Legume-based forage production systems reduce nitrous oxide emissions

• N2O-emissions were measured as a function of N-fertilization and soil compaction on a grassland site.
• N2O-emissions were influenced by the interaction N-fertilization × soil compaction.
• Soil moisture at the moment of soil compaction played a key role in this interaction.
• CO2eq emissions of legume-based and N-fertilized grasslands were compared.
• Legume-based swards emitted 67% less CO2eq than N-fertilized swards.

Nitrous oxide (N2O) emissions from agriculture demand attention because they are the main source of total global anthropogenic N2O-emissions. High N-fertilization and soil compaction are important factors that increase N2O-emissions. On intensively managed grassland sites both factors occur. Knowledge of the interaction of high N-fertilization and simultaneous soil compaction on N2O-emissions is therefore essential, but previous studies about this scenario are rare. In the presented study, N-fertilized grass swards (G) and unfertilized lucerne–grass mixtures (LG) were compared over a three-year period (2006–2008): N2O-emissions and dry matter yield were measured as a function of N-fertilization (0 (LG), 360 kg N ha−1 yr−1 (G) as CAN) and soil compaction (0 (C0), 321 kPa (C321)) on a loamy stagnic Luvisol derived from glacial till in northern Germany. CO2-equivalents (CO2eq) per hectare and per unit metabolizable energy (GJ ME) were calculated.N2O-emissions were significantly influenced by the interaction N-fertilization × soil compaction; emissions increased significantly when both factors were induced simultaneously (G/C0: 8.74, LG/C0: 2.46, G/C321: 13.31 and LG/C321: 2.22 kg N2O-N ha−1, respectively). Concerning the specific CO2-emissions, expressed in CO2eq (GJ ME)−1, the N-fertilized G swards emitted 67% more CO2eq than LG swards assuming that 50% of the field plots were compacted due to heavy wheel traffic, which are reliable figures from agricultural practice. Neither dry matter (DM) yield nor forage quality (MJ ME (kg DM)−1) differed significantly between fertilized G and unfertilized LG swards. Hence, legume-based instead of fertilizer-based forage production is a promising mitigation option without significant reduction of DM yields. In addition, results regarding soil compaction effects on GHG-emissions emphasize the urgent need to implement controlled traffic systems on intensively managed grassland in order to reduce the area affected by heavy wheel traffic.