Soil type affects partitioning of ruminant urine-15N due to variation in nitrification potential, immobilisation and drainage under autumn-winter conditions.

Abstract

There is a need to better understand how soil type affects ruminant urine-N and the in situ losses of nitrous oxide (N₂O) and dinitrogen (N₂) from pasture ecosystems. Similarly, the significance of processes responsible for these losses as N₂, such as codenitrification and denitrification, remain unclear. A 105 day in situ study quantified N₂ and N₂O fluxes from two contrasting soil types, well-drained (Allophanic) and poorly-drained (Gley), after synthetic ruminant urine application (800 kg N ha^-1, 40 atom % ¹⁵N) in late autumn. The well-drained soil promoted nitrification, as evident from inorganic-N dynamics, higher bacterial amoA gene copy numbers and faster pH decline. The N₂O emission factors from the poorly and well-drained soils were 1.48 ± 0.24% and 0.22 ± 0.03%, respectively. Denitrification N₂ fluxes were similar regardless of soil type, totalling 15% of ¹⁵N applied, and increased markedly after 60 days. While gene copy numbers of nosZI and nosZII varied temporarily, increasing as the poorly drained soil became anaerobic, there was no effect due to urine application, which may have been a consequence of denitrification occurring at depths below that used for microbial soil sampling. Codenitrification made a minor contribution to N₂ fluxes, accounting for ≤ 0.9% of N₂ fluxes in the well-drained soil and 12.1% in the poorly drained soil, confirming that codenitrification increases when soil conditions become anaerobic. In the well-drained soil, the ¹⁵N recovery of urine showed that more urine-N was lost via nitrate leaching, followed by immobilization in deeper soil depths. This highlights the importance of monitoring soil chemistry and biology in deeper soil layers when studying urine-N turnover and fate in grazed pasture soil. Despite identical historical grazing management these results show that soil type plays a determining role in urine-N transformation and fate, potentially due to differences in microbial activity, soil chemistry and drainage in deeper soil layers.