Removing Solids From Dairy Slurry: Effect On N Use Efficiency and Emissions of N2 O.

It is important to find ways to improve the efficiency of nitrogen use from animal manures.  Proper assessment of nutrient use must be carried out in long-term trials to ensure that the effects of the less available organically-bound nutrients can be assessed in terms of both crop production and possible impact on the environment. In this multi-year trial in a moist maritime climate (coastal British Columbia, Canada), we examined how removing solids form dairy manure slurry affects efficiency of N use of a perennial grass, tall fescue.  We also monitored the effect of the treatments on annual emissions of N₂O in the 8^th year of the trial.  Solids were removed by settling and decanting which reduced slurry viscosity, enhanced soil infiltration and reduced the organic N fraction. The grass receiving the separated liquid fraction had significantly greater (20-30%) N use efficiency and N recovery than grass receiving whole manure and the effect was most pronounced in summer. The improvement can be attributed to lower ammonia loss, a higher proportion of available N and a lower proportion of potentially immobilizing carbon in the liquid fraction. Over a full year of measurements, separation increased annual emissions of N₂O from about 0.8 to 1.3% of total applied N, with most of the difference occurring soon after applications; whole manure had greater fall/winter emission.  The results suggest that higher N use efficiency was not associated with lower N₂O emissions. However, there was no difference in emission per unit of N uptake for the two manures, so the benefit of separation was not substantially lessened by emissions of N₂O. The study is continuing.