The effect of animal trampling and DCD on ammonia oxidisers, nitrification, and nitrate leaching under simulated winter forage grazing conditions

Abstract

Purpose: Dairy winter forage grazing on free-draining soils is a common practice within the Canterbury region of New Zealand. The high stocking rates involved and the associated deposition of urine onto wet soils during winter create a high risk of nitrate (NO₃⁻) leaching from soil. The objective of this study was to determine the effect of animal trampling and the use of a nitrification inhibitor, dicyandiamide (DCD), on soil ammonia oxidisers, nitrification, and nitrate leaching under simulated winter forage grazing conditions. Materials and methods: A lysimeter trial was carried out using a Balmoral stony silt loam under a kale forage crop. Nitrate leaching losses, the effect of soil trampling, and the effect of DCD were measured. Soil nitrification rates and ammonia-oxidising community abundance and activity were measured in companion soil blocks under simulated dairy winter forage grazing conditions. Results and discussion: Animal trampling was found to reduce peak nitrate-N (NO₃⁻-N) concentrations in drainage water from urine patch areas by 43 %. In addition, animal trampling reduced the total amount of NO₃⁻-N leached from urine patches by 34 %. However, animal trampling did not affect the growth or activity of the ammonia oxidisers. The use of DCD was found to be highly effective in reducing the concentration and amount of NO₃⁻-N leached from urine patches. Dicyandiamide applications reduced peak NO₃⁻-N concentrations in drainage water by 66 %, and the total amount of NO₃⁻-N leached was reduced by 61 % under the simulated dairy winter forage grazing conditions. Ammonia oxidising bacteria (AOB) were more abundant than ammonia-oxidising archaea (AOA) and were responsible for mediating the nitrification process. Conclusions: These results suggest that both animal trampling and the use of DCD separately reduces soil nitrification rates and thus NO₃⁻ leaching.