Investigating “net” provenance, N source, transformation and fate within hydrologically isolated grassland plots

Abstract

Agricultural landscapes contain many different soil types with heterogeneous nitrogen (N) attenuation capacity. Typically, a zone of contribution (ZOC) surrounding a borehole is used to interpret subsurface hydro-biogeochemical functional capacity. This presents a “net” interpretation of source and attenuation within these calculated areas. Herein, we use the concept of ZOC commonly used for borehole screen intervals but for an end-of-pipe location within four hydrologically isolated plots. Water samples from end-of-pipe and piezometer locations are examined for nitrogen (N), biogeochemical, dissolved gas and isotopic viewpoints to elucidate multi-layered “net” water provenance, N source, transformations and fate. Results showed a nitrate (NO¯3-N) plume migrating in shallow groundwater (between 0.39 and 8.07 mg N/L), with low concentrations in the shallow artificial drainage system (below 3.22 mg N/L). Water provenance data showed distinct signatures of: precipitation and deep groundwater at 3–4 m below ground level (bgl) and water entering, migrating and discharging at the end of pipe location. The latter signature was caused by enrichment of δ¹⁸O-H₂O during migration. This means there was disconnectivity on site with no interaction between water migrating through the drainage pipe at 1 m and deeper groundwater migrating at 3–4 m depth. The analysis of NO¯3-N concentration and its isotopic signature (δ¹⁵N-NO₃¯ and δ¹⁸O-NO₃) identified further connections between screen interval depths and an up-gradient organic point source with elevated NO¯3-N migrating at this depth and different transformation processes occurring at different depths. Temporally NO¯3-N concentrations at this depth have decreased over time. Fenton et al. documented an average of 7.5 (±4.5) mg N/L whereas Ibrahim et al. documented an average of 6.8 (±3.7) mg N/L at this depth. The point source was removed in 2006 and NO¯3-N concentration in the present study have further reduced to an average of 3.9 (±2.8) mg N/L. End-of-pipe data at 1 m bgl highlighted connectivity with the overlying plot and showed different water attenuation functionality than the deeper system. End-of-pipe locations clustered together along the denitrification line. This highlighted a consistency of signals across the four plots in terms of what occurs in the soil profile above the drain installation depth of 1 m. At 3–4 m bgl however, samples varied spatially showing inconsistency between the end-of-pipe locations and plots indicating the occurrence of different processes. A fuller characterisation of dairy farm N sustainability can be deemed using the “net” provenance, N source, N transformation and fate methodology presented. Future work should investigate how drainage design (shallow and groundwater) affects N transformation and the “net” concept developed herein should be rolled out to rank dairy farms in terms of their N attenuation capacity.