio ds, respectively. Incorporating the ground pea residues reduced the to tal amount of NO3 leached by 15% during the first leaching period, ind icating that the pea residue may be an important factor in controlling leaching losses. Ryegrass (Lolium perenne L.) established on lysimete rs at the time of residue incorporation reduced the total leaching of N in the first leachi[tg period by 15%, but in the second and third le aching period grass eliminated NO3 leaching completely.