LOSS AND ASSIMILATION OF N-15-NITRATE ADDED TO A NORTH-DAKOTA CATTAILMARSH

Field N-15 balance studies were conducted to determine the fate of (KN O3)-N-15 added during summer to surface water in 0.27 m(2) plots withi n a North Dakota cattail (a mixture of Typha glauca Godr. and Typha an gustifolia L.) marsh. In a 1989 study, 50.4 mmol(15)NO(3)(-)-N (approx imately 1.1 mmol NO3- 1(-1)) disappeared within 8 days after a tempora ry build-up of NO2--N. Approximately 42% of the applied N-15 remained in the marsh. This residual N was found in cattail roots and rhizomes (9.5%), floating plants (Lemna minor L. and Spirodela polyrhiza (L.) S chleiden) (8.9%), soil (8.3%), litter (6.9%), surface water (4.6%) and cattail shoots (4.1%). The missing N-15 (58%) was presumably lost as a result of denitrification. In a 1991 experiment, 50.4 mmol of (KNO3) -N-15 was added to plots which had previously received additions of de ionized water or 153 mmol of unlabeled KNO3. All the labeled NO3- agai n disappeared within 8 days. The unaccounted-for N-15 was approximatel y 56% and 82% of the added (KNO3)-N-15-N in plots without and with the antecedent unlabeled KNO3 treatment, respectively. The difference in recovery (26%) was highly significant. Stimulation of dissimilatory NO 3- and NO2- reductases and/or suppression of dissimilatory reduction o f (NO3-)-N-15 to (NH4+)-N-15 are postulated as likely factors for the higher apparent denitrification loss associated with the antecedent KN O3 treatment.