Using delta15N and delta18O to evaluate the sources and pathways of NO3- in rainfall event discharge from drained agricultural grassland lysimeters at high temporal resolutions.
The origin of NO(3) (-) 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 delta(15)N-NO(3) (-) and delta(18)O-NO(3) (-) 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 m(3) of NH(4) (+)-N-rich agricultural slurry, during two rainfall events. Drainage started to occur 1 month after the application of slurry. The concentrations of NO(3) (-)-N from the two lysimeters were comparable; an initial flush of NO(3) (-)-N occurred at the onset of drainage from both lysimeters before levels quickly dropped to <1 mg NO(3) (-)-N L(-1). The isotopic signature of the delta(15)N-NO(3) (-) and delta(18)O-NO(3) (-) during the first two rainfall events showed a great deal of variation over short time-periods from both lysimeters. Isotopic variation of delta(15)N-NO(3) (-) during rainfall events ranged between -1.6 to +5.2 per thousand and +0.4 to +11.1 per thousand from the inter-flow and drain-flow, respectively. Variation in the delta(18)O-NO(3) (-) ranged from +2.0 to +7.8 per thousand and from +3.3 to +8.4 per thousand. No significant relationships between the delta(15)N-NO(3) (-) or delta(18)O-NO(3) (-) and flow rate were observed in most cases although delta(18)O-NO(3) (-) values indicated a positive relationship and delta(15)N-NO(3) (-) values a negative relationship with flow during event 2. Data from a bulked rainfall sample when compared with the theoretical delta(18)O-NO(3) (-) for soil microbial NO(3) (-) indicated that the contribution of rainfall NO(3) (-) accounted for 8% of the NO(3) (-) in the lysimeter drainage at most. The calculated contribution of rainfall NO(3) (-) was not enough to account for the depletion in delta(15)N-NO(3) (-) values observed during the duration of the rainfall event 2. The relationship between delta(15)N-NO(3) (-) and delta(18)O-NO(3) (-) 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 delta(18)O-NO(3) (-) in the inter-flow and delta(15)N-NO(3) (-) 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(4) (+)-N.
Granger
Steven J
SJ
Institute of Grassland and Environmental Research, North Wyke, Okehampton EX20 2SB, UK. steve.granger@bbsrc.ac.uk
Heaton
Tim H E
TH
Bol
Roland
R
Bilotta
Gary S
GS
Butler
Patricia
P
Haygarth
Philip M
PM
Owens
Philip N
PN
D000383:Agriculture; D017753:Ecosystem; D004784:Environmental Monitoring / Q000379:methods*; D009566:Nitrates / Q000737:chemistry / Q000378:metabolism*; D009584:Nitrogen / Q000032:analysis / Q000378:metabolism*; D009587:Nitrogen Isotopes / Q000032:analysis; D010100:Oxygen / Q000032:analysis / Q000378:metabolism*; D010103:Oxygen Isotopes / Q000032:analysis; D006109:Poaceae / Q000737:chemistry / Q000378:metabolism*; D011891:Rain; D021241:Spectrometry, Mass, Electrospray Ionization; D014872:Water Movements
D009566:Nitrates; D009587:Nitrogen Isotopes; D010103:Oxygen Isotopes; D009584:Nitrogen; D010100:Oxygen