Does Drainage of Poorly Drained Soils Affect Their Nitrogen Attenuation Capacity? Evidence from Six Dairy Farms in South Ireland.

In Ireland a high reliance on grazing and reduced mechanisation allows Irish milk to have the lowest carbon footprint within the European Union (EU). However, the efficiency of nitrogen (N) use within Irish pasture-based systems is variable and N leaching occurs. Heterogeneous soils create high and low denitrification areas, with attenuation capacity increasing with decreasing drainage class. However, to facilitate dairy expansion, poorly or imperfectly-drained soils are the first to be artificially drained. A major knowledge gap is whether such intervention disrupts natural attenuation capacity or simply transports attenuated water along the transfer continuum. The present multi-farm study (1 research and 5 commercial farms) was conducted on extensive monitoring networks (in-field drains, open ditches and groundwater). Water samples were taken for NO₃^--δ¹⁵N, NO₃^--δ¹⁸O, NH₄⁺-δ¹⁵N, N₂O-δ¹⁵N, dissolved-N₂O and excess-N₂ at multiple locations and allowed elucidation of N speciation, sources and transformational processes across these networks.

Organic NO₃^--N sources dominated monitoring points across all farms. On the research farm long term groundwater samples (2005-2014) exhibited high spatial NO₃^--N (0-25 mg/l) and NH₄⁺-N (0-23 mg/l) differences while concentrations in drainage systems were below EU standards. “Remediation” was shown with main transformational process indicative of denitrification but with a wide range of values (54.9-6.2‰, δ¹⁵N-NO₃^-; 28.2‰-2.3‰, δ¹⁸O-NO₃^-) suggesting variable attenuation and completeness. Commercial farms showed high NH₄⁺-N concentration (0-1.3 mg/l) but no NO₃^--N/l contamination (0-4.13 mg/l), again main transformational process was denitrification (nitrification on one farm) with smaller intra-farm ranges.

In conclusion across six sites artificial drainage systems installed in poorly or imperfectly-drained soils did not disrupt their inherently high attenuation capacity but instead acted as a conduit transporting attenuated water. However, some NH₄⁺-N breaches occurred, which need to be investigated further.

Future installation of land drainage should be guided by such information, avoiding areas of low attenuation capacity.