Nitrogen use efficiency and summer nitrous oxide emissions in a long term organic grass-clover ley system : implications of anaerobic digestion of cattle slurry on nitrogen management

Sammanfattning

This study investigated the long-term impact of anaerobically digested slurry (ADSH) on nitrogen use efficiency (NUE) and nitrous oxide (N₂O) emissions in comparison to untreated slurry (USH) and a control without fertilizer. The hypothesis was that ADSH would improve NUE and reduce N₂O emissions compared to USH but increase emissions relative to the control. The study was conducted on an organic dairy farm in Tingvoll, Norway, where three treatments (control, USH, and ADSH) were applied to experimental plots in a randomized block design. The main outcomes were forage yield, NUE, and N₂O emissions, with yield and NUE assessed over multiple years and N₂O emissions measured on residual effects from long-term application during the 2024 growing season when no fertilizer was applied. The results indicated that both USH and ADSH significantly improved yield compared to the control, with ADSH showing a slight yield increase over USH, although this difference was not significant. Yield variations were more influenced by seasonal factors, such as precipitation and temperature, than by fertilizer treatment. NUE was similar between ADSH and USH, and both treatments showed reduced NUE in the second cut compared to the first. A negative correlation between NUE and clover percentage suggested that biological nitrogen fixation may have reduced the efficiency of the applied organic nitrogen fertilizers. N₂O emissions were primarily driven by soil temperature and lagged precipitation, with no significant treatment effect on emissions. Interestingly, ADSH and USH exhibited similar emissions, with the treatment effect only interacting with temperature. The results suggest that environmental factors, particularly temperature and precipitation, have a stronger influence on N₂O emissions than fertilizer type. Limitations of the study include the short duration of N₂O measurements and potential disturbances from frame placement. Overall, the findings suggest that fertilizer type has a minor impact on NUE and N₂O emissions under cool, wet conditions, highlighting the importance of environmental factors in shaping agricultural emissions. Further research is needed to determine if these results hold in other climates or under different management practices.