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. N2O emissions after fertilization were larger in the 12 m system than in the 6 m system. Cumulative N2O 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 N2O emission factor was significantly higher with 6 than with 12 m drain spacing (1.4 versus 0.8 % N2O-N of N applied). The 6 m system acted as a net sink for CH4, whereas the 12 m system was a net CH4 source and had a higher climate forcing than the 12 m system (1390 versus 1110 g CO2 eq. m2 yr 1), but scaled for grass dry matter yield the climate forcing was similar. We conclude that larger N2O 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 N2O emissions from crop production in cool-humid climates.