Ammonia and nitrous oxide emissions from soils under ruminant urine patches and the effects of biochar amendment on these emissions and plant nitrogen uptake

Abstract

Urine and dung from ruminants contributes to emissions of both nitrous oxide (N₂O) and ammonia (NH3). These are important for a variety of agronomic and environmental reasons. Urine contributes a much larger fraction than dung to the total NH3 and N₂O emissions. Nitrous oxide is a potent greenhouse gas and its emission from grazing animal excreta is considered a major loss pathway for N₂O emitted from agricultural soils. Ammonia can be volatilised from the soil surface shortly after ruminant urination. In addition, a fraction of this NH3 may be converted into N₂O after NH3 is redeposited onto the soil. Biochar is produced as a by-product of the low temperature pyrolysis of biomass during bioenergy extraction. Incorporation of biochar into soil is of global interest as a potential carbon sequestration tool. Nitrogen (N) transformations in soil can be affected by the presence of biochar. This current research has been conducted to determine the effects of incorporating biochar into soil, on: ruminant urine-derived N₂O fluxes, NH3 volatilisation, N uptake by pasture, and pasture yield. The first experiment examined the effects of biochar incorporation (0, 15 and 30 t ha-1) on N2O emissions during an 86-day spring-summer field study following 15N-labelled ruminant urine application. The results showed that N2O fluxes were reduced by 70%, after incorporating 30 t ha-1 of biochar. The uptake of NH3 by biochar was proposed as a possible mechanism for the reduced N2O emissions. No differences occurred, due to biochar addition, with respect to dry matter yields, herbage N content, or herbage recovery of 15N. In the second experiment the capacity of four biochar types to take up NH3 was determined in order to define a possible mechanism for reduced N₂O emissions observed under 30 t ha-1 biochar in first experiment. The subsequent plant availability of biochar adsorbed NH3-N was then determined. The results showed that NH3-N adsorbed by biochar was stable, but readily plant available when placed in the soil. Plant dry matter yields were 2 to 3 times greater and N uptake by plants doubled when biochar containing adsorbed NH3 was incorporated into soil over a 25-day incubation study. The third experiment was conducted to measure NH3 volatilisation in-situ using micrometeorological methods during a 6-day summer field study. Soil temperature, pH, ammoniacal-N and moisture were measured in the top soil to provide input parameters for a volatilisation model. Cumulative NH3 volatilisation was 25.7 (± 0.5) % of the N applied. The results from this experiment were used as a maximum and in-situ measure of the NH3 volatilisation rate following urine application. The fourth experiment was conducted to determine if incorporating biochar (0, 15 and 30 t ha-1) actually reduced NH3 volatilisation from soil under ruminant urine application, and assessed the subsequent plant availability of this biochar adsorbed NH3-N. The NH3 volatilisation from 15N-labelled ruminant urine, applied to soil, was reduced by 45% after incorporating either 15 or 30 t ha-1 of biochar. When these urine-treated biochar particles were placed in fresh soil, subsequent plant growth was not affected but the uptake of 15N in plant tissues increased, indicating that the adsorbed-N was plant available. This thesis demonstrates incorporating biochar into the soil can significantly decrease NH3 and N2O emissions from ruminant urine with the captured N recycled to crops while simultaneously sequestering carbon in soils.