Impact of drain spacing on subsurface drainage and greenhouse gas fluxes in a grassland on a Mollic gleysol in western Norway

To study the effect of drainage intensity on GHG emissions and N drainage losses in cool-humid Norway, we established drainage systems with 6 and 12 m drain spacing in a previously undrained sandy loam (Mollic gleysol) collecting data in the years 2014–2016. After sowing a mixed grass ley, subsurface drainage was larger (1271 versus 699 mm) and mean ground water table (GWT) lower (102 versus 79 cm) with 6 than with 12 m drain spacing. Water filled pore space (WFPS) remained high throughout most of the year (> 80 %). It was highest in 12 m drain spacing, but shortly after fertilizations no differences between the two drainage systems were found. N 2 O emissions after fertilization were larger in the 12 m system than in the 6 m system. Cumulative N 2 O emissions in the 6 and 12 m system were 4.0 versus 2.5 kg N ha −2 yr −1 . N leaching for the entire observation period (29 months) was larger in the 6 m (42 kg ha −1 ) than the 12 m (19 kg ha −1 ) system . Grass yields, plant N-recovery and fertilizer N use efficiency was larger with 6 than 12 m. The mean N 2 O emission factor was significantly higher with 6 than with 12 m drain spacing (1.4 versus 0.8 % N 2 O-N of N applied). The 6 m system acted as a net sink for CH 4 , whereas the 12 m system was a net CH 4 source and had a higher climate forcing than the 12 m system (1390 versus 1110 g CO 2 eq. m −2 yr −1 ), but scaled for grass dry matter yield the climate forcing was similar. We conclude that larger N 2 O emissions with 6 m drain spacing were likely due to a combination of less complete denitrification and a naturally higher SOM content at this site, releasing extra mineral N. Our study can therefore not confirm that increased drainage intensity intrinsically reduces N 2 O emissions from crop production in cool-humid climates. • Reducing drain spacing from 12 to 6 m did not reduce N 2 O emissions. • Soil variation has a large effect on the impact of drain spacing on N 2 O emission. • Reduced drain spacing increased subsurface drainage and N-leaching. • Soil conditions were more reductive with 12 than 6 m drain spacing. • Reduced drain spacing changed CH 4 fluxes from net emission to net uptake.