The origin of NO yielded in drainage from agricultural grasslands is of environmental significance and has three potential sources; (i) soil organic mater (SOM), (ii) recent agricultural amendments, and (iii) atmospheric inputs. The variation in 15N-NO and 18O-NO was measured from the inter-flow and drain-flow of two 1 ha drained lysimeter plots, one of which had received an application of 21 m3 of NH-N-rich agricultural slurry, during two rainfall events. Drainage started to occur 1 month after the application of slurry. The concentrations of NO-N from the two lysimeters were comparable; an initial flush of NO-N occurred at the onset of drainage from both lysimeters before levels quickly dropped to <1 mg NO-N L-1. The isotopic signature of the 15N-NO and 18O-NO during the first two rainfall events showed a great deal of variation over short time-periods from both lysimeters. Isotopic variation of 15N-NO during rainfall events ranged between -1.6 to +5.2 and +0.4 to +11.1 from the inter-flow and drain-flow, respectively. Variation in the 18O-NO ranged from +2.0 to +7.8 and from +3.3 to +8.4. No significant relationships between the 15N-NO or 18O-NO and flow rate were observed in most cases although 18O-NO values indicated a positive relationship and 15N-NO values a negative relationship with flow during event 2. Data from a bulked rainfall sample when compared with the theoretical 18O-NO for soil microbial NO indicated that the contribution of rainfall NO accounted for 8% of the NO in the lysimeter drainage at most. The calculated contribution of rainfall NO was not enough to account for the depletion in 15N-NO values observed during the duration of the rainfall event 2. The relationship between 15N-NO and 18O-NO from the drain-flow indicated that denitrification was causing enrichment in the isotopes from this pathway. The presence of slurry seemed to cause a relative depletion in 18O-NO in the inter-flow and 15N-NO in the drain-flow compared with the zero-slurry lysimeter. This may have been caused by increased microbial nitrification stimulated by the presence of increased NH-N
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