Legume-based forage production systems reduce nitrous oxide emissions

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 (CO(2)eq) per hectare and per unit metabolizable energy (GJ ME) were calculated. N2O-emissions were significantly influenced by the interaction N-fertilization x 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 CO(2)eq (GJ ME)(-1), the N-fertilized G swards emitted 67% more CO(2)eq 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. (C) 2014 Elsevier B.V. All rights reserved.