Fate of the ¹⁵n-labelled faeces fraction of dairy farm effluent (DFE) irrigated onto soils under different water regimes

Irrigation of dairy farm effluent (DFE) onto pasture is the preferred treatment method in New Zealand for this very dilute organic effluent. Whereas the dynamics of the urine fraction is comparatively well understood, there is a lack of data on the fate of the mainly organic faecal fraction. To improve our understanding of the complex turnover processes, we labelled both the inorganic and organic N compounds of the faecal fraction of DFE with ¹⁵N. We then measured the ¹⁵N dynamics in various soil and plant fractions in a laboratory experiment at two water contents for up to 254 days. Feeding a dairy cow with ¹⁵N-labelled pasture yielded faeces that had a mean ¹⁵N abundance of 2.95 atom%. Unlabelled urine and water were added to the labelled faeces to construct the DFE, which contained 90.4% of the ¹⁵N in the organic nitrogen fraction (2.87 atom%) and 9.6% in the ammonium fraction (1.23 atom%). As N turnover and losses depend on the method of application and the soil structure, we simulated field conditions by surface-applying our DFE onto intact soil cores with pasture. Two soil water treatments were imposed; dry (30% water content) and wet (water table at 17 cm below the soil surface). The surface application resulted in filtration of the DFE, with a high proportion of the ¹⁵N remaining on the soil surface, where it was relatively unavailable for plant uptake but prone to gaseous and physical losses. Of the applied ¹⁵N, 9.9% in the dry and 13.5% in the wet treatment were still recovered as DFE on the soil surface at day 254. Plant uptake of faecal ¹⁵N accounted for 9.3% and 13.0% in the dry and wet treatments, respectively. The bulk of the ¹⁵N was recovered in the soil organic nitrogen fraction (35.1% in dry, 42.5% in wet), whereas ¹⁵N in inorganic and microbial nitrogen accounted for only very small amounts (< 2%). Total recoveries of the applied ¹⁵N in plant, soil and DFE remaining on the surface at day 254 were 58.4% in the dry, and 71.5% in the wet treatment. Separate analysis of the total and ammonium nitrogen contents and ¹⁵N enrichments of the constructed DFE and filtered subsamples (0.5 mm, 0.2μm) showed that the faecal fraction was not labelled homogeneously. Due to this heterogeneity, which was exacerbated by the filtration of DFE on the soil surface, it was difficult to calculate the turnover of the total faecal fraction based on ¹⁵N results.