Using [delta]15N and [delta] 18O to evaluate the sources and pathways of NO-3 in rainfall event discharge from drained agricultural grassland lysimeters at high temporal resolutions

Abstract

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