In situ N_2O emissions are not mitigated by hippuric and benzoic acids under denitrifying conditions

摘要

Ruminant urine patches deposited onto pasture are a significant source of greenhouse gas nitrous oxide (N_2O) from livestock agriculture. Increasing food demand is predicted to lead to a rise in ruminant numbers globally, which, in turn will result in elevated levels of urine-derived N_2O. Therefore mitigation strategies are urgently needed. Urine contains hippuric acid and together with one of its breakdown products, benzoic acid, has previously been linked to mitigating N_2O emissions from urine patches in laboratory studies. However, the sole field study to date found no effect of hippuric and benzoic acid concentration on N_2O emissions. Therefore the aim of this study was to investigate the in situ effect of these urine constituents on N_2O emissions under conditions conducive to denitrification losses. Unadulterated bovine urine (0 mM of hippuric acid, U) was applied, as well as urine amended with either benzoic acid (96 mM, U + BA) or varying rates of hippuric acid (8 and 82 mM, U + HA1, U + HA2). Soil inorganic nitrogen (N) and N_2O fluxes were monitored over a 66 day period. Urine application resulted in elevated N_2O flux for 44 days. The largest N_2O fluxes accounting for between 13% (U) and 26% (U + HA1) of total loss were observed on the day of urine application. Between 0.9 and 1.3% of urine-N was lost as N_2O. Cumulative N_2O loss from the control was 0.3 kg N_2O-N ha~(-1) compared with 11,9,12, and 10 kg N_2O-N ha~(-1) for the U, U + HA1, U + HA2, and U + BA treatments, respectively. Incremental increases in urine HA or increase in BA concentrations had no effect on N_2O emissions. Although simulation of dietary manipulation to reduce N_2O emissions through altering individual urine constituents appears to have no effect, there may be other manipulations such as reducing N content or inclusion of synthetic inhibitory products that warrant further investigation.