Slow pyrolysis liquid in reducing NH₃ emissions from cattle slurry - Impacts on plant growth and soil organisms

A substantial percentage of manure nitrogen (N) can be lost as gaseous ammonia (NH₃) during storage and field spreading. Lowering slurry pH is a simple and accepted method for preserving its N. Efficiency of slow pyrolysis liquid (PL) produced from birch (Betula sp.) as an acidifying agent, and its ability to reduce NH₃ emissions following surface application of cattle slurry, was studied in a field experiment. Untreated slurry (US) and slurries acidified with PL and sulfuric acid (SA) were applied to the second harvest of a grass ley. Immediate NH₃ emissions, grass biomass, N-yield and possible toxic impacts on soil nematodes and enchytraeids were examined. Furthermore, the effects on soil respiration, nitrogen dynamics and seed germination were studied in subsequent laboratory experiments. In the field, over one third of the water-extractable ammonium-N (NH₄-N) applied was lost through NH₃ volatilization from US. SA and PL acidified slurries reduced NH₃-N emission rate equally from 3.4 to <0.04 kg ha^-1 h^-1. Acidification with SA resulted in the highest and that with PL in the lowest grass dry matter (DM) and N yield. Neither SA nor PL acidification had negative effects on soil enchytraeids or nematodes. Reduced yield production, seed germination and delayed microbial activity after PL slurry application were most probably caused by the PL containing organic compounds. However, later increase in carbon dioxide (CO₂) production and improved seed germination suggest that these compounds were rapidly volatilized and/or degraded by soil microbes. Though PL efficiently cut NH₃ emission from surface-spread slurry, further studies on appropriate application methods and possible phytotoxicity are needed.