INFLUENCE OF RAINFALL AND IRRIGATION EVENTS ON NUTRIENT DYNAMICS IN DAIRY FARMS

Nutrient runoff from dairy farms is a well-known driver of eutrophication. While prior research on nutrient dynamics in agricultural systems has primarily focused on catchment scales, often overlooking localised nitrogen processing hotspots, our study investigates nutrient transformations at the farm scale, hypothesising that his approach will offer deeper insights into improved nutrient management strategies.  We measured nutrient concentrations (nitrate and ammonium) and isotopes (δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃⁻, δ¹⁵N-NH₄⁺) in farms’ runoff during rainfall and irrigation events. Significant variations of δ¹⁵N-NO₃⁻ in the farms’ runoff (-6.8 -23 ‰) compared to input (-3.8 to 7.1‰) strongly suggest that nutrients were being altered within the pasture before leaving the farm. Low δ¹⁵N-NO₃⁻ (as low as -6.8‰) in some runoff and manure samples (-3.8‰) were consistent with nitrification, while increases of δ¹⁵N-NO₃⁻ (23 ‰) suggested active denitrification. Very low δ¹⁵N-NH₄⁺ (as low as -10‰) coincident with moderate to high δ¹⁵N-NO₃⁻ (4.6 – 14‰) suggested dissimilatory nitrate reduction to ammonium (DNRA) was a significant process. Even during rainfall, significant isotope fractionation occurred, indicating complex transformations despite short water residence times. As such, the use of nitrogen isotopes to trace nitrogen sources should be undertaken with a detailed understanding of likely fractionation processes. This can help shed light on the times and locations of nitrogen transformation and loss processes to help improve management practices.

Presentation Preference: Oral

Primary Presenter: Justice Osuoha, Monash University (justice.osuoha@monash.edu)

Authors:

Justice Osuoha, Monash University  (Justice.osuoha@monash.edu)

Wei Wen Wong, Monash University  (WeiWen.Wong@monash.edu)

Anna Lintern, Monash University  (Anna.lintern@monash.edu)

Perran Cook, Monash University  (Perran.cook@monash.edu)