Fate of a dairy cow urine pulse in a layered volcanic vadose zone

Abstract

Nitrate-N leaching from dairy cow urine patches has been identified as one of the major contributors to groundwater contamination and degradation of surface waters in dairying catchments. To investigate the transport and transformations of nitrogen (N) originating from urine, fresh dairy cow urine was collected, amended with the conservative tracer chloride (Cl) and applied onto a loamy sand topsoil, underlain by gritty coarse sands and pumice fragments in the lower part of the vadose zone. The fluxes of the different N components and the conservative tracer leaching from the urine application were measured at five different depths in the vadose zone using three Automated Equilibrium Tension Lysimeters (AETLs) at each depth (max. 5.1 m). The uppermost part of the saturated zone was also monitored for the leached N and Cl fractions from the urine application. Textural changes and hydrophobicity in the vadose zone materials resulted in heterogeneous flow patterns and a high variability in the N and Cl masses captured. All three forms of potentially leachable N from the urine – organic-N (org-N), ammonium-N (NH4-N) and nitrate-N – were measured at the bottom of the root zone at 0.4 m depth. At the 1.0 m depth, effectively all of the captured N was in the mobile nitrate-N form. In the lower part of the vadose zone at 4.2 m, 33% of the applied urine-N was recovered as nitrate-N. This fraction was not significantly different from the corresponding fraction measured at the bottom of the root zone, indicating that no substantial assimilation of the nitrate-N being leached from the root zone was occurring in this vadose zone.